The vocal cords serve as the primary organ responsible for human vocalization. The vibrational signals generated by the vocal cords carry abundant textual information, much like those of speech signals. Vocal cord vibration recognition technology has the potential to effectively address the fundamental communication challenges encountered in the daily lives of individuals with language disorders. The utilization of flexible pressure sensors enables the detection of vocal cord vibrations and facilitates the discrimination of subtle variations within these vibrations. However, traditional electrical sensors are plagued by issues such as parasitic effects and electromagnetic interference, which significantly restrict their practical application in the detection of vocal cord vibration signals. In contrast, fiber optic sensors, particularly Micro-Nano Fiber (MNF) sensors, are more apt for detecting vocal cord vibrations owing to their small size, rapid response speed, and high sensitivity.At present, some researchers are engaged in using MNF for vocal cord recognition studies, although the intelligent classification and recognition of vibration signals as corresponding speech information has not yet been accomplished. Integrating wearable devices with deep learning presents a novel approach to accurately recognizing vocal cord vibration signals. This paper designs and fabricates a wearable flexible sensor featuring an S-shaped MNF, which consists of two layers of polydimethylsiloxane (PDMS) films with an S-shaped bent MNF embedded within it. The S-shaped MNF structure enlarges the contact area between the optical fiber and the vocal cords, thereby augmenting the efficiency and sensitivity of vocal cord vibration signal acquisition, enabling the capture of more stable and accurate signals. Additionally, this design makes the optical fiber's mode field more susceptible to vocal cord vibrations. When the vocal cords vibrate, the curved sections of the S-shaped structure experience minute displacements and deformations. These changes lead to substantial variations in the phase and intensity of the optical signals within the S-shaped MNF, which contain characteristic information regarding the vocal cord vibrations. Furthermore, combining the S-shaped MNF with flexible PDMS material effectively prevents local pressure or damage to the vocal cords during use, thus enhancing the safety and repeatability of the sensor. Subsequent in-depth simulation studies disclose that the evanescent field of the MNF increases as the MNF diameter and bending radius decrease. To balance the strength of the optical fiber with the contact area, the optimal sensor parameters were ultimately determined, specifically an MNF diameter of 4 μm and a bending radius of 1 mm.In terms of performance evaluation, this article has comprehensively investigated the response of MNF flexible sensors to both static and dynamic pressure as well as vibration. Experimental results show that the sensor's response time (222 ms) and recovery time (163 ms) are both under 300 ms, thereby demonstrating its rapid responsiveness to external stimuli and excellent durability. Across different frequencies, the sensor displays significant response variations at each test frequency, indicating strong frequency adaptability. In vocal cord vibration recognition, the sensor is worn in the vocal cord region of the human body. When the subject utters a sound, the vibrations of the vocal cords lead to changes in the transmitted light intensity of the sensor. A photodetector (PD, CONOUER, 200 kHz) converts the light intensity into corresponding electrical signals in real-time, and these signals are then transmitted to an oscilloscope for display and monitoring. The subject is required to repeatedly pronounce the 26 English letters, facilitating the acquisition of precise light-intensity spectral responses. A total of 1 660 datasets are obtained and combined with the target detection algorithm (YOLOv8) for the classification and recognition of vocal cord vibration signals, achieving an average recognition accuracy of 96.8%. To further assess the sensor's universality, vocal cord vibration data for four commonly used high-frequency phrases—“Ni Hao”, “Zao Shang Hao”, “Hello”, and “How Are You”—were collected from ten participants (5 males, 5 females). The YOLOv8 model was used to train and recognize 1 200 collected datasets. Among these phrases, the recognition accuracy for “How Are You” was the highest at 99%, with an average recognition accuracy across all phrases being 97.75%. These results suggest that the sensor attains high recognition accuracy across multiple individuals, highlighting its strong generalization capability. To comprehensively showcase the advantages of the proposed flexible wearable sensor based on MNF combined with deep learning technology, a detailed comparison was carried out with other sensors paired with different deep learning models regarding accuracy. The results disclose that the S-shaped MNF sensor combined with the YOLOv8 model used in this study achieves high accuracy in recognizing various characteristic signals (26 English letters). Moreover, even with 10 test subjects, the model maintains outstanding accuracy in identifying four commonly used phrases. Therefore, the proposed approach not only surpasses other methods in terms of accuracy but also exhibits significant advantages in meeting diverse requirements and demonstrating broad applicability.This study designs and fabricates a flexible wearable sensor that is based on a PDMS-encapsulated S-shaped MNF structure for the recognition of vocal cord vibration signals. Through theoretical modeling, simulation, and analysis, the main structural parameters of the sensor are rationally designed in order to achieve high sensitivity, reliability, and applicability. By utilizing the YOLOv8 deep learning model, the sensor successfully recognizes 26 English letters with an accuracy rate of 96.8%, thereby demonstrating its significant potential in vocal cord vibration recognition applications. To further enhance the generalizability of vocal cord vibration recognition, training is carried out using the pronunciations of four commonly used words from 10 different subjects, attaining a recognition accuracy of 97.75%. This effectively validates the sensor's universality and reliability among different users. The sensor is simple to fabricate, highly reliable, and possesses excellent resistance to electromagnetic interference, thus offering promising prospects in human-computer interaction applications. Future work will concentrate on testing across diverse populations and system optimization to establish a cloud database, further expanding its application potential in vocal cord vibration recognition, and holding prospects for assisting individuals with speech disabilities in daily communication.
Copper is a necessary trace element in the human body, with adults requiring about 2 mg per day. Functional materials containing Cu2? bring great convenience to life, but their released Cu2? from these materials are harmful to human health and the environment. Regulators have proposed allowable emission standards for copper (Ⅱ), among which “Drinking Water Sanitation Standard” (GB5749-2022) specifies 1.0 mg/L, with stricter standards in industrial emissions and sewage treatment plant pollutant discharge standards. The “Iron and Steel Industry Water Pollutant Discharge Standard” (GB13456-2012) sets 0.5 mg/L, while fishery requirements are the strictest under “Fishery Water Quality Standard of the People's Republic of China” (GB11607-89) limiting 0.01 mg/L. These regulations underscore the need for sensitive and selective detection of low concentrations of Cu2? for environmental, industrial, and human health applications.In this study, a tapered optical fiber sensor coated with CS-PVA composite coating is designed. The micro/nanostructure consists of a single-mode tapered optical fiber sensor whose tapered region generates evanescent waves. High-sensitivity detection of Cu2? in aqueous solution was achieved through coating a CS-PVA polymer composite film on the surface of the fiber's waist-cone region. Ion imprinting technology was applied to enhance the sensor, using glutaraldehyde as a crosslinker to strengthen the polymer network through the crosslinking agent, thereby enhancing metal ion adsorption. Regeneration using hydrochloric acid induces protonation of amine groups, creating repulsion between NH?? and adsorbed Cu2? for release.The ion-imprinted sensor demonstrated specificity when exposed to Cu2?, Cd2?, Fe3?, Co2?, and Pb2? confirming selective Cu2? detection. While the sensor primarily detects Cu2?, it could be adapted for other heavy metal ions through polymer template modification, showing great potential in heavy metal detection across environmental monitoring, industrial effluent testing and remote monitoring.Within 0~100 μmol/L Cu2?, the sensor reached 85.131 6 pm·μmol?1·L sensitivity with a limit of detection (LOD) of 0.388 μmol/L and linear response (R2=0.980 76). The improved ion-imprinted sensor exhibited specificity toward Cu2??. This easily fabricated sensor combines operational simplicity with high Cu2?? sensitivity while maintaining adaptability for other heavy metal ions through template ion substitution.The growing industrial demand underscores the importance of reliable Cu2?? detection for environmental and human health. We developed a simple, yet sensitive fiber-optic sensor for Cu2??. Three sensor configurations were fabricated via dip-coating: uncoated, CS-PVA-coated, and CS-PVA ion-imprinted variants. Experimental results demonstrate that, within 0~100 μmol/L Cu2?? concentrations, the CS-PVA-coated sensor achieved 85.131 6 pm·μmol?1·L sensitivity, an LOD of 0.388 μmol/L, and linear response (R2=0.980 76). The improved ion-imprinted sensor showed specificity toward Cu2?. Combining ease of fabrication with operational simplicity, this platform enables specific Cu2? detection while remaining adaptable for other heavy metal ions through imprinting modifications.
In recent years, safety accidents have occurred frequently in our country, including the collapse of some large facilities, the leakage of oil and gas pipeline, the damage of railway track and so on. These occurrences underscore the imperative for enhanced safety monitoring methodologies to mitigate risks associated with major industrial accidents. Particularly for extended infrastructure networks and cross-regional pipeline systems characterized by vast spatial coverage, distributed monitoring solutions are necessary. Distributed optical fiber vibration sensing system has the advantages of simple structure, long detection distance, and electromagnetic interference resistance. Consequently, it has been widely applied in engineering fields such as earthquake monitoring, intrusion detection, pipeline monitoring.As a key technology in distributed optical fiber vibration sensing system, phase-sensitive Optical Time Domain Reflectometry (φ-OTDR) has the advantages of high sensitivity and fast response speed. The predominant phase demodulation technique employed in φ-OTDR systems capitalizes on the linear proportionality between phase variations in Rayleigh backscattering light and external perturbation intensity, enabling quantitative reconstruction of disturbance waveform. Because φ-OTDR uses highly coherent detection light, the Rayleigh backscattering light at different locations will interfere with each other. The photocurrent signal formed by the photodetector will produce strong fluctuation, resulting in an extremely low signal-to-noise ratio at some interference fading points. Therefore, it may produce false phase, which makes the system give false alarms and affects the accurate judgment of the system.As a method to suppress interference fading, the rotated-vector-sum method needs to be provided with vector components of different strength, and the vector signals are rotated in the same direction to sum, so as to maximize the strength of arbitrary vector addition. In past studies, academics have taken different ways to generate or utilize multi-frequency signals. In this paper, a new combination scheme of multi-frequency signals and rotated vector is proposed. It can use quasi-orthogonal chirped pulse and matching filter to provide vector components with different intensity. The components are rotated to the same direction and then added to improve the intensity. We also study the filtering effect of the matched filter, which can be expressed by cross-correlation value. The cross-correlation value of signals with opposite chirp polarity in non-matched filtering are discussed from three aspects: the relationship between band overlap rate and cross-correlation maximum, the relationship between signal bandwidth and cross-correlation maximum, and the overall cross-correlation value. The cross-correlation value of the quasi-orthogonal chirped pulse formed by the selected parameters is lower than that of the orthogonal code and special optical pulse code. It verifies the feasibility of using quasi-orthogonal chirped signal for distributed fiber sensing. Furthermore, we compared and analyzed the intensity fluctuation, interference fading probability and the phase curve. This scheme can reduce the intensity fluctuation of Rayleigh scattering curve by 20 dB, and the interference fading probability is reduced to 0.25%, which greatly reduces the false phase values and successfully inhibits interference fading. At the same time, the signal to noise ratio of demodulated vibration signal can reach 36 dB. The distributed fiber optic vibration sensing scheme can reduce the influence of interference fading, improve the precision of system positioning and the accuracy of disturbed signal recovery, and has high application value.
Ocean waves are one of the most important and complex elements in ocean hydrology. Effective monitoring of ocean waves is crucial for various applications, including nearshore production, marine scientific research, and the prediction of undersea earthquakes. In this paper, a pressure-type ocean wave height sensor based on Fiber Bragg Grating (FBG) is designed for nearshore wave height measurement. The sensor's pressure measurement principle is based on the elastic diaphragm made of beryllium bronze. The external seawater pressure acts directly on the elastic diaphragm, which makes the center of the diaphragm change in deflection. Then it changes the effective length of the Pressure Fiber Bragg Grating (P-FBG), leading to the blue shift of its center wavelength, and detects the seawater pressure through the measurement of the wavelength change amount. For temperature compensation, a Temperature Fiber Bragg Grating (T-FBG) is positioned within the same cavity as the Pressure-Measuring Grating (P-FBG). The T-FBG is solely sensitive to temperature. By correlating the center wavelength drifts of the P-FBG and T-FBG through a dedicated formula, the influence of temperature variations on pressure measurements can be dynamically compensated, thereby enhancing measurement accuracy. Utilize the relationship between the underwater pressure wave and the surface wave height to achieve compensation of the wave pressure value, and then calculate the wave height through the upper spanning zero method. The optimal design parameters of the sensor are determined by combining previous laboratory research and simulation analysis. A finite element simulation analysis is conducted after constructing the sensor model to verify its feasibility. Three sinusoidal pressure signals with different amplitudes and periods are used to simulate small, medium, and large waves in the real ocean environment. The simulation results demonstrated that the elastic diaphragm exhibited good stability and responsiveness under the three types of positive pressure signals, with a response time of only 3.8 ms, significantly lower than the actual wave collection frequency, indicating that the response time had a negligible impact on measurement results. When the external ambient temperature changed abruptly from 15 ℃ to 20 ℃, the simulation results indicated that the average absolute temperature difference between the P-FBG and the T-FBG is only 0.03 ℃, suggesting that both experienced the same temperature change. Performance calibration experiments for pressure and temperature are conducted on the FBG ocean wave height sensor, revealing that the P-FBG wave sensor had a sensitivity of -9.486 nm/MPa, a linear correlation coefficient of 0.999 97, and a pressure resolution of 0.000 1 MPa (water depth resolution is 1 cm) within the pressure measurement range of 0 to 0.3 MPa. The temperature sensitivities of the P-FBG and the T-FBG were 24.9 pm/℃ and 30.6 pm/℃, respectively, with resolutions better than 0.04 ℃ and a linear correlation coefficient of 0.999 87 within the temperature measurement range of 0 to 35 ℃. In the hydrostatic test, the measured water level fitted the actual value with a degree of fit of 99.919%. The calibration tests confirmed that the FBG ocean wave height sensor can achieve high-precision wave measurement. This paper proposes a zero-line selection method based on the Smoothed Prior Approach (SPA), specifically designed to address the characteristics of measured ocean wave data. The proposed method effectively resolves the zero omission issue and exhibits superior applicability compared to conventional approaches. A five-day field test of the FBG ocean wave height sensor and the SBF3-2 wave buoy was conducted at the Luhaifeng Ocean Ranch. The results showed that the sensor and the SBF3-2 wave buoy obtained the same trend in wave height values, with a correlation coefficient of more than 0.85. The proposed pressure-type fiber grating ocean wave height sensor features underwater passivity, anti-electromagnetic interference, and fast signal transmission, offering a novel optical measurement method for nearshore wave height measurement and demonstrating promising application prospects.
Bridges are a crucial component of urban transportation systems, and their safety is directly linked to the life safety of road users. Real-time vibration and frequency monitoring of bridge structures can help identify potential issues at an early stage. By analyzing natural frequencies and vibration modes of bridges, early warnings for damage detection can be provided. While electronic accelerometers are commonly used, they are point-based and only offer localized measurements. Fiber optic strain sensors have recently attracted attention for vibration mode monitoring of bridges. To address challenges associated with high monitoring costs, complex sensor deployment, and intricate construction when using fiber optic sensors for bridge vibration mode monitoring, a monitoring system is proposed based on a localized array of Fiber Bragg Grating (FBG) strain sensors for the online vibration mode monitoring of medium- and small-span bridges. A high-resolution, long-range FBG strain gauge array is designed, consisting of a single optical fiber embedded with FBG sensors. The fiber is coated with a Glass Fiber Reinforced Polymer (GFRP) layer for protection and structural reinforcement. The sensor array contains seven monitoring points, each equipped with an FBG strain sensor. The sensors are mounted using a Fiber Bragg Grating (FBG) -based platform, which facilitates both sensor fixation and prestress adjustment. A Gray Wolf Optimization (GWO) -based Variational Mode Decomposition (VMD) method is introduced for the extraction of bridge modal parameters, including frequency, damping ratio, and mode shape. Through bridge model simulation experiments, the FBG strain gauge array is installed using a bonding technique. After extracting the bridge's resonance signals via the GWO-VMD method, a secondary interpolation process is employed to fit and extract strain values corresponding to the peak and trough points for the first-order modal analysis. The traditional Stochastic Subspace Identification (SSI) method is then utilized for first-order modal extraction, and the results from both methods are compared. The Pearson correlation coefficient between the two methods is 0.943 75, highlighting the effectiveness of the globally installed FBG strain gauge array in combination with the GWO-VMD method for modal parameter extraction. For practical validation, the system was tested on the Nansongshuigang Bridge along the national key highway from Rizhao to Nanyang. The FBG strain gauge array was positioned at the midspan of the bridge, with four sensor arrays evenly distributed along the bridge's underside. The results demonstrate that the designed FBG strain gauge array effectively captures strain signals induced by passing vehicles, achieving a dynamic strain resolution of 0.1 με. The GWO-VMD method successfully isolates the non-stationary vehicle-induced signals from the bridge's resonance signals. The second Intrinsic Mode Function (IMF2), which encapsulates the bridge's modal behavior, is used for modal analysis. Peak-trough analysis of IMF2 reveals a curve characterized by an initial increase followed by a decrease, with the peak corresponding to the midspan of the bridge and the trough exhibiting the opposite trend. Normalization of the results indicates that the first mode shape derived from the proposed method has a Pearson correlation coefficient of 0.95248 with the finite element simulation results. When compared to traditional electronic sensor monitoring methods, the proposed FBG strain gauge array demonstrates a first-order modal frequency error of less than 1% and a damping ratio error of less than 5% under vehicle excitation. The bridge's natural frequency exhibits a pattern of initial decrease followed by an increase, while temperature variations show an opposite trend. Additionally, the FBG strain gauge array is sensitive to small variations in the bridge's peak frequency due to temperature changes. The online monitoring system and methodology presented in this study enable the extraction of key bridge modal information-such as natural frequency, mode shape, and damping ratio-using a minimal number of strategically placed sensors. This approach facilitates real-time monitoring of the bridge's dynamic characteristics with low-cost implementation, providing a novel approach for assessing the structural health of bridges.
Humidity sensors are crucial in a multitude of sectors, including environmental monitoring, industrial processes, and life sciences. As technology advances, the demand for enhanced performance in humidity sensors has been on the rise. Compared to conventional capacitive and resistive humidity sensors, optical fiber humidity sensors have recently gained significant attention due to their compact size, swift response, robust anti-interference capabilities, extended transmission range, heightened sensitivity, and remarkable stability. Among them, Fabry-Perot microcavity sensors crafted on optical fiber facets have become a focus of research, particularly due to their straightforward fabrication and integration with humidity-responsive materials. Laser-induced waveguide self-growth technology, a method capable of fabricating beam-profile waveguides within photosensitive medium materials according to the intensity distribution of the incident beam, enables rapid preparation of sensors with polymer microcavity structures on optical fiber facets. The photosensitive polymers on the fiber facets undergo chain polymerization reactions induced by laser light, forming a highly cross-linked three-dimensional network structure. The humidity-sensing mechanism of polymer microcavity humidity sensors stems from the swelling or dehydration of the internal three-dimensional network structure upon environmental humidity changes, which leads to variations in the cavity length and refractive index of the polymer, thereby inducing spectral wavelength shifts in the interference spectra. Monitoring these wavelength shifts enables humidity sensing. This paper, embarking from the sensor's structure and principle, delves into the detailed investigation of the sensor's manufacturing process, humidity sensing performance, and respiratory monitoring. It proposes a cost-effective and rapidly fabricable humidity sensor with a polymer microcavity structure on the optical fiber facet. Leveraging laser-induced waveguide self-growth technology, the paper introduces a secondary self-growth method to achieve a higher surface-to-volume ratio and interference spectral contrast. Through precise control of laser power, exposure time, and utilizing the integrated stepper motor of a fusion splicer, a polymer microcavity with a cavity length of 47.65 μm was successfully created on the fiber facet. Changes in the cavity length and refractive index of the polymer microcavity in response to ambient humidity variations result in interference spectral shifts, enabling the accurate detection of relative humidity through spectral shift demodulation. The experimental results indicate that the sensor exhibits an interference spectral contrast of 21.6 dB and a free spectral range of 16.6 nm. It possesses an average sensitivity of 150 pm/%RH within a relative humidity range of 40%RH to 90%RH. Linear fitting of the mean wavelength shifts corresponding to different humidity points in multiple humidity rise-and-fall experiments yields a linearity of 0.985 83. During a 5-hour stability test, the maximum fluctuation in wavelength and reflected optical intensity was 0.208 nm and 0.082 dB, respectively. To further explore the sensor's practical application potential, a real-time breathing monitoring experiment lasting 3 minutes was conducted, revealing a response time of merely 0.8 s and a recovery time of 5.7 s. In summary, the proposed optical fiber humidity sensor, with its simple fabrication process, high sensitivity, rapid response, and excellent stability, shows great potential for applications in meteorological environmental monitoring and wearable healthcare fields. In the future, it is anticipated that the integration of polymers with micro- and nano-structured materials, such as graphene and MoS2, will be employed to fabricate end-face microstructures, thereby achieving an enhancement in humidity sensitivity.
In Brillouin sensing systems, single-mode fibers are commonly used. However, the input optical power of single-mode fibers is limited by the stimulated Brillouin scattering effect, which constrains their sensing performance. Compared with single-mode fibers, few-mode fibers have larger core radius, allowing higher input optical power and enabling the transmission of multiple relatively independent modes simultaneously, which holds the potential for improved sensing performance. Brillouin scattering in few-mode fibers can be characterized by parameters such as Brillouin gain peak, Brillouin frequency shift, and linewidth. These parameters vary with modes and exhibit different response characteristics to various physical quantities. Brillouin scattering in few-mode fibers offers the potential for simultaneous measurement of multiple physical quantities. Therefore, studying the Brillouin scattering characteristics of different modes in few-mode fibers is crucial, as it helps to better understand how each mode contributes to the overall scattering response and enables the optimization of the fiber's performance for multi-parameter sensing applications. However, existing research still has certain limitations. Reports on the full-mode acousto-optic mode coupling in few-mode fibers, as well as the effects of core radius and doping concentration on their stimulated Brillouin scattering, are scarce. To investigate the impact of core radius and doping concentration on the full-mode acousto-optic mode coupling of stimulated Brillouin scattering in few-mode fibers, this paper introduces the optical and acoustic waveguide characteristic equations of few-mode fibers under satisfying the core and cladding boundary conditions. Different optical and acoustic mode distributions and normalized field distributions are analyzed using characteristic functions and finite element methods. The relationship between the number of acoustic modes and the normalized acoustic frequency values in few-mode fibers is explored. The full-mode Brillouin gain spectrum of four-mode fibers is analyzed, and the effects of core radius and germanium doping concentration on the Brillouin frequency shift, linewidth, and gain spectrum in few-mode fibers are studied. The variation patterns of Brillouin frequency shift and linewidth in full-mode acousto-optic coupling of ten-mode fibers are summarized. In this paper, the simulations were implemented using MATLAB and COMSOL Multiphysics software. The research results indicate that there is a quadratic relationship between the number of acoustic modes in few-mode fibers and the acoustic normalized frequency value. The full-mode acousto-optic coupling in few-mode fibers can be divided into effective and ineffective acousto-optic mode coupling. Effective acousto-optic coupling can be divided into two cases: one is the coupling between the Lln mode family (l=0) and any LPmn mode, and the other is the coupling between the Lln mode family (l=2m) and the LPmn mode (m>0). When the germanium doping concentration is between 1.5% and 4.0%, the linewidth of the full-mode acousto-optic mode coupling increases with the increase of germanium doping concentration, and when the concentration exceeds 2.0%, the linewidth shows a linear increasing trend. Brillouin frequency shift variation: when the LPmn mode families are coupled with the L0n mode family, the Brillouin frequency shift decreases with increasing optical mode order; when the LPmn mode families (m>0) are coupled with the Lln mode families (l=2m), the Brillouin frequency shift approaches the total Brillouin gain spectrum frequency shift. Line width variation: when the LPmn mode families (m=0) are coupled with the Lln mode families (l=0), the line widths of the LP02 and LP03 modes are much larger than those of the LP01 mode; when the LPmn mode families (m>0) are coupled with the Lln mode families (l=0) and the Lln mode families (l=2m) respectively, and the linewidth of the same optical mode family LPmn increases as the optical mode order increases when it is coupled with the acoustic mode. The research results of this paper are expected to provide certain reference value for stimulated Brillouin scattering in few-mode fibers.
Optical fiber sensors are capable of converting the external environmental information they perceive into optical signal output in a specific pattern, which can provide the measured information while detecting and feeding back real-time changes in the external environment. When the environment changes, such as alterations in Refractive Index (RI), pH, temperature, humidity, biomolecules, etc, some characteristics of the optical wave signal inside the optical fiber sensing unit will also change. The fiber RI sensor based on the hybrid optical waveguide utilizes the cascading of different fiber waveguides to excite the intermodal interference effect, and has the advantages of small size and high sensitivity. However, there is still room for improvement in RI sensitivity, and one effective approach is to employ fiber micro-nano processing techniques to boost the evanescent field intensity, thus enhancing the RI sensitivity. Due to the merits of a simple manufacturing process, low cost, and high stability, this kind of sensor can be applied in a variety of complex industrial environments. This paper proposes and investigates a high-sensitivity RI sensor based on a Single Mode Fiber-Photonic Crystal Fiber-Single Mode Fiber (SMF-PCF-SMF) hybrid optical waveguide. This structure is axially fusion-spliced in sequence by SMF, PCF, and SMF, the length of the collapse area can be kept within the range of 150~160 μm through controlling the welding parameters, and enables the mutual excitation and coupling of the core mode and the cladding mode in the two-side collapse area. Theoretically analyze the sensing principle, and utilize the beam propagation algorithm to simulate the light-field energy distribution and exchange process, thereby obtaining its output spectrum. The simulation results indicate that when the light field enters the PCF from SMF, the high-order mode in the PCF is excited as it passes through the first collapsed area, with the energy being reduced and energy exchange taking place. After traveling a certain distance within the PCF, it reaches the second collapsed area. At the junction with the other SMF, the light converges and enters SMF for transmission, and the light energy is enhanced. Furthermore, when the PCF section is tapered, compressing the cladding thickness of the PCF section can increase the sensitivity of the structure to the change in the external RI, enhance the spectral redshift of the SMF-PCF-SMF hybrid optical waveguide structure, and raise its RI sensing sensitivity. The methods for achieving the enhancement of the evanescent field are investigated. Based on three micro-nano processing techniques, namely the fiber cladding polishing method, the hydrofluoric (HF) acid fiber cladding etching method and the fused biconical taper method, the device preparation is accomplished. The interference spectral characteristics of the SMF-PCF-SMF hybrid optical waveguide structure obtained through three processing techniques are experimentally tested and compared. Moreover, a microfluidic fixture (manufactured by using polytetrafluoroethylene material in combination with precision micromachining methods) is utilized to package the sensor. The RI sensing characteristics of liquid samples in SMF-PCF-SMF structures with three polishing depths, in SMF-PCF-SMF structures with HF acid etching, and in SMF-PCF-SMF structures where the cladding thickness of the PCF in the middle section is compressed by the fused biconical taper method are respectively tested and compared. The experimental results demonstrate that all three micro-nano processing methods have enhanced the linearity of RI sensing. Furthermore, by compressing the cladding thickness of the PCF in the middle section of the SMF-PCF-SMF structure through the fused biconical taper method, the obtained RI sensitivity of the liquid sample is the highest, reaching 1 405.36 nm/RIU, which is approximately 24 times higher. This RI sensor is characterized by its small size, easy integration and high sensitivity, and it has significant application value in the fields of RI sensing, industrial production, etc.
The rapid development of emerging technologies such as 5G, the Internet of Things, and artificial intelligence has created an ever-growing demand for high-capacity data transmission, increasing the pressure on current optical fiber systems. However, the bandwidth limitations of optical fiber amplifiers constrain the number of usable channels, creating a bottleneck in expanding system capacity. Enhancing the gain bandwidth of optical fiber amplifiers is therefore an effective approach to increase the transmission capacity. In long-distance fiber transmission systems, the C-band/L-band Erbium-Doped Fiber Amplifier (EDFA) configuration is often employed to amplify optical signals across parallel structure. However, traditional L-band EDFAs generally cover a gain bandwidth of only 40 nm (1 565~1 605 nm), while the International Telecommunication Union defines the L-band as 60 nm (1 565~1 625 nm). This has driven significant interest in extending the L-band gain bandwidth toward longer wavelengths. As a result, recent years have seen growing research efforts toward developing L-band extended EDFAs. However, the development of L-band extended EDFAs still faces challenges such as limited gain bandwidth, insufficient gain levels, and poor gain flatness.In this study, we explore the gain extension properties of bismuth-erbium co-doped fiber (BEDF), which offers promising potential for extending the gain bandwidth of L-band EDFAs. We designed and developed a two-stage L-band extended BEDF amplifier, which incorporates a double-pass amplification structure in the main amplifier stage. This configuration not only aims to broaden the gain bandwidth but also enhances the gain performance and suppresses noise figure effectively. Our research confirmed that BEDF can effectively extend the L-band gain bandwidth, providing new insights for further advancements in L-band extended EDFA technology. To achieve the L-band gain extension, co-doping erbium-doped fibers with elements such as phosphorus (P), ytterbium (Yb), and aluminum (Al) can shift the Excited State Absorption (ESA) spectrum of erbium ions, effectively broadening the gain spectrum into the longer L-band wavelengths. The addition of bismuth (Bi) ions into the fiber composition further enhances the emission cross-section of erbium ions, contributing significantly to the extended gain bandwidth. By incorporating Bi ions, the BEDF amplifies more effectively, as these ions both broaden the gain bandwidth and increase emission intensity.To enhance gain and suppress noise figure, we implemented a two-stage L-band extended bismuth-erbium co-doped fiber amplifier, where the main amplifier and pre-amplifier stages are connected via a circulator. The pre-amplifier uses a 2 m-long Er-doped fiber pumped by a 980 nm semiconductor laser at 100 mW. The main amplifier consists of a segment of BEDF that is bidirectionally pumped by two 1 480 nm semiconductor lasers through a Wavelength Division Multiplexer (WDM), with forward and backward pump powers of 400 mW and 500 mW, respectively. A Faraday Rotator Mirror (FRM) is positioned at the end of the amplifier, which reflects the amplified signal back through the BEDF, effectively enhancing the gain performance.The study tested BEDF lengths of 3.6 m, 5.8 m, and 6.8 m, analyzing gain and noise figure characteristics for each configuration. Results indicated that the gain spectra for all three fiber lengths appeared in the range of 1 560~1 620 nm, with notable shifts as fiber length increased. Specifically, longer BEDF lengths resulted in a gain peak shift towards longer wavelengths, while gain at shorter wavelengths diminished, leading to a narrowing of the gain bandwidth. At a BEDF length of 6.8 m, the gain level was significantly lower than that at 5.8 m, suggesting an optimal BEDF length for effective amplification. Across all BEDF lengths, the amplifier achieved a gain level above 11.36 dB at 1 620 nm, demonstrating effective L-band gain bandwidth extension, which is attributed to two intrinsic mechanisms: the broadening of the emission cross-section of erbium ions by Bi ions, and the energy transfer from Bismuth Active Centers (BACs) to erbium ions. This transfer enhances emission intensity and efficiency, especially in the presence of BAC-Ge, which exhibits an emission peak at 1 610 nm, directly supporting L-band gain bandwidth extension. The double-pass amplification structure further enhances gain and suppress noise figure. The signal reflected by the FRM undergoes a second amplification in the BEDF, effectively increasing the overall length of the gain fiber. Testing with double-pass BEDF lengths of 3.6 m and 4.8 m showed that the 3.6 m BEDF achieved a wider 20 dB gain bandwidth, higher maximum gain, and a lower minimum noise figure, although with slightly lower gain flatness compared to the 4.8 m configuration. The 4.8 m BEDF showed a slight reduction in 20 dB gain bandwidth and maximum gain, but with improved average gain and gain flatness.Comparison of the single-stage main amplifier and the two-stage amplifier within the 1 560~1 615 nm range revealed that the two-stage amplifier delivered a higher gain than the main amplifier, with comparable gains in the 1 615~1 620 nm range. This is because the pre-amplifier increases the overall pump power, resulting in a shift of the central wavelength towards shorter wavelengths and thus enhancing gain at shorter wavelengths while slightly reducing gain at longer wavelengths. In terms of noise performance, the two-stage amplifier exhibited a lower noise figure within the 1 560~1 610 nm range compared to the main amplifier, with similar noise levels in the 1 610~1 620 nm range. The noise figure improvement is attributed to the primary role of the first stage in determining noise figure, where the pre-amplifier contributes to increased gain and noise suppression.This study achieved a 20 dB gain bandwidth of 63 nm (1 555~1 618 nm), a maximum gain of 52.84 dB, a minimum noise figure of 4.23 dB, and a 3 dB flatness gain bandwidth of 35 nm (1 565~1 600 nm). Compared with recently reported L-band extended EDFAs, our configuration achieved a broader 3-dB flatness gain bandwidth, while maintaining high gain and wide high-gain bandwidth. These results underscore the potential of BEDF for advancing optical amplifier technology and supporting high-capacity, long-distance optical transmission systems. Further optimization in the co-doping composition and ratio of elements in BEDF is expected to extend gain bandwidth even further, paving the way for innovations in optical amplifier design.
Broadband Radio Frequency (RF) Arbitrary-waveform Generation (AWG) plays an important role in modern information systems, like high-speed optical communications, biomedical imaging, chemical coherence control, and advanced radar applications. Benefiting from a large bandwidth and compact configuration, the Time-domain Pulse Shaping (TPS) system provides possibilities for generating RF arbitrary waveforms based on the Fourier transform relationship between the input-output waveform pair. However, limited by the relatively low sampling rate and bit resolution of the Employed Electronic Arbitrary-waveform Generator (EAWG), the diversity and fidelity of the realized waveforms as well as its reconfiguration speed are constrained. To remove the EAWG′s limitation and realize dynamic real-time reconfiguration of RF waveforms, we propose and demonstrate a novel approach of RF arbitrary-waveform generation based on an improved TPS system with an integrated Dual Parallel Mach-Zehnder Modulator (DPMZM) and multi-tone inputs in this work. Different from the conventional TPS system, the proposed system is built using a mode-locked laser, a DPMZM, and a pair of conjugate dispersive mediums. One major difference lies in the employment of DPMZM, which can realize the Carrier-suppressed Single-sideband (CS-SSB) modulation mode to guarantee the one-to-one mapping from each frequency element to the output optical pulse. Another difference is the generation of the multi-tone RF inputs, which is provided by commercial sinusoidal signal generators instead of EAWG. Based on the Fourier transform relationship between the RF input signal and the output optical waveform, the spectrum of the applied multi-tone signals is linearly mapped into the temporal profile of the output optical signal, i.e., a list of discrete optical pulses with adjustable amplitudes and time intervals are generated. The following Photodetector (PD) and Low-pass Filter (LPF) detect and smooth the temporal envelope of optical pulses to realize the RF waveform generation. Note that the output optical pulses serve as the sampling points of the desired RF waveforms. In this design, by simply configuring the frequency spacing and amplitudes of the multi-tone RF inputs, the desired RF arbitrary waveform can be generated and reconfigured in real time.In order to verify the proposed RF AWG approach, a proof-of-concept experiment was successfully carried out. Firstly, the system dispersion matching was performed to ensure the fidelity of the output RF waveform. Secondly, the amplitude values of the three-tone input signals have been adjusted, a variety of customized waveforms were generated. Meanwhile, the frequency interval between adjacent frequency elements has been separately set as 4 GHz and 5 GHz, two square waveforms with different sampling rates up to 20 GSa/s have been achieved. The obtained results validate that the proposed approach can realize independent controlling over each sampling point of the desired output waveform by properly configuring the amplitudes, frequency interval or frequency values of the multi-tone inputs. In addition, the effects of higher-order dispersion from the dispersion medium on the fidelity of the output waveforms have been investigated. The simulation results show that as the input frequency increases, the peak amplitudes of the output pulses will decrease and the pulse width may slightly grow up. However, the amplitude deviation introduced by higher-order dispersion can be easily compensated by properly adjusting the power settings of the multi-tone inputs.In summary, a novel approach of RF arbitrary-waveform generation via TPS with an integrated DPMZM and multi-tone inputs has been proposed and experimentally demonstrated. By properly adjusting the DC bias of DPMZM, the CS-SSB modulation of RF input can be achieved, which guarantees the one-to-one linear mapping from each frequency element of RF input to the output pulses. Any desired waveform can be obtained by simply adjusting the frequencies or the amplitudes of the multi-tone inputs. Proof-of-concept experiments on different waveforms generation have been successfully carried out. Additionally, the impacts of higher-order dispersion on waveform diversity and fidelity are also investigated.
The tactile perception of a flexible humanoid robotic finger is essential for advancing applications in the fields of intelligent robotics, human-computer interaction, and prosthetics. This work presents a novel multi-parameter sensing method using fiber Bragg grating embedded in a cantilevered robotic finger structure. It aims to address the challenges posed by strain and temperature cross-sensitivity in tactile sensing systems. By embedding a differential sensing array of fiber Bragg grating sensors into the robotic finger, the design achieves accurate, real-time measurement of tactile force, contact temperature, and contact position while maintaining robustness and flexibility.The robotic finger is designed to closely replicate the structural and functional characteristics of a human finger, providing both dexterity and adaptability. The cantilevered finger bone is fabricated from polylactic acid through 3D printing, ensuring lightweight and high-strength properties. Six fiber Bragg grating sensors are symmetrically embedded within the cantilevered structure in pairs to form a differential sensing array. This differential configuration allows for the temperature compensation by sensing the same temperature and the opposite strain. This design effectively decouples strain and temperature effects, significantly improving the reliability of the sensor readings under varying environmental conditions.To ensure high precision, each fiber Bragg grating sensor in the array was carefully calibrated. Calibration experiments were performed to determine the stress and temperature sensitivities of the fiber Bragg grating sensors. The experimental results revealed that the embedded fiber Bragg grating array achieved a stress sensitivity of 112.898 pm/N and a temperature sensitivity of 81.185 pm/℃. Furthermore, the sensors demonstrated excellent accuracy and stability across a wide temperature range (10~50 ℃), with a minimum average measurement error of -0.025 6 nm. These results confirm the effectiveness of the differential sensing array in distinguishing between the effects of strain and temperature, making the system suitable for the tactile sensing tasks in practical applications.In addition to force and temperature sensing, the robotic finger can accurately detect the contacting position. By extracting the standard deviation features of the central wavelength shifts from the fiber Bragg grating sensors, we employed a support vector machine algorithm to classify different contact locations. The classification model was trained on data collected from multiple contact points across the finger's surface. The dataset is divided into a 70% training set (105 samples) and a 30% test set (45 samples) to ensure that the model's generalization ability. The support vector machine-based model achieved a contact position classification accuracy of 95.5%, demonstrating the system's capability to perform precise contact detection. This high level of accuracy is critical for applications that require fine manipulation, such as robotic surgery, where precise tactile feedback ensures safe and effective operation, and advanced prosthetic devices, where accurate sensory information enhances the user experience.In conclusion, this work demonstrates that fiber Bragg grating sensing technology, when integrated into a flexible robotic finger structure, provides a highly effective solution for multi-parameter tactile sensing. The system can accurately measure force, temperature, and contact position, featuring a lightweight design and high flexibility. This technology holds broad potential applications, particularly in scenarios where precise and reliable tactile feedback is essential. These include not only robotic surgery and advanced prosthetics but also collaborative robotics and human-computer interaction systems. Future research will focus on refining the design of the sensor array, optimizing the algorithmic approaches for data processing, and exploring additional applications in environments that demand precise tactile sensing.
With the continuous development of distributed optical fiber sensing technology, the Brillouin Optical Time Domain Reflectometry (BOTDR) system, due to its single-ended monitoring characteristics, electromagnetic interference immunity, and real-time sensing capabilities for temperature/strain changes, has become increasingly applied in structural health monitoring fields, particularly in large infrastructures such as bridges, dams, and subway tunnels. However, the weak spontaneous Brillouin scattering limits the performance of the BOTDR system. Increasing the input optical pulse energy enhances the scattering effect; however, if the energy exceeds a certain threshold, stimulated Brillouin scattering can deplete the pulse energy rapidly, reducing the sensing distance and impacting the system's performance. Factors such as the length, type, and condition of the distributed sensing fiber, as well as the linewidth and power of the light source in the BOTDR system, affect the stimulated Brillouin scattering threshold. Therefore, how to optimize the system's detection performance under varying stimulated threshold conditions is a key issue for compact and cost-effective BOTDR systems in practical engineering applications. This paper presents an automated detection method for identifying stimulated Brillouin scattering in optical fibers. The technique leverages Short-Time Fourier Transform (STFT)?-based optoelectronic demodulation and advanced signal processing. Its primary goal is to optimize the sensing distance for localized Stimulated Brillouin Optical Time-Domain Reflectometry (BOTDR) systems.The method uses an optoelectronic demodulation device to collect the raw time-domain Brillouin scattering signal along the fiber to be tested, applying STFT processing to obtain the Brillouin Gain Spectrum (BGS). By analyzing the peak intensity distribution of the BGS, a Brillouin peak intensity map along the fiber is plotted. A moving average method is used to remove random noise, followed by polynomial fitting to obtain the first- and second-order derivative curves. The zero-crossings of the second-order derivative are identified to locate the potential stimulated Brillouin scattering points. The first-order derivative values at these points are compared with a preset threshold σ to confirm the actual location of stimulated Brillouin scattering. To improve accuracy, the RMSE of the Brillouin peak intensity distribution in the denoised fiber section is calculated. The difference between the maximum and minimum values is computed and then divided by the window length h to derive the error threshold σ for linear fitting.In a 2 000 m fiber with a 20 ns pulse width and 20 kHz frequency, when the EDFA1 output power is 0.69 mW, stimulated Brillouin scattering occurred at a specific location of 1 230.2 m. The root mean square error of the Brillouin Frequency Shift (BFS) along the fiber indicated a deterioration in the signal-to-noise ratio after stimulated scattering, validating the effectiveness of the proposed signal processing method. The study further investigated the effects of pulse width, frequency, and fiber length on the location of stimulated scattering. Using a 2 000 m fiber with a 50 ns pulse width and 20 kHz frequency, with EDFA1 output power at 0.876 mW, the stimulated scattering occurred at 1 500.2 m. For a 2 000 m fiber with a 20 ns pulse width and 10 kHz frequency, with EDFA1 output power at 1.197 mW, the stimulated scattering occurred at 1 090.2 m. For a 1 009 m fiber with a 20 ns pulse width and 20 kHz frequency, with EDFA1 output power at 0.69 mW, the stimulated scattering occurred at 780.2 m. The results indicate that reducing pulse width or frequency advances the stimulated scattering position. Additionally, temperature experiments were conducted with a 2 000 m fiber, heating it to 50 °C in a water bath at 300 m and 800 m locations. The specific location of stimulated scattering was found to be 1 170.2 m.Using this method, it is possible to further determine the optimal sensing distance for low-cost BOTDR systems that utilize localized stimulated scattering, specifically for applications at construction sites. It can guide the configuration of the optimal sensing fiber length, determine the stimulated scattering threshold for the system, and fully leverage the performance of low-cost BOTDR systems. It overcomes the cost and applicability limitations of traditional technologies in practical applications, providing an economic, efficient, and adaptable monitoring solution for Brillouin fiber sensing technology in engineering applications. This method meets the needs of various standard construction monitoring applications.
Positioning algorithms based on visible light communication can help solve the problem of insufficient positioning accuracy in some special occasions such as indoors, basements, and underground. However, the accuracy of existing visible light positioning algorithms is difficult to improve further, and most of them remain in the simulation stage without experimental verification. In the previous visible light positioning algorithms based on deep learning, such algorithms can be divided into positioning algorithms based on received light signal intensity and positioning algorithms based on light source images according to the source of data. The positioning algorithm based on received light signal intensity receives the light signal intensity from each LED light source in turn, and obtains the positioning result based on the characteristics of the light signal intensity. Similarly, the positioning algorithm based on light source image receives the LED light source image and obtains the positioning result by analyzing the characteristics of the light source. This article proposes a positioning algorithm (MIF-VLP) based on the attention mechanism to fuse light Signal Intensity Information (RSS) and image information. The MIF-VLP algorithm uses ResNet-18 as the backbone network of the image, and maps RSS into a vector through word embedding, and then adjusts the output of ResNet-18 to make them have the same dimension. The fusion method of the attention mechanism is based on the image, so the input of the attention layer is multiple vectors, and the output is only one vector. The advantages of the algorithm are that, firstly, the algorithm uses both the light signal intensity information and the image information, which makes up for the overfitting problem caused by the single data, improves the generalization ability of the model and the final positioning accuracy. Secondly, the algorithm performs a permutation and combination preprocessing on the received light signal intensity, treats the input light signal intensity as a sequence, ignores the order and number of light signal intensities from multiple LEDs, and reduces the dependence on the environment to a certain extent. The traditional positioning algorithm based on the intensity of received light signals has strict requirements on the input order and number of data, because each light signal intensity represents the characteristics of an LED. In addition, the MIF-VLP algorithm can expand the size of the data set under the same experimental environment, so that the model can converge in fewer epochs and improve the generalization performance of the model. In comparison, the article selected the RSS-BP algorithm based only on RSS data and the CNN algorithm based only on light source images in the experimental stage. The CNN algorithm also uses ResNet-18 as the backbone network. The experimental results show that in the experimental environment of 2 m×2 m×1.8 m, the average positioning error of the MIF-VLP algorithm reaches 5 mm, which is 80.7% higher than that of the RSS-BP algorithm based on RSS information and 87.5% higher than that of the convolutional neural network algorithm based on image information; the maximum positioning error of the MIF-VLP algorithm reaches 8.9 cm, which is 41.4% lower than that of the RSS-BP algorithm based on RSS information and 19.1% lower than that of the convolutional neural network algorithm based on image information. The minimum positioning error of the MIF-VLP algorithm reaches 0.5 mm, which is much lower than the 1cm positioning error of the RSS-BP algorithm and the 2 mm positioning error of the CNN algorithm. Overall, among all the positioning points, the MIF-VLP algorithm has only one point with an error greater than 2 cm, which shows the stability of the MIF-VLP algorithm. Analysis shows that the reason for the large error at this point may be due to reflection or measurement error, because this point is located at the edge and is easily affected by reflected light.
To address the limitations of conventional optimization algorithms in Sensor-less Adaptive Optics (SLAO) systems, particularly the slow convergence and limited global search efficiency of Stochastic Parallel Gradient Descent (SPGD) algorithm and meta heuristic optimization algorithms, an Improved Dung Beetle Optimizer (IDBO) algorithm proposed by combining the Dung Beetle Optimizer (DBO) algorithm and the Osprey Optimization Algorithm (OOA) to significantly enhance wavefront correction in SLAO systems, achieving effective atmospheric turbulence suppression without the need for wavefront sensors. While the DBO algorithm is known for its strong optimization ability and fast convergence speed, it suffers from an imbalance between global exploration and local exploitation. To address this issue, we incorporate the Osprey optimization algorithm strategy to improve global exploration and enhance the local exploitation ability.The proposed methodology involves a model of an adaptive optics system using a 32-element deformable mirror for wavefront correction. The IDBO algorithm integrates the OOA to enhance the traditional Dung Beetle Optimizer (DBO), increasing population diversity and global search capabilities. Performance evaluations were conducted under various turbulence levels, utilizing wavefront aberrations as correction targets. We performed comprehensive comparisons of DBO, SPGD, and SA-SPGD algorithms, focusing on convergence speed, correction efficiency, and local extrema resistance. To evaluate the dynamic correction performance of the IDBO algorithm under different intensities of atmospheric turbulence, we conducted 20 simulations of the IDBO algorithm for atmospheric turbulence correction, with each numerical simulation introducing initial atmospheric turbulence of varying intensities.Numerical simulations results show notable improvements at an initial RMS value of 0.825 4, the IDBO algorithm achieves wavefront correction speeds that are approximately 14.0, 3.5, and 1.3 times faster than DBO, SPGD, and SA-SPGD, respectively. Under higher turbulence (initial RMS of 1.772 1), IDBO maintains superior performance with speeds 7.4, 1.1, and 0.8 times faster than the same algorithms. The system consistently delivers an 80% improvement in correction speed across different turbulence conditions while ensuring equivalent correction efficacy. IDBO also demonstrates enhanced convergence stability and robustness, significantly reducing the likelihood of entrapment in local extrema. The IDBO algorithm performs excellently under both weak and strong turbulence conditions. This hybrid algorithm has the advantage of rapidly correcting aberrations caused by turbulence of different intensities, resulting in better adaptive correction capability. The qualitative simulation results show the phase distribution before and after correction under weak and strong turbulence conditions.The integration of the OOA algorithm crucially improves algorithm performance by preserving diversity in the dung beetle population, averting premature convergence to local optimum solutions. The numerical simulations validate that the IDBO-based SLAO system outperforms mainstream algorithms in real-time control applications. With convergence speed, robustness, and dynamic correction performance amidst evolving turbulent conditions, IDBO algorithm presents a viable solution for wavefront correction in SLAO systems. This advancement offers a new approach to overcoming challenges in sensor-less adaptive optics, highlighting potential applications where traditional systems may falter. In the future, we intend to develop a high-performance processing platform based on FPGA and GPU, and apply the IDBO algorithm to dynamic aberration correction experiments.
In recent years, Unmanned Aerial Vehicles (UAVs) have been widely used across various fields due to their high mobility, flexibility, and cost-effectiveness. In the civilian fields, UAVs are utilised for activities such as agriculture, environmental monitoring, and search and rescue operations. Conversely, in the military fields, UAVs are employed for a range of purposes, including surveillance, reconnaissance, precision strikes, and target guidance. Ground-based battlefield reconnaissance sensor systems currently deployed by military forces include battlefield reconnaissance radars, magnetic sensors, infrared sensors, vibration sensors, acoustic sensors, and pressure sensors. UAVs are increasingly playing a crucial role in information collection for these ground sensors. Traditional communication methods for information collection typically rely on radio communication, which can be severely disrupted or rendered unusable in environments with electromagnetic shielding or interference. Solar-blind ultraviolet (UV) light, operating within the 200 nm-280 nm wavelength range, offers virtually no background noise in low-altitude airspace and provides all-weather, non-line-of-sight communication capabilities. This makes it an ideal communication method in electromagnetically challenged environments due to its excellent environmental adaptability, high confidentiality, and strong resistance to electromagnetic interference. Compared to line-of-sight UV communication, non-line-of-sight communication does not require precise alignment between the transmitter and receiver and offers greater flexibility in receiver positioning, making it more suitable for collecting information from ground sensors. However, traditional algorithms often face limitations in handling complex information collection tasks, particularly in terms of computational resources, adaptability, and real-time performance. Deep reinforcement learning (DRL) algorithms, as an emerging intelligent decision-making method, enable UAVs to autonomously complete tasks by learning and experimenting within the environment. This makes DRL an ideal approach for autonomous UAV navigation and data collection tasks.This paper addresses the challenge of UAV information collection in the presence of electromagnetic interference by employing an adaptive elevation angle UV non-line-of-sight communication method and utilizing DRL algorithms to tackle the information collection task. First, a UAV mobility model is established, followed by the proposal of a UV non-line-of-sight air-to-ground communication model with variable transmission and reception angles. Detailed modeling of the UAV's energy consumption is then carried out, considering flight energy consumption, the energy consumption of the electro-optical pod, and communication energy consumption. Subsequently, an information collection model is established. This integrated model balances task execution time, energy consumption, and communication quality during the information collection process. Given that the optimization problem is NP-hard, traditional polynomial optimization algorithms are inadequate for solving it. Therefore, this problem is formulated as a Markov decision process. To enable the UAV to make better decisions regarding flight direction, speed, and UV transmission and reception angles, a reward function tailored to the information collection task is designed. This reward function comprehensively considers time, energy, communication path loss, and the UAV's return to base. The Double Deep Q-Network (DDQN) algorithm, which separates action selection from evaluation, still faces overestimation issues in high-dimensional state and action spaces. This paper proposes that in the information collection scenario, the UAV must consider multiple directions and speeds of movement while adaptively adjusting the UV communication angles during the collection process. Compared to previous discrete environments with smaller action spaces, this scenario requires a larger action space. To better adapt to this scenario, improvements such as dual target networks, prioritized experience replay, and entropy regularization are incorporated into the classical DDQN algorithm, enhancing its adaptability and stability.To verify the effectiveness of the improved DDQN algorithm and explore the impact of different UV parameters, sensor quantities, and UAV flight altitudes on information collection time and energy consumption, comparative simulations with the classical DDQN algorithm are conducted. The proposed adaptive elevation angle DDQN algorithm effectively completes the information collection task, demonstrating at least a 13% improvement in time efficiency and a 14% reduction in energy consumption across multiple scenarios compared to the classical DDQN algorithm.
In recent years, intelligent navigation security is a hot spot in the field of water safety protection. Distributed optical fiber acoustic sensing technology based on phase sensitive optical time domain reflectometer can realize distributed monitoring of multi-point disturbance along optical fiber. However, due to the complex water environment and the fading of system signals, it is difficult to identify the disturbance signals stably and effectively. The main noise sources of distributed optical fiber acoustic sensing system are interference fading and polarization fading. Both of these phenomena greatly weaken the signal at the fading point, resulting in signal distortion. Under water, the cable is less coupled with the environment, and is more susceptible to the influence of water waves, currents and other factors. Therefore, timely and effective elimination of fading interference plays an important role in the identification of water security events. Combining fading mask, attention mechanism and self-supervised learning, this paper proposes a distributed optical fiber acoustic sensing system based on fading mask autoencoder for ship security event recognition in waters. This method is aimed at the ship event signal in the water area, and generates a basically noise-free signal by shielding fading noise into the deep learning model, so that the model can learn directly and greatly reduce the influence of fading noise on signal recognition. Mask autoencoder is an extensible self-supervised learner. It combines the attention mechanism in the form of a mask to achieve high-precision training with the simplest coding-decoding model structure, while it can also transfer learning only through the model weights of the encoder. On this basis, the fading mask autoencoder method is more helpful for the distributed optical fiber acoustic sensing system to achieve effective event recognition. Firstly, the amplitude signal of distributed optical fiber acoustic sensing is analyzed to determine the fading position. The model is then pre-trained using the upstream task of the fading mask autoencoder. The basic characteristics of distributed optical fiber acoustic sensing signal are learned by means of random mask. Finally, the downstream task of fading mask autoencoder completes the intelligent event recognition training by fading position mask. In this paper, four kinds of ship security events collected at the water test site are used as classification data, and the fading mask autoencoder is compared with the mask autoencoder and three related models. The results show that the average training accuracy of the fading mask autoencoder is 98.34%, which is 4.9% higher than that of the mask autoencoder. The average training loss was 0.1094, which was 0.095 less than that of the mask autoencoder. The average test accuracy was 93.01%, which was 6.45% higher than that of the mask autoencoder. Compared with the other three models, the fading mask autoencoder has higher training accuracy, lower Loss and faster convergence speed. Its average performance index is about 4.88%-7.62% higher than other models. Therefore, the fading mask autoencoder model based on the improved mask strategy can extract useful information from the signal more efficiently and accurately for training, and has better stability and generalization, which is suitable for the identification of navigation security events in waters.
Fiber Bragg grating (FBG) wavelength demodulation technique based on tunable Fabry-Perot (F-P) filters (FFP-TF) results in a degradation of the accuracy of the demodulation system due to the hysteresis and temperature drift characteristics of piezoelectric ceramics (PZT) inside the FFP-TF. Artificial intelligence machine learning algorithms can reduce the demodulation error of the system without increasing the complexity of the system. Therefore, this article proposes an FBG wavelength demodulation method based on Support Vector Regression (SVR) optimized by Sparrow Search Algorithm (SSA). The main purpose is to establish a nonlinear fitting relationship between F-P tuning voltage and transmission wavelength, replacing wavelength reference hardware. This method can save system costs, improve system demodulation accuracy, and contribute to the integrated development of demodulation systems. First, the reference grating, driving voltage, tuning time and F-P surface temperature are taken as the input features of the model, and the F-P transmission wavelength is taken as the output feature, and the F-P transmission wavelength compensation model is established by SVR. The SSA-optimized hyperparameters C and g are then input into the SVR model for training to obtain the target compensation model. Meanwhile, the training method of combining sliding window with SVR is proposed to improve the model's global optimization-seeking ability by updating the sliding window's size and sliding speed, which avoids the problem of the model's deterioration in generalization ability over time. After the model training, the correlation coefficient (R2), Mean Square Error (MSE), and Root Mean Square Error (RMSE) are introduced to evaluate the model and obtain the optimal compensation model. Finally, the FBG wavelength demodulation system based on SSA-SVR is built in Labview and the optimal training model is called. Then the real-time compensation ability of the model is checked by the stability experiment and the cooling experiment. The law of the wavelength compensation error in the frequency range of 0.5~2.5 Hz driving is explored experimentally, and the compensation ability of the SSA-SVR model is compared with PSO-SVR, LSSVR, and KRR models. The experimental results show that in the frequency range of 0.5~2.5 Hz, the error between the target value output and the real value of the SSA-SVR algorithm model decreases with the decrease of the driving frequency, which indicates that the demodulation system is more accurate under the low-frequency driving. The fitting coefficient R2 of the SSA-SVR algorithm reaches 0.999 99 in both training and test data, which is an improvement of 0.001 compared to the KRR algorithm, and the wavelength demodulation error is reduced by more than ten times. At 2.5 Hz driving frequency, the Mean Absolute Error (MAE) of SSA model is 38.689 pm, which is reduced by 28.078 pm compared to the PSO algorithm error, similarly, at 0.5 Hz driving frequency, the SSA model has a MAE of 19.83 pm, which is reduced by 48.8% compared to the PSO optimization algorithm error of 39.68 pm. In addition, the SSA model has the smallest error in all of the different drive frequency tests, showing better generalization performance, while the LSSVR model performs better at high frequencies and shows negative optimization in the low-frequency tests, and the KRR model has the largest error in a wide range of frequencies, indicating that this model has the worst fit. In the stability experiment at 25 ℃, the fluctuation of the demodulation value based on the SSA-SVR model is kept within 15 pm, and the average absolute error between the demodulation value and that of the sm125 demodulator produced by MOI company in the U.S.A. is 7.28 pm, whereas the stability of the traditional polynomial demodulation method of the reference grating is poorer, and the demodulation error of the SSA is reduced by 88.5% compared with it. For the cooling experiments from 40 to 90 ℃, the detuned MAE of the SVR compensated model is 7.44 pm, which is 27.6% lower relative to the 5.39 pm of the polynomial fitting method. The experiment proved through error analysis that this method is superior to the reference grating polynomial demodulation method in terms of real-time demodulation and stability, and the error between it and the sm125 demodulator remains within a small range. Compared with the traditional method, this method models the wavelength drift during the natural temperature change process of FFP-TF, realizes the nonlinear fitting between the F-P driving voltage and the transmitted wavelength over a wide range, and verifies that the fiber grating demodulation system can effectively reduce the grating wavelength demodulation error without the help of hardware reference.
The intrinsic signal stability is an important performance parameter for long-term stable operation and high-precision measurement in optical current sensing. If the intrinsic signal exhibits fluctuations, drift, or jitter, it can lead to cumulative measurement errors and increased instability in the system. This can result in incorrect current measurement values, misjudgment, and misleading system operation status assessment, thereby affecting the stability and safety of the system. Therefore, ensuring the stability of the intrinsic signal is crucial for maintaining long-term stability in optical current sensing systems.Jitter in the intrinsic signal has a direct negative impact on the measurement accuracy of optical current sensing systems. Jitter introduces fluctuations in the instantaneous signal values, leading to instability and inaccuracy in the measurement results. Particularly in applications requiring high-precision current measurements, the presence of intrinsic signal jitter introduces additional errors, reducing the measurement accuracy and reliability of the system. Currently, conventional methods such as dual-path techniques can not eliminate jitter in the intrinsic signal effectively. Despite the use of dual-path techniques, the intrinsic signal is still influenced by factors such as optical components, circuit noise, and environmental interference, and the jitter can not be completely eliminated. To address the issue of intrinsic signal jitter in optical current sensing systems and improve measurement accuracy, a debouncing Kalman-based method for high-precision extraction of the intrinsic signal is proposed. This method involves a detailed analysis of the noise characteristics and optical path structure of the optical current sensing system, followed by the establishment of a mathematical model for the intrinsic signal. Subsequently, a debouncing function is introduced to modify the Kalman gain K, resulting in a Debouncing Kalman (DBKalgorithm. The debouncing Kalman algorithm aims to address the severe estimation jitter in the state estimates caused by the initial state dependence and measurement process uncertainty sensitivity of the standard Kalman gain K.In this method, the debouncing Kalman filtering algorithm utilizes the debouncing function to modify the Kalman gain K, thereby providing denoising processing for the intrinsic signal. The introduction of the debouncing function allows the Kalman filtering algorithm to better adapt to the jitter characteristics of the intrinsic signal, reducing the impact of jitter on state estimation. Compared to traditional Kalman filtering algorithms, the debouncing Kalman filtering algorithm exhibits greater stability in the state estimation process and can effectively extract high-precision estimates of the intrinsic signal. Additionally, a recursive estimation of the noise variance is introduced to ensure real-time correction of the noise variance during the filtering process.The debouncing Kalman algorithm was validated and compared with the standard Kalman algorithm through simulations in MATLAB. The simulation results show that, under the same set of parameters, the relative error of the standard Kalman algorithm reaches approximately 11% after convergence, while the debouncing Kalman algorithm achievs a relative error of approximately 2% after convergence. This validates the feasibility of the proposed algorithm. Furthermore, the stability of the algorithm was derived and verified using the Lyapunov stability analysis method. Finally, an optical current sensing experimental platform was constructed, and the proposed algorithm was implemented in parallel on the LabVIEW FPGA hardware platform. The experimental results demonstrate that the amplitude error of the filtered intrinsic component is within 2%. This verifies the real-time performance of the algorithm and its ability to meet practical engineering requirements. The successful construction of the experimental platform and the parallel implementation of the algorithm on the hardware platform further demonstrate the real-time capability and feasibility of the proposed algorithm. It provides strong support for practical applications in the field of optical current sensing and offers a high-precision and stable solution to current measurement problems in engineering practice.
Amid the rapid advancements in optical fiber communications and sensing technologies, polarization-maintaining fibers have increasingly been utilized in fiber sensing systems. As a novel type of polarization-maintaining fiber, Side-Hole Fiber (SHF), with its unique microstructure and superior performance, holds broad application prospects in fields such as communication, sensing, and medical technology. The sensors made from SHF are highly sensitive, capable of monitoring multiple parameters simultaneously, and are easily integrated into the materials being measured. They play an extremely important role in the field of structural health monitoring, thereby attracting widespread attention. In recent years, many researchers have studied and analyzed the structure and birefringence properties of SHF. However, systematic analysis and research on the impact of pressure on the birefringence performance of SHF have not been reported.In this paper, the impact of radial pressure on the birefringence characteristics of SHF was analyzed systematically based on coupled mode theory and the photoelastic effect. To facilitate the experimental component of the study, a mechanical loading apparatus was engineered to apply varying levels of radial pressure on the SHF using different weights. Furthermore, we established an experimental system grounded in the principle of polarization interference, designed specifically to measure the birefringence of SHF under different pressure conditions. The experimental setup comprised a broadband light source, a polarizer, the SHF under test, and a spectrometer. Light from the broadband source, after passing through the polarizer, was transmitted through the SHF. The interference spectrum was subsequently captured by the spectrometer. Birefringence was quantified by analyzing the mean wavelength of troughs and the average interval between adjacent peaks within the interference spectrum.Experimental results indicated that while keeping the pressure magnitude constant, the birefringence values varied according to a cosine function with respect to the direction of application, achieving maximum and minimum values at even and odd multiples of π/2, respectively. When the direction of application was held constant, the birefringence values exhibited a linear relationship with the magnitude of pressure. Specifically, for angles θ within the range (kπ-π/4,kπ+π/4)(where k is an integer), birefringence values increased linearly with pressure. Conversely, for θ in the range (kπ+π/4,kπ+3π/4), birefringence values decreased linearly with pressure. At θ=kπ+π/4 the birefringence values remained essentially unchanged. The correlation coefficient r between the experimental and simulation results was 0.992 2, indicating a high degree of consistency within the permissible error range.
Distributed Brillouin fiber sensing technology enables continuous spatial measurement of parameters such as temperature and pressure, offering advantages over traditional point sensors in terms of wide range, long distance, and high capacity. Civil structures and large machinery inevitably face lateral pressures due to their own weight and external impacts during construction and use, necessitating reliable and efficient sensors for these forces. Additionally, temperature is a crucial physical parameter that often needs to be measured simultaneously with pressure. The use of Brillouin frequency shift in fiber optic distributed sensing for temperature or pressure is common, but its sensitivity to both parameters simultaneously complicates the measurement of multiple variables at once. Hence, this paper introduces a simultaneous demodulation method for temperature, fast-axis pressure, and slow-axis pressure. The numerical simulation and emulation were performed using the wave optics and solid mechanics modules within the COMSOL Multiphysics finite element analysis software. After setting the boundary conditions, pressure was applied to the photonic crystal fiber, and its deformation under pressure was calculated. The effective refractive index of the fiber was calculated using the wave optics module. By substituting into formulas, the birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth resulting from deformation were obtained. Demodulation was then employed to acquire the specific values of these three variables. To validate the reliability of the demodulation results, lateral pressure is applied to both the fast and slow axes, while simultaneously altering the temperature. Using the birefringence frequency shift, Brillouin frequency shift, and Brillouin linewidth at 0 MPa and 0 ℃ as reference values, simulations determined the variations in these parameters under different pressures or temperatures. These variations are then substituted into formulas to calculate ?P1', ?P2', and ?T'. By comparing these calculated values with the actual applied values of ?P1, ?P2, and ?T, the corresponding error values can be ascertained. The results indicate that the three parameters can be simultaneously demodulated with demodulation errors within 1 MPa and 1 ℃. The mean errors for fast-axis pressure, slow-axis pressure, and temperature were 0.21 MPa, 0.31 MPa, and 0.30 ℃, respectively, with standard deviations of 0.15 MPa, 0.21 MPa, and 0.21 ℃, respectively. When lateral pressures of 0 to 30 MPa and temperatures of 0 to 100 ℃ were applied, the pressure sensitivity in the fast-axis direction of the photonic crystal fiber was approximately -1.961 GHz/MPa, the pressure sensitivity in the slow-axis direction was about 1.356 GHz/MPa, and its temperature sensitivity was around 0.105 MHz/℃. Compared with the current optimal structure of the photonic crystal fiber, the pressure sensitivity is improved by -957 MHz/MPa. This paper presents a highly sensitive polarization-maintaining photonic crystal fiber that enables simultaneous demodulation of temperature, fast axis pressure, and slow axis pressure. Due to the sensitivity of the polarization-maintaining fiber to temperature, fast axis pressure, and slow axis pressure, this technology can be applied to the detection of high-precision fiber optic gyroscope rings. The proposed sensor and demodulation method offer significant reference value for the distributed monitoring of temperature and pressure in different directions during the construction and use of civil structures and large machinery.
During operation of a water turbine, the vibration of its mechanical structure may reflect its operating condition. If the peak value of the vibration displacement exceeds a certain range, the water turbine may malfunction, causing serious accidents resulting in casualties and property damage. Therefore, real-time monitoring of the vibration displacement of water turbines is of great importance to ensure the safe operation of water turbines. Existing vibration displacement calculation methods do not consider the influence of vibration frequency, resulting in significant errors when measuring complex vibration displacements. This paper proposes a grating wavelength conversion method based on Fourier series. First, the wavelength variation of the fiber-optic grating sensor feedback is decomposed into multiple wavelength components containing only a single frequency according to different frequencies. Calculate the vibration acceleration generated by each wavelength component corresponding to the vibration component based on the sensitivity of the sensor. Calculate the vibration displacement generated by each vibration component by quadratic integration of the vibration acceleration. Sum these vibration displacements to obtain the total vibration displacement. Perform displacement measurement for complex vibrations. In order to improve the accuracy of vibration displacement measurement, we conducted calibration experiments on the sensitivity outside the stable operating frequency band of fiber-optic grating sensors, because only a part of the corresponding relationship between vibration frequency and sensitivity is included in the stable operating frequency band. And the corresponding relationship between vibration frequency and wavelength change in this part of the frequency was determined by segmented fitting. Two experiments were designed to compare the ability of this method to calculate vibration displacement with traditional methods. They are the simple harmonic vibration experiment and the complex vibration experiment. The harmonic vibration experiment provides vibration excitation of only a single frequency by a vibration table. In the complex vibration experiment, the vibration excitation generated by the vibration table contains two different vibration frequencies, and there is a certain phase difference between these two different vibration frequencies. The wavelength change of the fiber-optic grating sensor is recorded, and the vibration acceleration and displacement are calculated using the above two methods. In the harmonic vibration experiment, the maximum vibration acceleration error of the traditional method is 4.6%, and the maximum vibration displacement error is 4.6%. The maximum vibration acceleration error of the method in this article is 2.1%, and the maximum vibration displacement error is 3.74%. In complex vibration experiments, the traditional method cannot accurately measure the vibration displacement, and the maximum error of this method is 8.45%. The experimental results of harmonic vibration show that both traditional methods and grating wavelength conversion methods based on Fourier series can accurately measure vibration displacement when there is only a single frequency vibration in the environment. The results of complex vibration experiments show that when there are multiple vibrations of different frequencies in the environment, the grating wavelength conversion method based on Fourier series can accurately decompose and reconstruct the grating wavelength signal, and has higher accuracy and stronger stability in measuring multi-frequency vibration displacement.In addition, we also used these two methods to measure the vibration displacement peak-to-peak values of the stator end and upper frame data of the water turbine, and compared them with the results of the electrical sensors. The experimental results show that the vibration component at the stator end of the water turbine is single, and the vibration displacement peak-to-peak measurement results are similar. The vibration components at the upper frame of the water turbine are complex, and the vibration displacement peak-to-peak measurement results of the traditional method are less than 5 μm. The vibration displacement peak-to-peak measurement results of the electric sensors and the grating wavelength conversion methods based on Fourier series are both around 45 μm. This indicates that the grating wavelength conversion method based on Fourier series has certain practical application value.
Transverse Mode Instability (TMI) and Nonlinear Optical Effects (NLE) prevent high-power all-fiber lasers from further power scaling. Low Numerical Aperture (NA) Large Mode Area (LMA) active fibers can maintain large effective areas while suppressing the Higher-Order Modes (HOMs) and increasing the thresholds of NLE and TMI. Additionally, it is easier to match the passive component for low NA LMA fiber because its structure is simpler and consistent with the step-index fiber structure. Mode Field Adapters (MFAs) match the mode field between LMA fibers and Single-Mode Fibers (SMFs) and are crucial passive components in fiber laser systems. The insertion loss and beam quality of MFAs significantly affect the power scaling and beam quality of laser systems. This paper made MFA based on tapered low NA LMA fiber with NA=0.05 and Thermally Expanded Core (TEC) fibers, maintaining high coupling efficiency and improving the output beam quality. The impact of mode field mismatch, core offset, and angular misalignment between LMA fibers with different NAs and SMF on coupling efficiency and beam quality of MFA was studied theoretically and experimentally. First, theoretical models for TEC and tapered LMA fibers were built based on diffusion and adiabatic criterion equations. The mode field distribution and propagation characteristics of TEC and tapered LMA fibers were simulated based on the beam propagation method to optimize the device structure and preparation parameters. Second, a simulation model was created to analyze the insertion loss and beam quality degradation caused by mode field mismatch, core offset, and angular misalignment between LMA fibers with different NAs and SMFs. The simulation results show that reducing the NA of LMA fibers helps suppress HOMs and reduces the insertion loss and beam quality degradation of MFAs caused by core offset and angular misalignment. During the experimental process, the SMF was heated with an H2-O2 flame to expand the Mode Field Diameter (MFD) of SMF without causing transmission loss. The MFD of the TEC SMF under different heating times was measured using the far-field method. Two MFAs were prepared by LMA fibers (25/400 μm) with NAs of 0.06 and 0.05, respectively, to TEC SMF (5.3/125 μm, NA=0.14). The insertion loss and beam quality factor (M2) of the devices were measured. The forward insertion loss decreased from 4.50 dB to 0.29 dB, and the difference in bi-directional insertion loss decreased from 2.50 dB to 0.19 dB when the LMA fiber (NA=0.06) for MFD matched with the SMF. The experimental results show that matching the MFD of SMF and LMA fibers effectively reduces the insertion loss and the difference in bi-directional insertion loss. The taper ratios of the LMA fibers are both 2, and the heating times for the SMF are 25 min and 20 min, respectively, when the MFD is matched between the LMA fibers with NAs of 0.05 and 0.06 and SMF. Due to the MFD matching between the LMA fibers and SMFs, only the unavoidable core offset and angular misalignment during the fusion process, which affect the coupling efficiency and beam quality, are considered, and these misalignments are random variables. The impact of misalignment on beam quality and coupling efficiency in LMA fibers with different NAs was indirectly reflected by performing multiple measurements and calculating the mean and standard deviation. The forward insertion loss decreases to 0.29 dB with a standard deviation of 0.085, and the bi-directional insertion loss difference is 0.19 dB with a standard deviation of 0.077 when the MFD is matched between the LMA (NA=0.06) fiber and the SMF. The forward insertion loss decreases to 0.23 dB with a standard deviation of 0.024, and the bi-directional insertion loss difference is 0.06 dB with a standard deviation of 0.011 when the MFD is matched between the LMA (NA=0.05) fiber and SMF. Cladding modes caused the difference in bi-directional insertion loss, and HOMs were not stripped by the cladding light strippers as they do not propagate in SMF. Therefore, this difference is positively correlated with the beam quality of the MFA. The difference in bi-directional insertion loss for the MFAs based on LMA fibers with NAs of 0.05 and 0.06 are 1.76 dB and 2.50 dB, and the M2 are 1.88 and 2.15, respectively, when the MFD of the SMF and the LMA fiber are not matched. This difference for the MFAs based on LMA fibers with NAs of 0.05 and 0.06 is 0.06 dB and 0.19 dB, and the M2 value is 1.15 with a standard deviation of 0.017 and 1.26 with a standard deviation of 0.092, respectively, and when the MFD of the SMF and the LMA fiber are matched. The experimental results show that the LMA fiber with NA=0.05 contains fewer HOMs and reduces beam quality degradation and insertion loss caused by core offset and angular misalignment during the splicing process, resulting in higher beam quality and lower insertion loss of the MFA. These conclusions are consistent with the theoretical analysis and simulation results. The MFA based on LMA fiber with NA=0.05 has promising application prospects in single-mode output high-power fiber lasers due to its low insertion loss and high beam quality advantages.
Underwater wireless optical communication has garnered significant attention in the wireless communication field due to its high data rate, enhanced security, and lightweight nature. However, seawater can induce absorption and scattering of light. Absorption results in a reduction of the received optical power at the receiver, which is an irreversible process, while scattering causes alterations in the received photons at the receiver. Moreover, the ocean typically contains turbulence, a phenomenon caused by temperature variations and irregular movements, leading to random fluctuations in the optical signal. Consequently, the underwater channel is intricate and challenging to predict. To achieve reliable communication performance, a more dependable signal detection method is required at the receiver. In this study, a deep learning-assisted signal detection method is proposed for underwater optical communication. A convolutional neural network (a specialized form of deep neural network) is developed to directly detect the Original On-off Keying (OOK) signal, and two distinct training methods for the Deep Neural Network (DNN) are proposed during the training phase. Initially, an indoor underwater optical communication experimental platform is designed and constructed, incorporating three types of water tank channels (flowing water, turbid flow 1, turbid flow 2). The attenuation coefficients and probability density functions of the channels are measured. Subsequently, a simulated underwater optical channel is derived based on the measured channel mathematical models, and a simulated dataset of OOK signals for the neural network is obtained. The proposed methods are tested using the dataset, and the performance of the two different DNN training methods and the adaptive threshold method is simulated under different simulated channels. The proposed methods exhibit an improvement in Bit Error Rate (BER) compared to the adaptive threshold method at any signal-to-noise ratio in the three channels. The improvement is most notable in the simplest flow channel, with up to a two-order-of-magnitude enhancement, and it increases with higher signal-to-noise ratios in the relatively complex turbid flow channel 2. Additionally, due to DNN training method 1 learning multiple datasets from different channels, it exhibits worse BER performance compared to training method 2, which only learns one channel dataset. However, thanks to the powerful fitting capability of DNN, the BER is still superior to the adaptive threshold method. To validate the simulation results, experimental datasets of OOK signals are obtained based on the experimental platform. The DNN is retrained and tested using the experimental datasets, and the BER performance of the two different DNN training methods and the adaptive threshold method is experimentally studied. For 5 Mbps communication transmission in the three water tank channels, the DNN method achieves a reduction in BER of two orders of magnitude, one order of magnitude, and one order of magnitude, respectively, compared to the adaptive threshold method. The trend of the experimental results is consistent with the simulation. For turbid flow 1 and different communication rates (5 Mbps, 10 Mbps, 25 Mbps), the DNN method achieves a reduction in BER of one order of magnitude at all three rates, and the proposed method requires lower received optical power compared to the adaptive threshold method when the BER is the same. The simulation and experimental results demonstrate that the proposed method enhances the performance of underwater wireless optical communication in complex channels compared to the adaptive threshold method, validating the reliability of the method. Therefore, the method can offer valuable insights for the design of high-speed and reliable underwater wireless optical communication systems.
The lateral creep of hydrate-bearing layers associated with natural gas hydrate decomposition can easily cause geo-engineering security risks. The triaxial shear instrument is an experimental apparatus for simulating lateral creep of the lateral creep of hydrate-bearing layers. To obtain the displacement of this creep accurately and in real time, a kind of displacement sensing scheme based on fiber Brag grating peak counting of strain shift curve with high noise resistance is proposed. The sensor is mainly composed of fiber Bragg grating, cantilever, transmission system, gear, rack, base and metal-tube. Inside of the transmission system is composed of gears with different teeth and installed on the base with bolts. Using the rack as the probe to engage with the input gear of the transmission system, another side of output shaft is fixed with the gear securely. One end of the cantilever beam press is fixed to the side plate of the base, while the other end presses against the gear. One end of the fiber Bragg grating is encapsulated in a metal tube using epoxy resin, and the tube is welded vertically to the free end of the cantilever beam press plate using a laser, while the other end is fixed in a similar manner on the side plate of the base after a pre-tension is applied. The interval between two metal tubes is 41 mm. The rack and transmission system are used to convert the lateral displacement of the hydrate-bearing layers into the rotation of the output gear. During the rotation of gear, the tooth of gear pulls on the free end of the cantilever to produce periodic stretching and resetting of the fiber Bragg grating. The average and minimum wavelength shift of the reflection center is 1405 pm and 1263 pm, respectively. By calculating the product of the number of peak jumps in the wavelength drift curve of the reflection center of the fiber Bragg grating and the standard step length of the rack passing between adjacent peak jump, we can determine the lateral displacement of the geological layer. Furthermore, we can build a linear relationship between the frequency of peak jumps and speed. The coefficient of sensor's transmission system is 2 mm/r and tooth number of gear is 20, respectively. In the measuring range of 40 mm, displacement sensor has a resolution up to 0.1 mm, maximum error of only 16 μm, and the range is adjustable through the rack length. The sensor displays excellent repeatability and multi-parameter detection capabilities, greatly enhancing the sensor's noise immunity. When the rack is pushed at three different speeds for 6 mm, it is found that the wavelength drift for the same number of gear teeth is only 28 pm, which is significantly smaller than the minimum wavelength drift of the reflection center 1 263 pm. In situations where the displacement change speed far exceeds temperature variations, temperature only offsets the entire wavelength drift curve of the reflection center. This offset is directly related to the temperature sensitivity of the fiber Bragg grating and range, temperature does not impact the wavelength drift of the reflection center resulting from displacement or the displacement of the rack between adjacent peaks. In this case, the fiber Bragg grating is only affected by noise sources like vibration and demodulation instrument error. The minimum optical signal-to-noise ratio achieved is 43.966 dB, meeting the requirements for hydrate-bearing layer creep monitoring and enabling simultaneous measurement of displacement and velocity. It's worth noting that when the displacement change speed matches the temperature change specifically, when the temperature changes rapidly form 0 ℃ to 20 ℃, the optical signal-to-noise ratio decreases from 43.966 dB to 14.497 dB, greatly impacting displacement measurement accuracy. Therefore, it's essential to incorporate a free-state fiber Bragg grating for temperature compensation, ensuring the sensor maintains a high noise resistance. At this point, the sensor utilizes two fiber Bragg gratings to achieve three-parameter measurement of displacement, speed, and temperature, while maintaining a high cost-efficiency. Finally, four kinds of displacement sensors' performance are compared, with displacement sensor base on fiber Brag grating peak counting of strain shift curve standing out for its superior anti-noise and multi-parameter detection capabilities.
The Panda-type polarization-maintaining fiber, as a typical stress-type fiber relied on stress birefringence, is utilized for maintaining the polarization state of the transmitted light. An ideal polarization performance is achieved by increasing refractive index difference along two orthogonal axes resulting from stress formed into the fiber core. The Panda-type polarization-maintaining fiber has been widely used in fiber optic gyroscopes, telecommunications, fiber optic sensors, and high-speed optical communication systems owing to its advantages of high polarization extinction ratio, low polarization mode dispersion, and low insertion loss. Currently, improving the birefringence of Panda-type fiber is a significant research direction. Various studies have focused on changing the core shape of the fiber, such as using elliptical, leaf-shaped, and square-shaped cores to enhance birefringence, but with limited effects. An alternative method is to change the shape of stress regions to improve the birefringence of the fiber, such as Knot-type polarization-maintaining fibers and elliptical cladding polarization-maintaining fibers. Remarkably, the Knot-type polarization-maintaining fiber exhibits the best polarization-maintaining performance due to its larger effective stress regions. The most commonly used fiber has a cladding diameter of 80 μm to achieve miniaturization of fiber coils. However, for high-precision satellite positioning, unmanned aerial vehicles, and automotive navigation, research on 60 μm thin-diameter polarization-maintaining fiber is urgently needed. As the cladding diameter decreases, the study of coating thickness becomes challenging because thinner coating layers are difficult to maintain the excellent transmission performance of the fiber. In this paper, the COMSOL finite element analysis software is utilized to propose a method to enhance the birefringence of Panda-type fiber by adjusting the material properties of the stress regions. By changing thermal expansion coefficients of materials from 2×10-6 K-1 to 7×10-6 K-1, the Young's modulus from 2×1010 Pa to 12×1010 Pa, and the Poisson's ratio from 0.1 to 0.5, the impact of the stress regions on effective refractive indices of the fast and slow axes of the fiber core is enhanced, and thus improving the birefringence of the fiber. For the miniaturization of fiber coils, this study simulates the effect of reducing the outer coating diameter from 165 μm to 135 μm on the fiber's transmission performance. Furthermore, a complete physical model of a 32-layer, 82-turn fiber coil is built, where point loads applied to the boundaries of each turn of the fiber is used to simulate the real internal stress during fiber winding. The stress of each turn in the fiber core is then extracted as the basis for judging the output error of the fiber coil. To reduce the error caused by winding tension, the study discovers an optimal ratio of thickness between the inner and outer coatings by analyzing different material properties and effects. This improved thickness ratio reveals an excellent suppression effect of winding tension by approximately 10% compared to the original fiber. The simulation calculates the Young's modulus and Poisson's ratio of the double-coatings, with the inner coating's Young's modulus varying from 1.56 MPa to 15.6 MPa and the outer coating's Young's modulus varying from 1 GPa to 4.68 GPa. Both the inner and outer coatings have Poisson's ratios ranging from 0.25 to 0.45, and the conclusion is drawn that the material properties of the coatings also have a significant effect on suppressing winding tension. In summary, this paper proposes methods to enhance the birefringence of Panda-type polarization-maintaining fiber by changing the structure and material parameters of the stress regions. Additionally, it demonstrates that reducing the coating thickness of the fiber effectively enhances birefringence performance under low-temperature environments. Finally, to reduce the error caused by fiber winding tension, it suggests optimizing the thickness ratio of the fiber coatings and the material parameters of the coatings.
In severe cold climates, the bearing capacity of ice body depends on its thickness. As a carrying medium, the ice body with enough thickness expands the human activity area. However, when the bearing capacity of the ice body is insufficient, brittle failure will occur, leading to serious consequences such as casualties and property damage. Therefore, the research on the strain of the ice body measurement technology is of great significance to ensure the reasonable bearing capacity of the ice body. The traditional electromechanical strain measurement systems have some disadvantages, such as large volume and difficult sensor installation and disassembly. The current measurement methods of spectroscopy mainly focus on theoretical simulation and exploration of ideas, while there are few specific plans and experimental studies. In this paper, the Single-mode No-core Single-mode (SNS) fiber optic sensor for strain measurement of ice bodies is proposed. SNS fiber optic sensor is implanted into ice bodies in a layered freezing manner. When the strain of ice changes, it modulates the sensor, causing the wavelength of the interference spectrum to shift. By monitoring the spectral wavelength, ice strain measurement is achieved. By utilizing the similar temperature sensitivity of adjacent spectral troughs, the wavelength difference between adjacent troughs can be applied to strain measurement, which is not affected by changes in ice body temperature. The length, width, and height of the ice body are 250 mm×150 mm×16 mm. When the temperature of ice body increases from -20 ℃ to 0 ℃, the wavelength shifts of Dip1 and Dip2 in the spectrum are very close. The temperature sensitivities of Dip1 and Dip2 are 9.8 pm/℃ and 9.5 pm/℃, respectively, with a relative difference of only ~2.5%. Therefore, by using the wavelength difference between Dip1 and Dip2 as the measurement factor for ice bearing capacity, temperature independent bearing capacity measurement can be achieved. Establish an experimental system to study the strain response of ice under different bearing capacities. Place weights at the center of the upper surface of the ice body, to apply bearing capacity to the ice body. The weights are loaded from 0 g to 600 g. Results show that when the bearing capacity of the manufactured ice body exceeds 500 g, its strain significantly increases. When the bearing capacity exceeds 600 g, the ice body undergoes brittle fracture, and the sensor effectively extracts the strain signal during the ductile-brittle transition process of the ice body within this range. The melting experiment shows that the sensor can monitor the complete strain change process during the natural melting of ice, and the measurement results are not affected by the temperature changes inside the ice. For existing ice body in actual testing environments, fiber optic cannot be pre-embedded inside. Improvement is needed for the layered freezing method. Firstly, determine the location to be monitored on the ice surface, known as the monitoring point. Place the fiber optic in a natural straight state on the ice surface, aligning the SNS sensor on the fiber optic with the monitoring point. Place ice block on the fiber optic on both sides of the SNS sensor to secure the fibers. During the fixation process, slowly inject water into the gap between the ice block and the ice surface, and naturally freeze for 5 min. The ice block and ice surface are completely frozen, thus achieving the fixation of the fiber optic. Afterwards, slowly inject water into the ice body until the ice blocks are submerged. After 1 h of natural freezing, the injected water is frozen together with the original ice body, thus completing the implantation of optical fibers into the existing ice body. It should be noted that the ice body itself is a 3D structure, and when a single sensor is implanted, only the strain information of the monitoring point where the sensor is located can be obtained. To accurately describe the overall strain of the ice body as much as possible, research can be conducted through fiber optic measurement schemes with good reusability such as fiber Bragg grating.
Sound source localization is a pivotal research area within the field of acoustics, finding extensive applications in domains such as Unmanned Aerial Vehicle (UAV) navigation, intelligent traffic systems, medical imaging, and structural health monitoring. Traditional sound source localization methods typically rely on arrays of multiple microphones or sensor networks. Nevertheless, these conventional approaches are beset by challenges related to complex installation, intricate data processing, and poor resistance to interference. In recent years, there has been considerable attention directed towards the emerging field of optical fiber-based acoustic localization, within which most optical fiber-based detection systems have employed Fiber Bragg Grating (FBG) sensor arrays due to their wavelength-based multiplexing capabilities. However, FBG sensors exhibit limitations in sensitivity. In contrast, optical fiber Extrinsic Fabry-Perot Interferometer (EFPI) sensors, with their probe-like structure and advantages in terms of high sensitivity and structural simplicity, are better suited for sound source localization. In this research endeavor, we have introduced optical fiber collimators within EFPI sensor arrays to develop a self-collimating optical fiber-based EFPI acoustic sensor array. The primary objective is to augment the sound pressure sensitivity and detection range of the sensor array. The designed sensor array exhibits elevated acoustic sensitivity and an expanded spatial detection range, thus holding immense potential for applications in sound source localization and the detection of partial discharge phenomena.Firstly, the optical field distribution of a quarter-pitch-length gradient multimode optical fiber was verified using Rsoft software. Subsequently, an EFPI (Extrinsic Fabry-Perot Interferometer) acoustic sensor with a self-collimating optical fiber was designed. To assess whether the proposed sensor exhibits enhanced sensitivity to sound pressure, it was compared to an EFPI acoustic sensor without the self-collimating feature. Next, three EFPI acoustic sensors with identical structures and self-collimating optical fibers were fabricated for sound source localization experiments. Prior to conducting the localization experiments, the consistency of sound pressure sensitivity and sound source directionality among the three sensors with self-collimating optical fibers was verified. Subsequent to these preparations, time-delay signals were acquired using an intensity demodulation technique and recorded on an oscilloscope. The time-delay signals were processed using conventional cross-correlation algorithms to calculate the time delays between pairs of sensors. Finally, based on the geometric positions of the sensor array, an estimation of the approximate sound source location was determined.The experimental results show that the interference spectrum FSR of EFPI sensor with collimator is 5.25 nm, and the maximum fringe visibility is 14.96 dB. The EFPI sensor without a collimator has a FSR of 5.18 nm and a maximum fringe visibility of 9 dB. The FSR of them is almost the same, but the interference spectral intensity of the former is increased by about 6 dB. In addition, the EFPI spectral slopes were 6.5 dB/nm and 10.2 dB/nm, respectively, without and with collimators, and the spectral slope of the latter increased nearly twice as much as the former. In the response characteristic experiment for the single sensor, EFPI acoustic sensor with collimator is superior to EFPI acoustic sensor without collimator in sound pressure response waveform and sound pressure sensitivity test. EFPI acoustic sensor with collimator has sound pressure sensitivity of 185 mV/Pa. The minimum detectable sound pressure is 52.7 μPa/Hz1/2@500 Hz, and the signal-to-noise ratio reaches 62 dB. In the experiment of sound source directionality, the designed sensor showed good performance under different sound pressure directionality. When the sound source was placed directly in front of the sensor, its sound pressure sensitivity reached 185 mV/Pa. When the sound source was set on the side of the sensor (90°, 270°), its sound pressure sensitivity could still reach 177 mV/Pa. This indicates the ability of the sensor array to achieve sound source localization within a wide-angle range. In the two-dimensional plane sound source location experiment, the signal delay in the time domain signal is extracted by the correlation algorithm, and the two-dimensional plane sound source location within the range of 200 cm×200 cm is finally realized. The theoretical spatial resolution is 0.71 cm, and the maximum positioning error of the system is no more than 2.8 cm. Finally, the performance comparison with other EFPI acoustic arrays shows that the system has the advantages of high sensitivity, low production cost, simple demodulation system and large detection range.
To enhance the bandwidth and real-time capabilities of Radio Frequency (RF) spectrum analysis, the optical real-time Fourier transform method has been proposed. The optical real-time Fourier transform method based on the temporal Talbot effect has the advantage of a simple structure by using optical pulse sampling and dispersion delay structure. However, its frequency measurement bandwidth is limited by the optical pulse's repetition rate. Addressing this limitation, a real-time spectrum analysis scheme of wideband RF signals based on the fractional temporal Talbot effect is proposed and demonstrated. Based on the sampling and dispersion structure, the scheme realizes the mapping of the RF signal frequency to the optical pulse time interval. At the same time, the repetition rate of the optical pulse before sampling is multiplied by passing through a dispersive element satisfying the fractional Talbot distance in advance. The frequency measurement bandwidth of the system can be significantly improved by using the fractional Talbot effect. A proof-of-concept experiment is carried out to test the performance of the proposed scheme. The repetition period of the optical pulse is set to 151.5 ps, that is, the repetition frequency is 6.6 GHz. Each pulse has a Gaussian shape and the full width at half maximum of the pulse is approximately 30 ps. Dispersion compensation fiber is used to provide dispersion for the system. The total dispersion value of the two sections of dispersion is about 3 650 ps2, which is about 0.1% different from the theoretical result. The RF signal to be measured is generated by a RF signal generator. The output optical signal is converted into an electrical signal by a 40 GHz bandwidth photodetector and recorded by a sampling oscilloscope with a bandwidth of 50 GHz. Comparing the experimental results under integer-order, 3rd-order fractional, and 9th-order fractional temporal Talbot conditions, it is verified that the measurement frequency bandwidth of the system increases with the order of the temporal Talbot effect. Real-time spectral analysis of single-tone and two-tone RF signals within a 29.7 GHz bandwidth is achieved using the 9th-order fractional temporal Talbot effect. Numerical simulation is carried out to achieve time-frequency analysis of a large-bandwidth linear chirp signal. Based on the 3rd-order fractional temporal Talbot effect, a linear chirp signal with a frequency range of 2~13 GHz and a chirp rate of 2.2 GHz/ns is successfully identified. Numerical simulation results further verify that this scheme can effectively analyze frequency transient signals. The main causes of frequency measurement errors include the time jitter of the input optical pulse train, the limited bandwidth of the pulse detection system, the deviation between the dispersion value and the theoretical value, high-order dispersion terms, etc. In the experiment, the time jitter root mean square value of the optical pulse is approximately 1 ps, which is 0.066% of the period of the optical pulse train. When the frequency measurement bandwidth is 29.7 GHz, the frequency measurement error caused by the time jitter is about 200 MHz. In order to improve frequency measurement accuracy, methods such as reducing optical pulse jitter, increasing the bandwidth of the pulse detection system, and compensating for high-order dispersion can be used. It should be noted that the increase of frequency measurement bandwidth will sacrifice the frequency resolution of the system. In practical applications, the requirements of system bandwidth and frequency resolution should be fully considered to select the order of the fractional temporal Talbot effect. With its advantages of simple structure, large bandwidth, and real-time processing, this scheme has potential application value in the fields of broadband radar, cognitive radio, and other fields.
Loosing of the bolt connection structure affects its working and operation safety. The main reasons for the loosing coming from loading, vibration, and friction. Consequently, loosing is inevitable and its monitoring is important for its application. At present, the theory and technology of testing the tightness state of bolt connection are still not mature. The detection of small torque is a technical difficulty in this field. In this study, for the structure to be tested on the uneven surface in the narrow space, the Bragg Fiber Grating (FBG) was used as the sensor to identify the small torque of the bolt connection. In the testing, a periodic vibration in the tested structure with bolt tightness information was excited and employed for the identification. One tail of the suspended- FBG was sticked on the tested structure, and the vibration yielded periodic strains in the tail of the FBG, which acted as the source of the elastic longitudinal wave propagating along the optical fiber with a FBG written in it. The edge-filter method was used to demodulated the signals in the FBG sensor to satisfy the high frequency signals. The information coming from the FBG was used to be identified. Firstly, the Empirical Mode Decomposition (EMD) method was used to decompose the original signal, basing on which, we removed the unstable components and noise by calculating the correlation function of each component and the original signal. Then the signals were restructured for later identification. The dimensional features (standard deviation, residuals, peak- peak value, and energy) and dimensionless features (skewness, kurtosis, waveform factor, amplitude factor, impact factor and margin factor) of the signals were exacted, and were inputted to the recognition system based on the Support Vector Machine (SVM) finally, where we used the ten-fold cross-validation algorithm and Gaussian kernel function SVM for higher accuracy. Results show that the recognition accuracy reaches to 97.2% and the torque recognition ability is on the order of N·cm. This study proves that optical fiber is a good acoustic waveguide, and the installation technique of suspended FBG effectively mitigates spectral distortion resulting from uneven stress due to direct adhesion, thereby decreasing the complexity associated with sensor installation. At the same time, because the optical fiber as an acoustic waveguide does not sense the torsional displacement, the bending stress wave cannot form an effective transmission in the optical fiber, the FBG only senses the vibration displacement along the optical fiber axis that causes the longitudinal wave. Therefore, the signal deviation caused by the excitation and sensor setting in the actual test process is relatively small and limited, which can reduce the difficulty of the signal processing. On the other hand, the study identifies that the signal processing and identification method are suitable for the non-linear, non-stationary and small sample test data in this study. This study presents a new detection method for the bolted state, especially for the detection of small torques in small mass structures on the uneven surface in the narrow space.
Fiber optic interferometer has the advantages of small size, light weight, anti-corrosion, anti-electromagnetic interference, high sensitivity, etc., and is widely used in the measurement of temperature, humidity, magnetic field and other parameters. In recent years, researchers have dramatically improved the measurement sensitivity of interferometric fiber-optic sensors by cascading or paralleling two fiber-optic interferometers to produce an optical Vernier effect. When the free spectral ranges of the two fiber optic interferometers are close but not equal, the resulting Vernier effect is called the normal Vernier effect, when the free spectral range of one fiber optic interferometer is about an integer multiple of the other fiber optic interferometer, the resulting Vernier effect is called the harmonic Vernier effect.In this paper, a parallel optical fiber temperature sensor based on two Sagnac interferometers is proposed, where the interferometers SI1 and SI2 are connected to the two outputs of the fiber-optic coupler C3, where SI1 is a reference interferometer and SI2 is a sensing interferometer. When the length of the Panda fiber in the SI2 interferometer is approximately i+1 of the length of the Panda fiber in the SI1 interferometer (i is 1,2,3 …) times, the two interferometers will produce an i-order harmonic Vernier effect. When i is 0, it produces an normal Vernier effect, at which time there will be a single envelope in the interference spectrum. When i is 1, it produces a first-order harmonic Vernier effect, at which time there will be a double envelope in the interference spectrum. When i is 2, it produces a second-order harmonic Vernier effect, at which time there will be a triple envelope in the interference spectrum. In other cases, the order is analogous.We have numerically simulated the theoretical analysis and the free spectral range of the interferometric spectrum of the length interferometer SI1 with a Panda fiber of 520 mm at constant temperature is 9.13 nm. The SI2 interferometers with lengths of 572 mm, 953 mm, and 1 430 mm of the Panda fiber have free spectral ranges of 8.30 nm, 4.98 nm, and 3.32 nm, respectively. When the temperature is increased from T0 ℃ to T0+1 ℃, the interference spectra of SI2 interferometers with different Panda fiber lengths are all shifted to the short-wave direction, and the shifts are all about 1.89 nm, which is consistent with the theoretical analysis. The parallel interference spectra of interferometer SI1 and SI2 with Panda fiber lengths of 572 mm, 953 mm and 1 430 mm show single, double and triple envelopes respectively, indicating that the two interferometers produce the normal Vernier effect, first-order and second-order harmonic Vernier effects, respectively, and from the theoretical calculations. It can be seen that the amplification of the normal Vernier effect, first-order and second-order harmonic Vernier effects are all 11 times. When the temperature increases from T0 ℃ to T0+1 ℃, the single envelope moves in the short-wave direction, while the double and triple envelopes both move in the long-wave direction, which is opposite to that of the single SI2. In addition, the shifts of the single, double and triple envelopes are all about 20.7 nm due to the fact that the Vernier magnification is the same for the normal Vernier effect, first-order and second-order harmonic Vernier effects.It is experimentally concluded that the interference spectra of SI2 are blueshifted in the temperature range from 40 ℃ to 50 ℃, and the shifts are all about 1.89 nm , which is consistent with the theoretical analysis and simulation results. The temperature sensitivity of the sensor corresponding to the normal Vernier effect is -20.67 nm/℃, the temperature sensitivity of the sensor corresponding to the first-order harmonic Vernier effect is 21.34 nm/℃, the temperature sensitivity of the sensor corresponding to the second-order harmonic Vernier effect is 21.18 nm/℃, and the fiber optic sensors corresponding to the harmonic Vernier effect and the normal Vernier effect have almost the same temperature sensitivities, which are both about 21 nm/℃. This results are consistent with the theoretical analysis and simulation results. The above experimental results show that the temperature sensitivity of the SI2 interferometer is independent of the length of the Panda fiber, although the magnification is the same, the harmonic Vernier effect and the normal Vernier effect correspond to the detuning of the length of the Panda fiber are obviously different, the detuning corresponding to the normal Vernier is 52 mm, and the detuning corresponding to the first-order and second-order harmonics is -87 mm and -130 mm, respectively. This shows that the higher the order, the larger the detuning amount, which is approximately a multiple increase. The above experimental results are consistent with the theoretical analysis. Since the larger the detuning amount, the easier the Vernier magnification can be controlled and realised, the harmonic Vernier effect is obviously superior to the normal Vernier effect from the preparation point of view. This study can provide an important reference for the subsequent study of optical Vernier effect.
The increased power of the motor directly results in a higher temperature rise effect on the rotor. High temperatures can cause turn-to-turn short circuits or permanent demagnetization of the motor rotor, which can seriously affect the reliability of the generator operation and the stability of the combat system. Therefore, the research of online rotor temperature measurement technology is of great significance. Compared to electronic sensors, Fiber Bragg Gratings (FBGs) are resistant to electromagnetic interference, small in size, require no power supply, and can be used for quasi-distributed measurements. This is a great advantage for monitoring the motor rotor temperature. However, research on FBG rotor temperature monitoring systems is still relatively rare. The state of motion of the motor rotor is an important factor affecting the accuracy of the system. The effect of rotor whirling on measurement accuracy due to unbalanced mass is even more difficult to ignore and has not been studied.In this paper, a model of FBG scanning spectra under rotor whrling conditions is developed by combining the transmission matrix theory of FBG and the coupled transmission theory of self-focusing lenses. The scanning error of the center wavelength and its influencing factors have been investigated in conjunction with relevant experiments. The results show that the whirling of the rotor leads to aberrations in the scanning spectra of the FBG, which are mainly manifested in the offset of the reflection peaks and the reduction of the 3 dB bandwidth. The main factors affecting the temperature measurement error of the system are the rotor whirling frequency, the demodulator scanning frequency, the radial displacement of the rotor at the end face, the axial ratio of the axial trajectory and the deflection angle of the axis. As the ratio of the coupling loss period to the spectral scan time (q-value) increases, the maximum center-wavelength scan error and the peak-seeking error both show a rapid decrease, followed by a slow decrease to stability. The key to ensuring a low level of temperature measurement error in the system is to ensure that q>10. The peak-finding error of the Gaussian curve fitting method is reduced to the level of the centroid method when q>40. When the rotor radial vibration amplitude is 200 μm, the axis ratio of the axial trajectory is 3, and the axis deflection angle is 0.1°, for a typical demodulator operating bandwidth of 40 nm and FBG bandwidth of 0.3 nm, if it is hoped that the measurement error of the polyimide-coated FBG to be less than 0.5 ℃, it should be ensured that the q value reaches 115 or more. The corresponding scanning frequency must be approximately 1.74 times higher than the whirling frequency. When using the centroid method or the Gaussian curve fitting method for peak finding, it is only necessary to make the scanning frequency about 0.21 and 0.44 times higher than the whirling frequency, respectively. The centroid method is more advantageous than the Gaussian curve fitting method for rotors with strong whirling.In addition, a study was conducted to investigate the effect of the intensity of the rotor whirling on the maximum scanning error at the center wavelength of the FBG and the system temperature measurement error. The results show that the maximum scanning error at the center wavelength increases slowly and then linearly as the rotor radial displacement amplitude or axis declination amplitude increases. The peak-finding error of the centroid and Gaussian curve fitting methods increases slowly and then dramatically. As the axial ratio of the rotor axis trajectory increases, the maximum scanning error and the peak detection error at the center wavelength both increase dramatically and then slowly increase to a steady state. When the amplitude of radial displacement is less than 200 μm and the axial deflection angle is less than 0.167°, the maximum temperature measurement error caused by whirling motion is 2.9 ℃, which is reflected by 15 sampling spectra under the condition of q=10 and the axial ratio of n=3. When peak detection is performed using the centroid method or the Gaussian curve fitting method, the temperature measurement error of the system is reduced to 0.6 ℃ and 1.1 ℃, respectively.
In recent years, optical fiber shape sensing technology has been widely studied in various fields, and has been widely used in robot, medical, aerospace, industrial equipment structure monitoring and submarine cables. With the change of application scenarios and the gradual improvement of measurement performance requirements, the research needs of optical fiber shape sensing technology are becoming increasingly urgent. At present, the research on fiber shape sensing is mainly divided into two directions. One is the shape sensing technology based on FBG, which takes advantage of the wavelength drift of FBG under strain and realizes shape measurement by writing FBG on multi-core fiber, which has the advantages of high precision and simple data processing. In this direction, some scholars have done more in-depth research, but this technology is limited by the number and interval of FBG writing, and cannot achieve long-distance distributed shape measurement. The other direction is the shape sensing based on the distributed optical fiber measurement system. As a medium of shape sensing technology, optical fiber is small in size, light in weight, and has strong electromagnetic interference resistance and corrosion resistance. It can be either a transmission medium or a sensing medium. When the light wave is transmitted in the optical fiber, the optical intensity, phase, frequency and other parameters of the optical fiber will change with the change of environmental parameters such as strain and temperature. The data processing equipment is used to demodulate the modulated light, and then the information of strain and temperature of the optical fiber is obtained. In this paper, the Brillouin scattering in the fiber is used to reconstruct the shape of the fiber or the measured object in contact with it, and the strain change values of more than two fiber cores in the shape sensor are measured at the same time. Then the shape reconstruction algorithm is used to reconstruct the shape of the sensor or the measured object. In this paper, the BOTDA system is built with a spatial resolution of 1 m. A homogenous low-crosstalk seven-core fiber from Changfei Company is selected as the distributed shape sensor. The total length of the fiber is 300 m, the core diameter is 8 μm, the cladding diameter is 150 μm, and the protective layer diameter is 245 μm. The remaining six cores are located at a distance of 42 μm from the middle core and are symmetrically distributed around each other at 60°. At the same time, the seven pigtails of the multi-core fiber are labeled and separated by a fan-in fan-out coupler. By using the BOTDA system, the Brillouin gain spectra of the intermediate core and the off-core are measured, and it is verified that the intermediate core is not affected by bending, and the strain values of each two symmetric off-core are negative to each other. Three unsymmetrical cores with 120° distribution were selected, and the intermediate cores were used as temperature compensation to demodulate the induced variables of each core at different curvature radii. Finally, parallel transmission frame shape reconstruction algorithm is used to reconstruct the shape of seven-core fiber when the curvature diameter is 0.112 m and 0.052 m. When the curvature diameter is 0.112 m, the curvature reconstruction error is 0.375%, which is mainly due to the low spatial resolution of the construction system and the torsion problem in the winding process. Distributed fiber shape sensing technology has a very large application prospect, but there are still many technical difficulties that need to be overcome by researchers. The work in this paper has laid the research foundation for the subsequent distributed fiber shape sensing, and has certain practical significance.
As the germanium (Ge) core is mostly in an amorphous or polycrystalline state after fabrication, laser annealing is an effective way to improve the properties of semiconductor core fiber. During the laser annealing process, the axial scanning velocity of the laser along the fiber is an important parameter that affects the properties of the annealed fiber. Therefore, it is of great significance to investigate the modification mechanism of laser annealing on the Ge core to improve the properties of annealed fibers.In this study, three sets of Ge core fibers with different outer diameters (OD) and the same inner diameter (ID) were annealed by CO2 laser at different scanning velocities. The laser annealing experiments were carried out on Ge core optical fibers with an ID of 41~43 μm and the ODs of 188 μm, 251 μm, and 270 μm, respectively. The Ge core fibers were annealed by the SK-3D30 CO2 laser. The laser spot is 1 mm in diameter, the output power is 0~30 W, and the laser wavelength is 10.6 μm. The scanning region along the fiber axis is 1 mm×50 mm, which can completely cover the Ge core fiber, and the laser scanning in this region was reciprocated along the fiber axis during the annealing time. After laser annealing, samples were analyzed by using the spectrometer. Raman experiments were carried out on the cross-section of the Ge core fiber to collect the Raman peak frequency information. The obtained data was processed into mapping by MATLAB software. The optical transmission loss of Ge core fiber was measured by the cutback method. The system consists of a laser, photodetector, and optical power meter. The samples were cut off at 5 mm each time and measured 3 times per fiber. All measurements were made at room temperature.Three sets of experiments were carried out, the laser frequency is 50 kHz, the laser power is 20%(6 W), and the laser scanning time is 20 s. 1) The Ge core fiber with an OD of about 188 μm was annealed by laser, and the scanning velocities were set at 8 mm·s-1, 10 mm·s-1, 12 mm·s-1, and 14 mm·s-1. The Raman frequency distribution and average value at 10 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 3.435 dB·cm-1. 2) The Ge core fiber with an OD of about 251 μm was annealed by laser, the scanning velocities were set at 10 mm·s-1, 12 mm·s-1, 14 mm·s-1, 16 mm·s-1, and 20 mm·s-1. The Raman frequency distribution and average value at 14 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 2.147 dB·cm-1. 3) The Ge core fiber with an OD of about 270 μm was annealed by laser, and the scanning velocities were set at 12 mm·s-1, 14 mm·s-1, 16 mm·s-1, and 18 mm·s-1. The Raman frequency distribution and average value at 16 mm·s-1 laser scanning velocity closest to Ge bulk crystal and optical transmission loss values was 3.578 dB·cm-1. The experimental results show that under the same OD conditions, the laser annealing effect becomes better first and then worse with the increase in laser scanning velocity, and the scanning velocity for obtaining the optimal annealing effect increases with the increase of the OD of the fiber. Temperature variation at fixed points on the surface of the Ge core on the laser-irradiated side during the annealing process was simulated by COMSOL Multiphysics. The simulation results indicated that under the same OD conditions, the faster scanning velocity leads to the formation of denser temperature pulses, so that the Ge core is in the relatively high-temperature region most of the time, and the strength of the modification effect of this temperature field structure on the Ge core is more enhanced.The experimental results and the simulation of temperature variation indicate that the laser scanning velocity is an important factor affecting the annealing effect of Ge core fiber. The annealing intensity of the laser-annealed Ge core fiber can be enhanced as the laser scanning velocity is increased.
To solve the problems of inaccurate results in the theoretical calculation of Fiber Bragg Grating (FBG) sensors for strain testing at high temperatures, and the complexity of the preparation process of special fiber gratings, moreover, to enhance the operability of strain testing in practical engineering by using FBG, this paper proposes a bare FBG sensors testing method based on the clamp-adhesive type, and combines the temperature compensation method, to achieve the accurate measurement of 3 000 με within 250 ℃.Firstly, for the problem of temperature-strain cross-sensitivity, the temperature compensation method is proposed to remove the effect of temperature, and the formula is derived. Based on this formula, the temperature compensation based FBG sensor temperature decoupling method is by using another FBG sensor that only responds to temperature. Then, after the two FBG sensors are integrated with the substrate, a suitable paste method can be selected so that one sensor can only sense temperature, and another sensor can sense temperature and strain, and the real strain value can be obtained after the temperature value brought into the formula.Secondly, the strain transfer rates of two FBG integration methods (clamp-adhesive and surface-adhesive) are compared by finite element analysis, and the simulation results indicate that the strain curve of the clamp-adhesive method fits the substrate strain curve better, and the strain transfer rate is 99.47%. In contrast, the gap between the strain of the surface pasted method and the substrate strain is larger, and the strain transfer efficiency is 95.8%. Therefore, the strain transfer efficiency of the clamp-adhesive sensor is higher than that of the surface-adhesive FBG sensor. In addition, the surface-adhesive type needs to cover the grating of the Bragg grating sensor with glue, and the change of the material properties of the adhesive layer in the high temperature environment has a large impact on the reflection of the grating, so the penetration of the glue into the fiber grating area will make the grating fail, which has a large impact on the strain measurement.Thirdly, with reference to the strain transfer theory of clamp-adhesive FBG sensor, the influence factors of the scale ratio on the strain transfer rate are analyzed. It can be concluded through simulation that when the scale ratio is greater than 1, the strain transfer rate decreases at a faster rate. When the scale ratio is less than 1, the strain transfer rate only differs by 0.1%. Considering the influence of the coverage area of the adhesive layer on the adhesive firmness and the fluidity of the adhesive, the chosen gauge scale is 1.Finally, in the experiment, the scale ratio of the clamp-adhesive FBG sensor is 0.87 due to the glue fluidity and manual operation. Meanwhile, a fiber grating temperature sensor is connected as temperature compensation. Then, the temperature/strain calibration tests are carried out, resulting in the fiber temperature sensitivity coefficient of FBG1 (12.393), the strain sensitivity coefficient (1.596) and the fiber temperature sensitivity coefficient FBG2 (12.293). The high-temperature tensile test of 3 000 με within four temperature gradients from 100 °C to 250 °C (100 °C, 150 °C, 200 °C and 250 °C) is carried out in a tensile testing machine. The results show that the strain measured by the fiber grating sensors based on the temperature compensation decoupling is similar as data from the standard high-temperature strain gauges, in which maximum relative average error of 2.26%. The results of the research present the reference significance on the engineering application of the strain measurement and integration method of FBG sensors according to a high temperature environment.
In order to further improve the detection sensitivity of multi-component trace gases, an acoustic resonant cavity and an interferometric fiber optic acoustic wave sensor were used to enhance the excitation and detection of the Photoacoustic (PA) signal. A fiber optic PA sensing system was designed to achieve high-sensitivity detection of C2H2 and CH4 gases. To increase the amplitude of the PA response and reduce the cross-interference of other gases, two DFB lasers with center wavelengths of 1 532.83 nm and 1 650.96 nm were selected as the excitation light sources for C2H2 and CH4. Through the digital-to-analog converter, the working parameters such as modulation frequency, bias current and modulation depth of the two DFB lasers were controlled by FPGA. Two near-infrared lasers with different wavelengths were coupled through a wavelength division multiplexer and then incident into the PA cell, achieving dual-component gas excitation of C2H2 and CH4. To further improve the detection limit of gases, a multi-pass device composed of two coaxial concave mirrors combined with a resonant PA cell formed a multi-pass resonant PA cell. The excitation light was reflected multiple times in the multi-pass resonant PA cell, which increased the interaction length between the target gas and light, increasing the effective power of the excitation light. The Micro-electro-mechanical Systems (MEMS) cantilever in the fiber optic acoustic sensor was used as the acoustic sensitive element, and a fiber optic Fabry-Perot (F-P) interference structure was designed to convert the deflection displacement of the cantilever into the change of the F-P cavity length. A superluminescent diode with a central wavelength of 1 550 nm was used as the detection light source. The emitted broadband light entered the acoustic wave sensor after passing through the optical fiber circulator. The F-P interference spectrum containing PA information was detected by the miniature optical fiber spectrum module. The signal processing circuit performed high-speed acquisition and real-time processing of the F-P interference spectrum. By using high-resolution spectral demodulation technology, ultra-high sensitivity PA signal detection based on fiber optic F-P sensor was realized. In order to obtain the highest detection sensitivity, the relationship between the PA response and the modulation parameter was measured by adjusting the modulation current of two DFB lasers. The PA response was measured under different currents, and the system obtained the best detection performance when the modulation currents of the C2H2 laser and the CH4 laser were set to 8.5 mA and 4 mA, respectively. The frequency response of the system was tested to obtain the best PA signal amplitude. In the range of 500 Hz to 1 250 Hz, the modulation frequencies of the two DFB lasers were adjusted, and the resonance peak appeared at 1 660 Hz in the two frequency response curves. The linearity of the sensing system was evaluated. The gas mixtures of C2H2/N2 and CH4/N2 with different concentrations were flushed into the PA cell, and the PA response of the system to C2H2 and CH4 was analyzed. In the concentration range of 0 ppm to 100 ppm, there was a good linear relationship between the excitation PA signal amplitude and the concentration of the two mixed gases, and the linear responsivity of the system to C2H2/N2 and CH4/N2 were 7.39 pm/ppm and 5.67 pm/ppm, respectively. Pure N2 gas was pumped into the PA cell to test the noise level of the system and evaluate the stability of the sensor. Two sets of noise data were tested repeatedly, the deviation (1σ) were 0.364 pm and 0.365 pm, and the average value was 0.36 pm. Based on sensitivities of 7.39 pm/ppm and 5.67 pm/ppm for C2H2 and CH4 gases, detection limits of 48.7 ppb and 63.4 ppb were obtained, respectively. The detection sensitivity and stability of the system were evaluated by Allan-Werle deviation analysis. When white noise dominates, the Allan-Werle variance value decreased as the averaging time increased. With an average time of 400 s, the results of Allan-Werle analysis of variance showed that the detection limits of the system for C2H2 and CH4 gases reached 2 ppb and 3 ppb, respectively. The Normalized Noise Equivalent Absorption (NNEA) coefficient normalized the absorption line intensity and the effective power of the excitation light. The output powers of C2H2 and CH4 lasers were 14.7 mW and 21.9 mW respectively. Therefore, the calculated NNEA is 8×10-10 cm-1 WHz-1/2. The designed optical fiber PA sensing system realized the high sensitivity detection of C2H2 and CH4 gas.
With the development of smart robots, intelligent tactile sensing is increasingly applied in industrial production, which can greatly improve efficiency and accuracy. Compared with traditional electrical sensors, optical fiber Bragg Grating (FBG) sensors have significant advantages, such as flexibility, electromagnetic immunity, and small size. They also demonstrate high sensitivity and rapid response in perceiving strain and pressure. Current researches on FBG-based tactile sensing mainly focus on strain, temperature, sliding positioning and contact force deduced from the Bragg wavelength shift of FBG. However, there are relatively few researches on combining feature extraction, machine learning, and other cutting-edge technologies to achieve more sophisticated intelligent perception, such as material recognition.In this work, we presented a FBG based sliding-tactile sensing and classification training method for online material recognition by the differential properties of contact surface materials, such as roughness and stick-slip phenomenon. We developed a horizontal two-layer silicone rubber covered FBG sensing unit and its sliding-tactile perception system. When sliding on the certain material, a continuous strain exerts to FBG through the silicone rubber sensing unit and FBG's response changes.To classify efficiently, this paper extracted the mean maximum difference λB(max)-λBˉ, extreme difference λB(max)-λB(min), and standard deviation Δλ(std) of the FBG's wavelengths as the three-dimensional feature for mapping the material properties. And the classification training of the Support Vector Machine (SVM) algorithm and its classification model was developed. The results show that the classification accuracy is 96.6% for rough cloth, PLA and 800-grit sandpaper under the mixed dataset of 5 cm/s, 10 cm/s and 15 cm/s sliding speeds. Compared with the direct wavelength and traditional mean/median feature classification methods, this three-dimensional feature-based method exhibits superior classification capability and adaptability.In order to achieve further intelligent applications, this paper also designs an interactive computer control system, including wavelength acquisition, speed control and material recognition result display. It can control the sliding speed and online material recognition as well. Utilizing the prediction function trainedModel.predictFcn(t_test), the corresponding predicted results were presented after extracting three-dimensional features. In 36 tests, 5~15 cm/s random sliding speed (3 types of materials×3 samples×4 times slip) were carried out, and the correct predictions were 34 tests, which verifies that this method is effective and accurate.This work indicates that the FBG sensor has great potential in the field of material recognition by slip-tactile sensing. The research results can provide a novel online material recognition method for intelligent sensing robots.
In practical applications such as slope instability deformation, random cracking of concrete and collapse of wind turbines, position tracking always requires two-dimensional sensing. The fiber-optic displacement sensors have been widespread applied in civil engineering field due to their intrinsic advantages, including electromagnetic interference immunity, miniature size, electrically-passive operation, and multiplexing capability, however, they are not able to retrieve the displacement direction and amplitude simultaneously. In view of this reason, a vector displacement measurement sensing device based on Fiber Bragg Grating (FBG) with large range and simple structure is proposed to identify the displacement magnitude and direction of the monitored structure simultaneously. The sensing device is mainly constructed of four FBGs, a base, an upper free rotation rod, springs 1 and 2, self-made U-shaped structures 1 and 2. The pre-tensioned FBGs are respectively pasted on the inner and outer sides of center position of the U-shaped structure as the sensing unit. When the deformation occurs, the screw plays a connecting role, and the spring is not affected by the screw. The bottom spring of U-shaped structure 2 is connected with the monitored point, and the movement of the monitored point will cause axial tension of U-shaped structure 2. The force will be applied to the spring and the U-shaped structure. The internal and external sides of the U-shaped structure are subject to tension and compression, respectively. Additionally, the movement of U-shaped structure 2 causes the upper rod to rotate around the center point, which further makes the spring of U-shaped structure 1 elongate and produces tension on the U-shaped structure. FBGs are bonded to both the upper and lower surface of the stainless steel plate of the U-shaped structure for temperature compensation. Even if temperature change occurs in an FBG sensor unit, strain in the upper and lower two FBGs bonded to both surfaces of the stainless steel plate are equal, so thermal strain can be neglected. The sensing principle of determining the displacement direction and amplitude simultaneously is introduced, and its expression is also derived. Calibration experiments of six sets of auxiliary structures (i.e., core sensing elements) were conducted. The experimental results showed that the sensing element is characterized by a superb linearity, a measurement range of 0~140 mm, a sensitivity of 4.362 pm/mm, a hysteresis error of 3.25%, and a repeatability error of 6.62%, respectively. Additionally, an indoor accumulation slope model test was performed to verify the performance of the FBG displacement sensing device in monitoring the continuous sliding deformation process of soil. The displacement values calculated by FBG-based sensor is basically consistent with that measured by Particle Image Velocimetry (PIV) technology, with an average relative error of 5.63%. The maximum relative error of horizontal displacement is 10.83%, the minimum value is 0.11%, the maximum relative error of vertical displacement is 11.17%, the minimum value is 0.67%, which can meet the measurement requirement of the sensor in slope monitoring. The errors of some measuring points exceeding 10% may be due to the fact that the probe of FBG displacement sensing device was buried shallowly in the soil and not fully in accordance with the soil deformation. Meanwhile, the relative error of displacement azimuths calculated by two technologies is basically within 10%, with a maximum error of 10.31% and a minimum error of 0.09%, which is basically the same as the error analysis of above displacement. The error analysis of displacement azimuth calculated by two techniques also proves the reliability of the FBG displacement sensing device developed in this paper for monitoring vector deformation of soil slopes. This capability demonstrates its broad application prospects in the field of intelligent monitoring.
With the advancement of the construction of space, space and ground integrated information networks, satellite communication systems nowhave a higher requirements for information transmission rate, satellite node storage capacity, satellite coverage and security. Traditional microwave communication methods are limited by bandwidth, speed, geographical location, spectrum, etc., and will be difficult to meet the ultra-high speed and ultra-large capacity communication requirements of multimedia broadband services for satellite networks. At the same time, laser communications are gradually becoming an important technical means for satellite communications due to its advantages of high transmission rate, high security and reliability, strong confidentiality, small terminal equipment, light weight and low power consumption. To achieve all-round coverage of communication signals, laser networking based on dynamic satellites and the establishment of high-speed, low-latency, high-reliability and large-capacity satellite communication systems will become the future development trend of satellite communication. In the future, space will inevitably gather a large number of products of human space activities, including rockets, satellites, and rocket ejections. As humans develop space, the increase in these space debris will also bring a series of hazards. Existing space debris research mainly focuses on how to avoid collisions with satellites and spacecraft in orbit. In addition, these space debris move randomly in space, which will block point-to-point laser communications. Therefore, more effective research on the reliability of satellite laser communication systems is needed.In order to solve the problem of inter-satellite link interruption that may be caused by space debris in low-orbit satellite laser communications, this paper proposes a Direction-enhanced Link State (DE-LS) routing algorithm. Firstly, the network topology of satellite communication is built. The polar orbit constellation model is selected. According to the orbital plane and the number of satellites, an initial and constant address is set for each satellite in the polar orbit constellation. Based on the changes in the satellite node addresses of the starting and ending points in different transmission tasks, the Direction Influencing Factor (DIF) is introduced. Then, based on the celestial motion patterns of satellites and space debris in polar orbit constellations. A joint simulation model of space debris and satellites is constructed to obtain the relative positions of satellites and debris at a given moment and to perform inter-satellite visibility analysis.. Based on the inter-satellite visibility data, a Direction Enhancement Index (DEI) is proposed corresponding to the four directions of each node. The direction impact factor and direction enhancement index are combined with the inter-satellite link distance and transmission delay to comprehensively represent the link cost. The cost is used as a measure to select the shortest path, and the shortest path is selected between each pair of satellite nodes in turn, and the number of routing hops is used as the evaluation index. The simulation experiment is carried out in the Walker constellation. Space debris and satellites are jointly modeled and simulated first. Then, in this environment, two situations are selected: the theoretical minimum number of hops in the same orbit is 4 hops and the theoretical minimum number in different orbits is 7 hops. Taking satellite communications No. 21 and No. 25 and satellite No. 21 and No. 55 as examples for routing selection, routing hop count and transmission delay are used as evaluation indicators, and compared with the Dijkstra routing algorithm, which also solves the shortest path. The simulation results show that the DE-LS algorithm can maintain the theoretical minimum number of hops when the link is interrupted. At the same time, it saves 14% of the hops and reduces the transmission delay by 17% compared with the Dijkstra algorithm, which reflects the effectiveness of DE-LS algorithm in avoiding faulty links.
Existing optical fibers struggle to support broadband dense wavelength division multiplexing and coarse wavelength division multiplexing transmissions, necessitating the development of optimized fibers with moderate dispersion, low dispersion slope, enlarged effective area, and low attenuation. Currently, None Zero Dispersion Shifted Fiber (NZDSF) fibers have a small effective area incompatible with conventional fibers, emphasizing the need for precise design control of the refractive index profile. Therefore, creating a S+C+L band NZDSF with a large effective area and low slope is crucial for meeting the escalating demand for bulk data transmission.The dispersion slope, a crucial parameter in optics, is determined by the interplay between waveguide and material properties. The effective area, a metric that signifies the fiber's optical performance, relies heavily on the refractive index profile and the chosen input wavelength. In order to find the appropriate dispersion slope and effective area, we need to find a suitable refractive index profile. In our research, we have employed a refined profile structure modal, featuring a triangular+ring core configuration embellished by a central depression and fabricated by a two-step process to prepare the core and cladding by using the Outside Vapor Deposition (OVD).Through experiments, adjustments were made to the doping levels in the core, thereby modifying the relative refractive index Δn1 and radius R1 of the first core layer. This enabled the formation of a triangular cross-sectional structure. Simultaneously, the relative refractive index Δn3 and thickness R3-R2 of the third core layer were also adjusted, resulting in distinct refractive index waveguide configurations. This approach strucks a balance between achieving low attenuation, a large effective area, a reduced dispersion slope, and an appropriate zero dispersion wavelength. After optimizing the preform preparation and drawing process, the optical fiber cross-section obtained has a high matching with the designed cross-section. The triangular structure of the first core layer has a relatively straight slope, and Δn1 is between 0.52% and 0.57%. In the third core layer, a slightly curved convex structure is formed due to the diffusion of GeO2, and Δn3 is between 0.13% and 0.17%. In line with the experimental findings, it has been observed that when the first core layer radius R1, the third core layer R3, and the second core layer's relative refractive index Δn2 remain relatively constant, an increase in Δn1 and a subsequent decrease in R2/R1 lead to a gradual reduction in the zero dispersion wavelength λ0 and a corresponding decline in the effective area Aeff. Our experimental target is to achieve a zero dispersion wavelength λ0 below 1460 nm, even approximating 1 420 nm, while maintaining a significant effective area Aeff. To balance these parameters, it is necessary to slightly reduce Δn1 to the range of 0.52% to 0.53% and adjust R2/R1 to approximately 2.6 to 2.7. By these adjustments, we can achieve a suitable equilibrium between the effective area Aeff and the zero dispersion wavelength λ0.The experimental fiber design achieved a mode field diameter of 9.35 μm and an effective area Aeff of 68 μm2. Additionally, the zero dispersion coefficient exceeding 1.5 ps·nm-1·km-1 at 1 460 nm, well-suited for S-band wavelength division multiplexing applications while effectively suppressing four-wave mixing in the S-band. Furthermore, the fiber exhibited a low dispersion slope of only 0.059 ps·nm-2·km-1, providing relatively suitable dispersion characteristics in the C and L bands. The fiber also exhibited superior attenuation coefficients of 0.276 dB·km-1 at 1 383 nm, effectively mitigating the impact of water absorption peaks. The attenuation coefficients at 1 550 nm and 1 625 nm were 0.195 dB·km-1 and 0.205 dB·km-1, respectively, facilitating extended transmission distances. Through comparison, it was confirmed that this S+C+L band NZDSF with low dispersion slope and large effective area is well-suited for high-speed, high-capacity, and long-distance optical communication systems.
The channel fading generated by seawater absorption and scattering, oceanic turbulence, and pointing error will severely reduce the communication quality of an Underwater Wireless Optical Communication (UWOC) system. Many scholars directly transplanted the weak atmospheric turbulence model to depict the oceanic turbulence statistics, and this had been proven to be incorrect by a series of laboratory measurements and associated data-fitting tests. So, it is obviously of great significance to study and evaluate the effects of a composite fading channel on the key performance of the UWOC system, especially with a proper weak oceanic turbulence model.In this paper, the Generalized Gamma Distribution (GGD) verified by a series of experimental tests was selected to characterize the weak oceanic turbulence. Then, a new hybrid fading channel model was proposed to more reasonably simulate the communication environment in the ocean, which had integrated the GGD weak turbulence, the zero/nonzero boresight pointing error, the implicit path loss and multipath propagation effect characterized by Fading Free Impulse Response (FFIR). Next, the mathematical expressions of the Probability Density Function (PDF) considering GGD weak turbulence and zero/nonzero boresight pointing errors were derived using higher transcendental Meijer-G and Whittaker functions. Subsequently, based on this, the closed-form expressions of the average Bit Error Rate (BER) were derived for the serial-relayed UWOC systems with both zero and nonzero boresight pointing errors, respectively. Finally, the accuracy and rationality of the derived closed-form formulas for the average bit error rate of the relaying UWOC system derived above were verified by some Monte Carlo numerical simulations; meanwhile, the influences of different key parameters on the system BERs were also investigated.The results show that the introduction of serial relaying nodes can effectively improve the end-to-end BER performance of the UWOC systems in a long-distance communication environment. With the increase of the relaying nodes number, the system's BER decreases rapidly at the same transmission power, indicating that the serial-relayed scheme dramatically improves the performance. For instance, if the end-to-end distance is fixed as 45 m and the target BER is set to be 10-3, under the zero boresight pointing error condition, the required node transmission power will be reduced to 24 dBm, 12 dBm, and 6 dBm, respectively, when the relaying node number is assumed to be 1 to 3 individually. Similar to the working situation of zero boresight pointing error, when there is a non-zero boresight pointing error, the serial relaying node still effectively improves the system's BER performance. Unfortunately, the delayed spread expansion of the FFIR caused by the increase of the initial divergence angle of light source, that is, the so-called Inter-symbol Interference (ISI), will seriously degrade this performance improvement. For example, when the initial divergence angle is increased from 0.01° to 3°, the transmission power needs to be increased in general by about 10~15 dBm to obtain the same BER compared with the original transmission one. In addition, the jitter standard deviation and the initial boresight displacement associated with the pointing error will also significantly impact the BER performance of the relaying UWOC systems. For instance, if the jitter standard deviation is enhanced from 10 cm to 20 cm, the node transmission power needs to be increased by nearly 20~25 dBm to achieve the same BER value; meanwhile, when the initial boresight displacement increases from 5 cm to 10 cm, the transmission power needs to be added by about 5 dBm to reach the same BER target. Therefore, when one considers the BER evaluation of the serial-relayed UWOC systems, it is necessary to consider the impacts of the initial divergence angle, the jitter standard deviation, and the initial boresight displacement on performance thoroughly.The analytical results of this paper can provide calculation support for analyzing the BER performance of the relaying UWOC systems.
In order to improve the accuracy of shape sensing, this paper optimizes the sensing position based on the Non-dominated Sorting Genetic Algorithm-II (NSGA-II), and uses the Radial Basis Function-Particle Swarm Optimization (PSO-RBF) neural network algorithm to improve the accuracy of structural reconstruction. In this study, the goal was to reconstruct the shape of a 150 mm×150 mm×0.5 mm nitinol version. Firstly, the finite element model of the nitinol version was established by using ANSYS workbench software. After a series of operations such as meshing, adding constraints, adding materials, and modal analysis, the surface strain modal matrix and displacement modal matrix of the model were extracted. According to the modal analysis results and the principle of modal reconstruction, 8 sensing points can be selected to realize the shape reconstruction of the model. The strain mode matrix is used as the input matrix of the NSGA-II algorithm. According to the modal confidence criterion, the conditional number criterion and the modal mode shape similarity criterion, three objective functions were obtained. The NSGA-II multi-objective optimization algorithm, which introduces fast non-dominance sequences, business strategies and congestion operators, was used to select the best sensing location. It not only reduces the computational complexity of the algorithm, but also better retains the excellent individuals. Then, the wavelength of the center of the Fiber Bragg Grating (FBG) was demodulated by the SM125 interrogator, and the linear relationship between the wavelength change and curvature of the eight FBG centers was obtained by linear fitting. Since epoxy resin has a high strain transfer rate, the FBG was glued to the selected optimal sensing position. The nitinol plate was bent into different arcs to obtain FBG strain data. The displacement and shape of the nitinol plate at this time were recorded. The strain-mode mode shape, displacement mode mode and FBG strain data were input into the reconstruction algorithm. According to the modal reconstruction algorithm, the shape reconstruction was preliminarily realized, and the best sensing position point reconstruction results obtained by the K-means++ algorithm were compared. Finally, the PSO-RBF neural network algorithm was used to fit the nonlinear relationship between the reconstruction error and the reconstruction displacement. The PSO-RBF neural network algorithm has strong nonlinear fitting ability, which can avoid falling into local optimum. The ratio of the training, validation, and test sets is 6∶2∶2. In this way, the prediction of the reconstruction error can be realized, and the accuracy of the shape reconstruction can be improved. The NSGA-II algorithm was used to optimize the sensing position, and the FBG strain information was collected to reconstruct the structure shape, and the reconstruction effect was better than that of the K-means++ algorithm, and the root mean square error was reduced by 30% and the maximum error was reduced by 15% compared with the K-means++ algorithm. After fitting the nonlinear relationship between the reconstruction error and the reconstruction displacement by PSO-RBF, the root mean square error and the maximum error are reduced by 90% and 70% respectively compared with the non-error compensation, and the reconstruction shape is almost the same as the structural shape, which can achieve high-precision reconstruction of the structural shape. This paper successfully realizes the high-precision shape reconstruction of the nitinol version. By optimizing the optimal sensing position, the root mean square errors are 0.500 mm, 0.561 mm and 0.636 mm, and the maximum errors are 2.102 mm, 2.315 mm and 2.561 mm, respectively, when the bending curvature radius of the nitinol plate is 200 mm, 180 mm and 160 mm, respectively. When the bending curvature radius is 180 mm and 160 mm, the root mean square error is 0.038 mm and 0.046 mm, and the maximum error is 0.686 mm and 0.778 mm, respectively.
With the advantages of high sensitivity,simple structure,and low cost,the multi-longitudinal mode beat frequency technique has attracted considerable attention since it was first proposed in 2010. The fundamental mechanism behind this technique is that numerous multi-longitudinal modes within the fiber laser bandwidth are stimulated and beat with each other to generate a Beat Frequency Signal (BFS). The Free Spectrum Range (FSR) of the BFS is related to the propagation time in the fiber laser cavity,thus the applied parameters on fibers can be recovered by tracking the change of selected BFS. Compared with traditional optical fiber sensors interrogated in the optical domain,the BFS can be detected in the frequency domain. Benefitting from the large frequency gap between the optical signal and frequency signal and the mature electronic instrument,the interrogation resolution and speed can be enhanced greatly. The sensitivity of BFS is proportional to the selected frequency. In other words,in order to improve the sensitivity,high-frequency BFS should be chosen,which brings instability and low measurement range.The vernier effect is an effect method to improve the sensitivity in optical fiber interferometers. The vernier envelope can be generated by combining two interference spectrums with different FSRs. Hence,the measurements can be recovered by monitoring the vernier envelope with an enhanced sensitivity. Up to now,researchers have applied the vernier effect to measure parameters,including temperature,strain,refractive index,etc. It should be noted that the vernier effect magnifies the sensitivity but also the measurement error,resulting that the vernier effect is appropriate for the applications with a damage threshold. Essentially,the vernier effect is formed by two periodic signals with different periods,it also can be generated in the frequency domain.In this paper,the vernier effect is introduced to the multi-longitudinal mode beat frequency technique to enhance the sensing sensitivity. To verify the proposed method,optic-fiber axial strain measurement is experimentally carried out. Two linear-cavity fiber lasers with different lengths are formed by a pump laser (pump),a Fiber Bragg Grating (FBG),a section of Erbium-Doped Fiber (EDF),and two Faraday reflector mirrors. With the help of the pump and the EDF,FBG-based laser with the same wavelength of the FBG is emitted and picked by a Photodetector (PD) to achieve photoelectric conversion. Benefitting from the large bandwidth of the FBG-based laser,numerous longitudinal modes are emitted and beat with each other to generate BFS,which are recorded by an Electrical Analyzer (ESA).Firstly,the axial strain sensing characteristic of cavity 2 is investigated. Breaking FRM 1 and FRM 2,the BFS with FSRs of 6.97 MHz and 7.52 MHz with signal-to-noise ratios of 20 dB are generated. Keeping FRM 2 connected and FRM 1 disconnected,a section of single mode fiber with a length of ~55 cm in cavity 2 is fixed between two move stages. The strain is applied to the fiber from 0 ~1 090.8 με with a step of 90.9 με by pulling one move stage. The BFS with the frequency of 1.506 94 GHz generated in cavity 2 is chosen as the tracking signal. Results show that with the increased axial strain,the selected BFS shifts to a lower frequency range and the slope can be calculated as -1.228 kHz/με by linear fitting test data,and the R2 value is 0.998.Secondly,the characteristic of the vernier is investigated. By connecting FRM 1 and FRM 2 at the same time,an obvious vernier effect is generated. In order to get the upper envelop curve of the BFS,extremum points are extracted and polynomial fitting is performed. The bandwidth is enhanced and the FSR is 95.06 MHz,agreeing well with theoretical analysis. Thirdly,the axial strain sensing sensitivity is experimentally tested. The strain is performed as above and the envelope with frequency of 1.524 24 GHz is selected as the tracking signal. Results indicate that the envelope shifts linearly with the increased axial strain and the sensitivity is -17.23 kHz/με with an R-square of 0.993. Compared with the sensitivity in cavity 2,the amplification is 14.09,agreeing well with the calculated value. Meanwhile,Root Mean Square (RMS) method is employed to get the frequency of the envelope,and the results shown that the sensitivity is -18.47 kHz/με,which is slightly larger with the peak sampling method.Finally,the stability in two situations is tested. In this step,the axial strain applied on fiber is kept at 90.9 με and the frequency change is recorded every 1 minute in 10 minutes. Experimental results show that the maximum frequency fluctuations are ±3.13 kHz and ±101.17 kHz,corresponding to measurement errors of ±2.55 με and ±5.74 με,respectively. It proves that the vernier not only enhances the sensitivity but also enlarges the measurement error. This study provides an important reference to enhance the sensitivity of the multi-longitudinal mode beat frequency technique and has a potential application in other parameters sensing scenes.
Airflow velocity measurement is important in many fields such as meteorological monitoring and wind power generation. Currently,it is common to utilize ultrasonic,differential pressure,eddy current,or heat transfer methods for measuring. However,in some extreme environments,these measurements may be encounter interference,resulting in less accurate or non-functional results. Fiber optic anemometers have attracted much attention because of their advantages of electrically passive operation,resistance to electromagnetic interference,high sensitivity,integrated structure,and the ability to measure over long distances. In order to enhance the performance of fiber optic anemometers,this paper proposes a hot-wire anemometer based on fiber grating Fabry-Perot Interferometer(FPI) structure.Fiber grating FPI anemometers are prepared by sandwiching a section of cobalt-doped fiber with a length of 500 μm in the middle of two Fiber Gratings(FBGs). The cobalt-doped fiber in the interference cavity is heated with a 1 480 nm laser to form a hot wire. Due to the effect of thermal expansion and thermo-optic,the optical range difference of the FPI cavity length changes with the temperature of the cobalt-doped fiber,which brings about the wavelength redshift of the interference spectrum. As air flows through the FPI anemometer,the heat dissipation from the FPI is accelerated. The temperature of the FPI decreases,causing a blue shift in the interference spectrum,and the airflow velocity value can be calculated by measuring the change in the interference spectrum.We first performed temperature calibration experiments on the FPI anemometer probe by placing the anemometer probe in a temperature-controlled box. The output power of the broadband light source was set to 5 mW,and the output of the 1 480 nm laser was turned off to avoid the heat generated by the cobalt-doped fiber absorbing the laser. The temperature of the temperature control box was adjusted to gradually increase from -20 °C to 110°C in 10 °C steps,and the interference spectral drift of the anemometer in this temperature range was measured. Thus,the wavelength response of the FPI anemometer to temperature is obtained. The temperature sensitivity obtained by the linear fitting is 11.8 pm/°C,and the linear R2 is 0.997. To explore the impact of laser power on the initial temperature of the anemometer,we recorded the interference spectra of the FPI when the laser power was increased from 0 mW to 500 mW at a step of 10 mW. The experimental results show that the peak wavelength of the FPI interference spectrum shifts by 1.71 nm when the laser power is increased to 500 mW,which corresponds to an increase of 144.92 °C in the initial temperature of the anemometer. Finally,we carried out airflow velocity measurement experiment at different laser powers of 150,330 and 500 mW. The airflow velocity was changed from 0 to 9 m/s. The experimental results indicate that the sensitivity of the FPI anemometer increases with laser power but decreases with airflow velocity. It reached -1 053.86 pm·m-1·s at a airflow velocity of 0.1 m/s when laser power was 500 mW. It was decreased to -746.47 and -15.15 pm·m-1·s for airflow velocity of 0.5 and 5 m/s,respectively. The sensitivity of an anemometer exhibits an exponential decay trend due to Newton's law of cooling,which states that the rate of temperature decrease of a hot wire is proportional to the temperature difference between the wire and the surrounding environment. In low airflow velocity conditions,the temperature difference between the cobalt-doped optical fiber and the airflow is significant,resulting in a high rate of temperature decrease. However,as the airflow velocity increases,the temperature difference between the cobalt-doped optical fiber and the airflow sharply decreases,leading to a reduction in the sensitivity of the anemometer.The fiber grating FPI anemometer proposed in this paper has the advantages of high sensitivity,simple and compact structure,easy fabrication and low cost when compared with the previously reported optical fiber hot-wire anemometers. It is expected to be widely used for airflow velocity measurement in the related fields.
As an important carrier of artificial intelligence,robots exhibit many abilities that humans do not possess. Robotic hands are an important component of robots and an important subfield of the intelligent robotics industry. Robotic hands surpass human hands in accuracy and stability. It can perform precise welding and assembly processes in the production workshop,replace humans in completing dangerous tasks such as explosive dismantling,and can also be used for deep-sea and aerospace exploration. However,traditional robotic hands are purely functional and lack the ability to perceive the surrounding environment,making it difficult to respond promptly to changes in the working environment. The application of sensor technology is a prerequisite for achieving robot control,interaction,and intelligence. The integration of sensor technology is crucial for robot control,human-machine interaction,and intelligence. Sending sensor exported data to the feedback control system improves the operational efficiency of the robotic hand. Electrical sensors have been widely used in other fields. Although the technology is mature and capable of sensing physical quantities such as force and temperature,but due to their poor flexibility,susceptibility to electromagnetic interference,difficulty in wiring,and poor flexibility,they can not fully meet the needs of intelligent sensing for robotic hands. FBG sensors use wavelength encoding,so the energy loss and electromagnetic field changes of the optical fiber can not interfere with the detection results. They also have the advantages of high accuracy,fast response speed,and low loss. FBG sensors exhibit excellent flexibility and scalability,with minimal impact on the maneuverability and agility of robotic hands. Therefore,FBG sensors are an ideal choice for sensor applications in robotic hands.In this paper,a flexible sliding sensor is proposed for detecting the sliding direction and speed of robotic hands. The sensor uses silicone rubber as the packaging material,which can improve sensitivity of the sensor and protect the FBG. Firstly,FEA was conducted using ANSYS software to obtain theoretical values of strain changes over time at different positions during the sliding process. Then,based on the simulation results of FEA,a sliding sensor was designed and the sensor fabrication was completed. Finally,a sliding detection platform was built to experimentally verify the simulation results. The experimental setup includes an FBG demodulator,sensors,PC,motor,weights,and pulleys. The motor is used to horizontally and uniformly pull weight through the surface of the sensor.The experimental results show that the sensitivity of each FBG sensor encapsulated in silicone rubber has sensitivity to pressure of 9.26,7.83 and 7.76 nm/MPa,and the sensitivity to shear force is 1.74,3.17 and 3.29 nm/MPa,respectively. When the slider slides over the sensor surface in different directions,the wavelength shift curve of each FBG shows different trends of variation. By using SVM model and 1D-CNN network to identify the sliding characteristics,the accuracy of sliding direction judgment reaches 91.0% and 93.5%,respectively. The sliding time of the slider is obtained from the wavelength shift curve,and the sliding speed can be obtained by dividing the sliding distance by the sliding time. The maximum relative error of sliding speed calculation is 9.29%. The research results of this paper have certain practical value for the research and promotion of robotic biomimetic skin.
Pressure detection under high temperature environments has an urgent need in aerospace, equipment development, petrochemical and other fields. Especially, with the development of space industry and the research of new generation engines, in-situ pressure measurement of key parts such as pipes, chambers and combustion chambers plays an important role in engine test, combustion instability analysis, mode switching of engine systems and so on. Consequently, it is highly necessary to research on the design, manufacture and testing methods of the sensors. Compared with the traditional electrical sensors, fiber-optic sensor has the advantages of small size, long transmission distance and immunity to electromagnetic interference. Through the analysis of the high temperature pressure sensors based on fiber-optics Fabry-Perot principle, the operating temperature is mainly limited by the sensor composition material, sensitive unit processing method, signal transmission mode and demodulation method. The high-temperature resistance performance of sensitive materials directly determines the upper temperature limit of the sensor. Thermal stress mismatch is another significant factor causing the failure of sensors in harsh environments. In addition, the transmission and extraction methods of the characteristic signal are the difficult points of high-temperature pressure sensors. To address the above limitations, there are many researches on quartz, silicon, silicon carbide, sapphire and other high-temperature resistant materials as sensitive unit in high temperature region to improve the operating temperature. Meanwhile, MEMS and femtosecond laser processing methods have great advantages in sensor consistency and thermal stress matching manufacturing. Among the single-crystal oxide materials, single crystal magnesium oxide material shows high application value in the fields of high-temperature fiber-optic sensing because of its ultra-high melting point, excellent optical and mechanical properties. In this paper, combining the advantages of magnesium oxide material and MEMS processing technology, a magnesium oxide wafer optical fiber Fabry-Perot pressure sensor based on MEMS for harsh monitoring is proposed. The square Fabry-Perot cavity is designed to improve the pressure sensitivity. The thermal stress matching machining of the sensitive element is realized by developing the wet etching and direct bonding technology of magnesium oxide. Meanwhile, the optical fiber is integrated for pressure detection in high-temperature environments. The structural parameters of the sensor are optimized by the mechanical, thermal and modal simulation results. The pressure experiments at room temperature and high temperatures are carried out to verify the large pressure range and temperature response performance of the sensor. The experimental test results demonstrate that the FP cavity length of the sensor during the increasing and decreasing pressure over three cycles varies linearly with the pressure in the range of 15 MPa at room temperature with a nonlinear error of 0.75%FS. Additionally, error bars diagram is drawn to analyze the uncertainty and reliability of the sensor test results, which indicates that the sensor test results are relatively reliable. The high temperature pressure test results show that the sensor can be effectively measured in the range of 22 ℃ to 800 ℃. For each temperature, the FP cavity length decreases with the pressure, and the cavity length approximately linearly changes with the pressure over the entire test range, even up to 800 ℃. Therefore, the experiment results demonstrate that the sensor can stably operate at an environment of 22~800 ℃ and 0~0.7 MPa. This work is of fundamental importance in realization of pressure detecting in ultra-high environments. And the application of magnesium oxide in the field of optical sensing provides a new way to solve in-situ pressure measurement at high temperatures, narrow spaces or other related positions of aeroengines.
Microwave Photonic Filters (MPFs) can find applications in the fields of optical communications, radar, optical fiber sensing. Due to the implementing of high-frequency optical signals and different optical processing devices, MPFs can achieve low loss, large bandwidth and compatible communication. To construct a finite impulse response MPF, researchers have proposed a variety of tapping and delay structures. An MPF can be classified as coherent one or incoherent one, according to the coherent characteristic of the adopted optical carriers. A simple approach of Phase-Modulation to Intensity-Modulation(PM-IM) conversion is usually used to construct a coherent MPF, but the frequency response of the MPF is limited by the adopted optical filter. The principle of the incoherent MPFs is based on the impulse response filters, which are constructed through multiple taps and time delays. One of the important indicators to evaluate the performance of the MPFs is the reconfigurability. In terms of tap implementation, using physical paths with equally spaced time delay is a common method, but this requires complicated switching structures to tune the length of each optical path. Another common approach is to use multi-wavelength sliced broadband optical sources or incoherent laser arrays, combined with dispersion fibers to achieve time delay. Usually, the positive coefficient MPFs require incoherent light sources to reduce interference between the adjacent taps, but it will increase the system noise. Therefore, how to achieve a high signal-to-noise ratio incoherent filter with tunable and reconfigurable performance is one of the critical issues that needs to be solved urgently in the field of microwave photonic signal processing.In this paper, an MPF with a simple structure and a high reconfigurability is demonstrated based on an optical Recirculating Frequency Shifting Loop (RFSL). By adding a Single-Sideband(SSB)modulator to an optical fiber loop, a new optical frequency component will be generated each time when the optical carrier passes through the SSB modulator which is modulated by a microwave signal. As a result, the proposed RFSL not only has an equally spaced comb-like spectrum in the frequency domain but also an equally spaced time delay between the adjacent optical carriers in the time domain. It can be found that a traditional MPF based on an RFSL mainly utilized its multi-carrier characteristics in the frequency domain and the time delay between the taps is achieved by using dispersive optical fibers. However, the time domain characteristics of the RFSL have not yet been effectively developed. In fact, the equally spaced time delay characteristic of the RFSL can be also used to introduce a time delay between different taps. Since the frequency difference between the multiple optical carriers generated by the RFSL is larger than the bandwidth of the photodetector, the intensity interference caused by the high coherence of the optical carriers can be avoided, and a stable spectral response can be achieved. By using a Programmable Optical Filter(POF) to shape the spectrum generated by the RFSL, different tap numbers can be controlled, and the weight of each tap can be precisely adjusted, thereby obtaining an effective tuning of the Free Spectral Range(FSR), tap number and weight.In the experiment, the output optical signal from the RFSL is delivered to the POF which is a Waveshaper, for spectral shaping. The POF provides a series of discrete frequency channels for the filtering of each carrier and modulation sideband of RFSL. After the k-th optical carrier is attenuated to a certain extent, any adjustable tap weight can be obtained. In terms of tunable reconfigurability, we propose to use the POF to change the delay between taps, and allocate new channels to each comb group (both carrier and sidebands) in the original RFSL. We experimentally demonstrated the switching of the tap number between 2, 5, and 10 with an FSR of 3.85 MHz, and the switching of the FSR between 3.85 MHz, 1.925 MHz, and 770 kHz with a tap number of 2. Through spectral shaping, we have achieved an MPF with a reconfiguration of the tap number and the FSR. Moreover, when the tap number is 5 or 10, the MPF with reconfigurable tap number has a Main to Secondary Sidelobe Ratio(MSSR)of 11 dB. On the other hand, the optical amplifier in the RFSL is an Erbium-Doped Fiber Amplifier(EDFA)which can bring a high small signal gain, the long erbium-doped fiber length will also increases the cavity length of the RFSL, which decreases the FSR in the experiment. The cavity length can be further reduced through using a Semiconductor Optical Amplifier(SOA), and other devices such as Polarization Controllers(PCs), Tunable Optical Filters(TOFs), pigtails and jumpers such as Dual-Parallel Mach-Zehnder Modulator(DPMZM)can all be optimized to reduce the optical length. Different from non-frequency-shifting loops, RFSL does not require much incoherence of the optical carrier source. Even under a small loop length, a stable multi-tap signal can still be obtained. Through further optimization, the FSR of the MPF we proposed can be increased to 100 MHz.
Multicore Fiber (MCF)/imaging fiber bundle is a key device of flexible optical endoscope. In imaging applications, MCFs are widely used in the non-coherent imaging which transmits the intensity distribution only. The bending sensitive distortion of phase plane and inter-core crosstalk bring challenges in the coherent imaging application. In this paper, a Helical-Core MCF (HC-MCF) is designed for the application of coherent imaging.Due to the intricate nature of HC-MCF, neither semi-analytical models nor empirical methods can fully resolve the modal properties. Consequently, a full-vector finite-element method is employed for numerical simulation of HC-MCF. By utilizing the optical transformation technique, HC-MCF in the natural space is equivalently represented in the helical coordinate maintaining the translation invariance. The original isotropic permeability and dielectric constant (both scalars) of the optical fiber material are adjusted to equivalent dielectric constant and equivalent permeability values. This simplification can effectively reduce the computational complexity of the field distribution and equivalent effective refractive index of fundamental mode in HC-MCF. By using this method, the inter-core group delay differences of HC-MCF is simulated for optimization of fiber design.Then, an optimized design of HC-MCF is proposed. In order to minimize the distortion of phase front after transmission in HC-MCF, each core of HC-MCF should have a similar optical path. An appropriate core spacing should be selected to balance between the spatial sampling density and crosstalk among fiber cores. The helix period is preferred smaller than the critical bend radius in the application. Our final design of HC-MCF are arranged in a densely stacked hexagonal configuration, comprising 6 layers with a total of 91 cores. The radii of the fiber cores are 4 μm, 3.3 μm, 3.1 μm, 3 μm, 2.9 μm and 2.8 μm from the inside to the outside, with a core pitch of 20 μm and helical pitch of 10 cm. The inter-core group delay difference per unit length of straight HC-MCF is calculated and the maximum is found to be 6 fs/m. When the bending radius is significantly larger than the wavelength, the change in mode equivalent refractive index caused by bending is disregarded, and the variation in group delay difference resulting from core bending is determined solely by changes in core geometry length. The trajectory equation of the bent core is derived to obtain its geometry length, enabling determination of the corresponding change in group delay. Calculations are performed for two different bending radii (0.5 m and 0.05 m) to assess variations in group delay difference per unit length for helical fibers under these conditions. Remarkably, similar changes are observed under both bending scenarios, indicating that alterations in bend state do not induce significant phase modifications within transmitted light fields. By carefully designing the structure of HC-MCF, excellent bend performance can be achieved.At last, the bend induced inter-core crosstalk of HC-MCF is calculated. The crosstalks between cores of adjacent layers for HC-MCF with a total length of 100 m, torsion rate of 20 π/m, and core spacing of 20 μm are calculated and compared. Due to slight variations in mode phase mismatch between different layers during bending, there exists a maximum crosstalk value when phase matching conditions are satisfied. When the bending radius is smaller than that at which phase matching occurs, inter-layer core crosstalk becomes insensitive to bending radius and maintains a consistently low level. In this design, featuring slightly varied core sizes and a helical structure within each layer, and an exceptionally low level of crosstalk (-550 dB/100 m) is achieved. This remarkably reduced crosstalk could significantly enhance the imaging quality.Due to the helical core design of the designed helical MCF, the complex random disturbance of the optical field phase transmitted by the multi-core fiber under the bending condition is reduced, and the group delay difference caused by the bending between the cores is effectively suppressed. HC-MCF can help to reduce the complexity of the coherent image restoration, which finds useful applications in fiber optic micro-imaging, ultrafast laser imaging and other fields.
As one of the most important energy storage technologies today, the safety and reliability of lithium batteries have always been of great concern. The thermal stability and pressure stability of lithium batteries are important parameters that affect their safe and reliable operation. The internal electrochemical reactions will cause changes in temperature and stress during operation. Abuse of lithium batteries can cause rapid increases in temperature and stress of electrodes, leading to degradation of battery performance and even safety accidents such as combustion or explosion. Therefore, real-time monitoring of internal temperature and stress changes in lithium batteries is crucial for the long-term safe and stable operation of lithium batteries. However, current monitoring methods used for temperature and stress inside lithium batteries just focus on single parameters or external measurements, which have problems such as poor resolution and limited accuracy, making it difficult to monitor the changes inside the battery. In order to improve the healthy level of lithium-ion batteries monitoring, this paper proposes to use fiber Bragg grating sensing technology to monitor the changes. The gratings are implanted to collect real-time temperature and stress changes of the battery anode, realizing an optical channel for in-situ monitoring of the lithium-ion battery anode. Furthermore, combined with the battery test system, the connection between electrical and optical sensing signals is established. In the system, the temperature sensitivity of the FBG sensors is 9.3 pm/℃, and the stress sensitivity is 1.2 pm/με. The FBG sensors are mounted in different ways to achieve accurate measurement of dual parameters. Both ends of FBG1 are fixed for strain measurement. FBG2 fixes single end to monitor temperature and functions as temperature compensation for FBG1 at the same time. FBG3 is outside the battery, which is used to measure the external temperature of working environment. The experimental results show that the FBG sensors can remain good sensing performance at 400 ℃. The implantation has no effect on pouch cell performance, nor does it affect the sensing performance of FBG sensors. During the working cycles of lithium batteries, the detachment and embedding of lithium ions can cause temperature changes, resulting in a sensor wavelength shift of 100 pm, which means temperature increases by 11.1 ℃. The coefficient of thermal expansion of anode is 25.5 με/℃. After temperature compensation, the stress change of anode can be observed, indicating that the change in stress is influenced by current. In other words, the hop of current can cause the anode to contract and the resulting stress will result in a wavelength drift of 21.96 pm at most, which is approximately 18.3 με. According to our research, different charge and discharge rates have different effects. The faster the rate, the greater the variations in temperature and stress. The temperature change is 2.8 times and the stress is 4.4 times higher at 10 mA than at 2.5 mA. If the rate of charge or discharge further increases to 50 mA, the operating temperature will exceed 45 ℃. After 300 cycles at 45 ℃, the volume expansion rate of battery is about 10%, and the battery is likely to malfunction. The implantable grating monitoring system in this paper can not only measure the temperature and stress changes caused by electrochemical reaction with high precision, but also has fast demodulation speed, which is conducive to real-time and accurate monitoring of the thermal runaway and deformation bulge failure of lithium batteries. The research results are conducive to quantifying and evaluating the possible thermal runaway and volume expansion problems in the electrochemical process, which is expected to provide an effective experimental reference for the safe use of lithium batteries.
As basic physical quantities, time and frequency are the research basis of many applied fields, such as verification of basic physics, clock-based geodetic survey, positioning and navigation. Optical fiber is an ideal medium for high stability time-frequency transmission due to its advantages of low loss, large bandwidth and strong anti-electromagnetic interference. At present, mainstream frequency transmission schemes can be roughly divided into optical frequency, optical frequency comb and radio frequency transmission. However, most of the existing time-frequency transmission systems can only guarantee a constant phase difference during the operation of the system, and ignore the phase difference change that may occur after relocking when the system is restarted or the system link length is changed. This situation cannot meet the needs of some coherent applications, such as distributed phased array radar, radio telescope arrays. These applications not only require stable phase difference during operation, but also require that the value of phase difference does not change after multiple restarts, which can provide reference signals of the same frequency and phase between different sites to achieve more effective coherent processing.In this paper, an absolute phase transfer scheme for optical fiber radio frequency based on adjustable optical delay line is proposed. The scheme makes full use of the round-trip transmission delay of the time signal to determine the integer period of the phase of frequency signal. Combined with the high precision phase measurement of microwave frequency, a microwave frequency signal with a fixed phase difference between the remote signal and the local signal can be obtained when the system is shut down and restarted for many times. In this scheme, 1PPS signal is not directly used as a time reference signal. First, RC differential circuit and avalanche triple laser are used to convert 1PPS signal into narrow pulse signal, and then surface acoustic wave filter is used to filter the narrow pulse signal to obtain the narrow band time reference signal. By means of frequency division multiplexing, time signal and microwave frequency signal are coupled in the same wavelength channel for transmission so as to avoid the delay difference introduced in different wavelength transmission. The system uses the way of wavelength division multiplexing to transmit. The return time frequency signal is obtained by a loop method and compared with the local reference signal to obtain the round-trip link delay and link cumulative phase. The cumulative phase of the link is used as an error signal to control the optical delay line to stabilize the link, and then the absolute phase transmission is realized by using the calculated link delay to calibrate the integer period of frequency signal. Compared with other time-frequency co-transmission modes, the system of this scheme is simpler. It does not need to adopt multiple modulation modes, nor does it directly modulate 1PPS signal. Narrow-band filter can be used to separate the time signal from the microwave frequency signal so that they do not affect each other.The experiment was carried out on a 60 km optical fiber link. After transmission, the system obtained a frequency stability of better than 4×10-14@1 s, 5×10-17@10 000 s. After the link is stabilized, the measured stability of time transfer (TDEV) is 10 ps@1 s and 0.3 ps@10 000 s. The results show that this scheme has good link compensation effect. When the system is restarted several times, the maximum inconsistency of the average phase difference is 0.008 rad, corresponding to 0.15% of the whole cycle, which can ensure the realization of good phase consistency.
While effectively enhancing the transmission capacity of current commercial single-mode optical fiber communication systems, Polarization Division Multiplexing (PDM) technology also faces serious challenges from Polarization Mode Dispersion (PMD) and Rotation of State of Polarization (RSOP). PMD causes the broadening of optical pulses, resulting in crosstalk and distortion of the signals, which may significantly increase the Bit Error Rate (BER) of the system. Also, RSOP might cause rapid change in the polarization state of optical signals up to several hundred krad/s, preventing the two polarization signals from being correctly separated at the receiver. In practical optical fiber links, PMD and RSOP usually coexist and impose significant impairments on the system performance. The main work of this paper is to build an impairment model of PMD and RSOP and to conduct a joint compensation of the polarization-relevant impairments.Traditionally, the Stokes formalism is employed to analyze the PMD and RSOP in commercial single-mode fibers that actually support two orthogonal polarization modes. The corresponding Stokes vector is a 3-dimensional real vector with clear physical meaning, and can be intuitively represented in the geometrical Poincare sphere. However, the Stokes formalism requires 3 auxiliary 2×2 Pauli matrices, and when extended to the treatment of Modal Dispersion (MD) and Mode Coupling (MC) in an N-mode optical fiber (i.e., a modal space of dimension N), a number of N2-1 auxiliary N×N Gell-Mann matrices are required, which can drastically complicate the analysis of MD and MC effects as N increases. Recently, borrowing methodology from quantum mechanics, we proposed and developed the Density Matrix (DM) formalism for the MD and MC in a modal space of arbitrary dimension N≥2. Without requiring any auxiliary matrices, the DM formalism is simple and straightforward in formulation and application, and is thus particularly suitable for the study of modal properties and signal compensations in the optical communication system.In this paper, we apply the DM formalism in the PDM system to construct a joint polarization-impairment compensation scheme. We establish a polarization impairment model for coexisting PMD and RSOP based on traceless Hermitian impairment matrices. By tracking the corresponding independent parameters in the matrices, we achieve the joint impairment compensation of PMD and RSOP. To verify the proposed scheme, we build a 28 GBaud PDM quadrature phase-shift keying coherent optical communication transmission simulation system, and implement a joint impairment-compensation for Differential Group Delay (DGD) over a wide range of 30~170 ps and fast RSOP over 300 krad/s~2 Mrad/s. In simulations, with about 100 iterations, the tracking and compensation have already converged for the impairment of 100 ps Differential Group Delay (DGD), and with only 0.5 dB and 0.7 dB optical Signal-to-noise Ratio (OSNR) cost, joint compensation of RSOP can be achieved in typical (600 krad/s) and extreme conditions (2 Mrad/s), respectively, both in the presence of 100 ps DGD. When 170 ps DGD and 2 Mrad/s fast RSOP coexist, the BER is 3.22×10-3, which still meets the criterion for error-free transmission of signals. The fast convergence, the small OSNR cost, and the stable BER performance verify the validity and efficacy of our polarization impairment model and the corresponding joint compensation scheme in optical communication systems.Furthermore, taking advantage of the DM formalism, our joint compensation scheme can be readily generalized to modal spaces of arbitrary dimension N for the impairment analysis and signal compensation of MD and MC, simply by the extending the relevant matrices from 2×2 to N×N. Therefore, our work potentially provide a simple and effective theoretical approach for the impairment analysis and compensation of optical-signals in more general mode-division multiplexing communication systems.
Fiber optic Extrinsic Fabry-Perot Interferometers (EFPI) are frequently utilized in many acoustic sensing scenarios due to their simple structure, ease of fabrication, high sensitivity, high phase consistency, and strong resistance to electromagnetic interference. However, the cavity length of the EFPI sensor is susceptible to environmental variables such as temperature and air pressure, and the drifting of the orthogonal working point caused by the change of cavity length will lead to signal fading and distortion. Nevertheless, several demodulation methods are less practical or even ineffective when dealing with small signals: Dual-wavelength method for Mach-Zehnder interferometer is inconvenient to apply to the EFPI demodulation; the Ellipse-Fitting Algorithm's (EFA) Lissajous figure will degenerate into a straight line for small signals, and there are also the disadvantages of poor real-time performance and slow demodulation speed; the second-order Differentiate-and-Cross-Multiply (DCM) operation has wide applicability, but the Direct Current (DC) term must be accurately removed, for small signals, the removal of DC term is difficult; the Bessel method has the same difficulties as DCM, and it can only demodulate single-frequency signals; The method of using tunable laser feedback to control the orthogonal working point has the drawback of high cost, and lasers with wavelength scanning function have high requirements for hardware reliability; Phase Generated Carrier (PGC) technology requires a complex carrier modulation system with a limited frequency response range, and the system is complex and large when PGC uses piezoelectric transducer to generate phase carriers. In contrast, intensity demodulation has the advantages of a linear transfer function, simple signal processing, high sensitivity and is suitable for the detection of high-speed and small signal. JIANG Yi et al. proposed a Symmetrical Demodulation Method (SDM) suitable for unstable cavity length and unknown cavity length, which constructs two equal interference phase differences between the three output signals by selecting specific three wavelengths, and then recovers the phase of the signal through mathematical operations. This method has the advantages of large dynamic range and simple operation, and it is more suitable for the detection of large signals. However, in the case of small signals, the SDM algorithm may lead to increased noise and error in the demodulation result.If the wavelength of the light source is 1 550 nm, the interference phase of EFPI changes 0~2π, corresponding to the cavity length variation range of 775 nm. When the cavity length change caused by vibration is less than 30.11 nm, that is, the radian value in the interference phase is less than 0.22 rad, the approximate error of sinx and x is less than 1%. Under this condition, we can directly remove the DC term of interference signal reflected by EFPI to avoid the difficulty of distinguishing DC terms in small signals, Bsinφ4πns(t)λ can be regarded as the approximation of an interference signal. Three specific wavelengths are selected to construct two equal interference phase differences between the three output signals, and on the basis of these three signals, the two intermediate formulas can be compensated for each other to avoid the cancellation phenomenon, it ensure that at least one of the intermediate formulas has a better waveform, so as to obtain an intensity demodulation result with better Signal-To-Noise Ratio (SNR) and this approach is summarized as a Modified Symmetrical Demodulation Method (MSDM).Both theoretical analysis and numerical simulation demonstrate that MSDM performs better than SDM for small signals. Theoretical analysis indicates MSDM has a smaller and smoother error bound and relative condition number than SDM. In the numerical simulation, MSDM has less error than SDM in cavity length, frequency, and signal amplitude. After adding Gaussian white noise to the simulated signal, more high-frequency noise appears in SDM, while MSDM achieves superior demodulation results for the signal waveform. In the experiment, the SNR of the three signals ranges from 60~65 dB, and the experiment results align well with the simulation. Due to the complicated noise in practical applications and the high sensitivity of SDM, high-frequency noise appears in the demodulation results of SDM, resulting in a decrease in SNR. MSDM effectively improves the SNR of the demodulation results, the power spectra of MSDM's results are smoother than those of SDM, and the SNR near the main frequency increases from 74 dB to 86 dB. Values of cosδ measured in experiments consistently maintain between 0.55~0.7, in agreement with the theoretical predictions, thus confirming the reliability of MSDM. Additionally, the outputs of MSDM perform good linearity with the sound pressure of the speaker, and the linearity coefficient reaches 0.995 11. When the signal frequencies are 100 Hz, 500 Hz, 1 kHz, and 10 kHz, respectively, the MSDM demodulation results still have good SNR.An improved three-wavelength demodulation method for small signals of EFPI sensors is proposed by enhancing the existing three-wavelength phase demodulation algorithm. Our research group uses the approximate relationship of the sinusoidal function under small signal conditions to calculate the phase difference by calculating the intensity of the three signals, thereby solving the fading problem of EFPI interference signals. Through numerical analysis, simulations and experiments, it is proved that the proposed method has higher algorithm stability and smaller error in demodulation for small signals and better recovery on waveforms. Besides, the algorithm also theoretically has certain demodulation capabilities for non-periodic signals, which can potentially expand its application in the future.
With the rapid increase of electrical power demand, the voltage level of power grid also improved. The traditional current transformer is difficult to meet the needs of power grid because of its easy saturation, poor insulation, narrow frequency band, etc. The optical fiber current sensor can solve these problems. Therefore, it has wide application prospects in power monitoring and smart grid construction. However, the nonlinear response of magneto-optical crystal is the main factor that limits the application of the magneto-optic fiber current sensor. Nowadays, research on the nonlinear error of the Fiber Optical Current Sensors (FOCS) mainly focuses on the nonlinear dependence of the Verdet coefficient of the Magneto-optical (MO) materials on the external temperature. The change of Verdet coefficient will lead to the crosstalk of magnetic field and temperature, which is one of the main factors limiting the practical application of the FOCS. Nevertheless, when the temperature is fixed, the output signals of the MO materials is also not changed linearly with change of the external magnetic field. To improve the accuracy of the sensors, we studied the source of the nonlinear response of the MO crystal theoretically and experimentally. In this paper, we demarcated the output curve of garnet crystal from no magnetic field to saturated magnetic field. Through simulation and experiment, we proved that the nonlinear response of MO fiber optical current sensor is caused by the diffraction characteristics of magnetic domain of the MO material. The model of magnetic domain effect proposed by us conforms to the output response of the MO sensors. Through the dual-channel demodulation method, we can calculate the intensity of the output light through the 45° polarization beam splitter according to Malus law. And we obtain the Faraday rotation angle according to the demodulation algorithm of difference division sum. The theoretical results show that the influence of nonlinear error caused by magnetic domain effect can be ignored with the method of the two-channel demodulation. And in this way, the influencet of light source fluctuation can be eliminated. Simulation and experimental results show that the nonlinear error at a fixed temperature is mainly due to the voltage conversion coefficient mismatch of the two photodetectors and the assembly error of angle between polarizer and polarization beam splitter. Although the nonlinearity seems slight in macro performance, it is the key to affect the performance of the sensors in the case of high accuracy requirements. When the temperature changes, the nonlinear error of the MO materials is mainly due to the temperature dependence of Verdet coefficient on temperature. In order to solve the above problems and improve the accuracy of the MO fiber optical current sensor, we propose a nonlinear compensation model based on quadratic fitting. By comparing the four error models between the compensation model and the traditional demodulation model, the experimental results show that this method has good fitting accuracy and is suitable for nonlinear compensation of FOCS based on the MO material. In the actual experiment, we adjust the two magnifications to make the two conversion signals equal without applying the magnetic field. It means that the initial assembly angle error is compensated by adjusting the magnification. In this case, the result of the sensor output is the best. After that, the nonlinear law of the sensor is calibrated by the experimental data, and the external magnetic field is obtained by solving the quadratic function, which can significantly improve the accuracy of the sensor. This method has a simple structure and high fitting precision, and can meet the real-time acquisition of the MO fiber optical current sensor.
With the further development of intelligent robots, bionic touch sensing has become an important means for the robotic dexterous hand to interact with the external environment, making it a hot topic. However, in the existing research on tactile perception of robotic hands, most of the sensing units have complex structures and are difficult to integrate in the hand.For touch sensing of the dexterous robot hand, this work employs optical Fiber Bragg Grating (FBG) as the information transmission and sensing carrier and develops a wearable touch fingerstall. The thumb fingerstall contains two segments: fingertip and finger pulp. The fingertip is composed of a curved surface and a plane surface. The curved surface can fit the thumb shape better, and the plane surface can provide a more stable touch. Specially, we use a cylindrical pin as the knuckle to connect the fingertip and finger pulp. The touch sensing of three contact positions of the fingerstall have been studied:position 1 is the vertical fingertip, position 2 is the 15° fingertip, and position 3 is the 30° finger pulp. Firstly, the fingerstall model was simulated using the finite element method to verify its structural feasibility. Then, the fingerstall was loaded using an electronic tension machine, ranging from 0 N to 10 N with a 1 N increment, and each load was held for 25 s. For each test position, the experiment was repeated three times, and the responses of FBG were recorded. Analysis of the touch sensing experimental data shows that the sensing fingerstall has good sensitivity and linearity within the 0~10 N touch pressure range. The average sensitivities at position 1, position 2, and position 3 are 24.119 6 pm/N, 10.338 3 pm/N, and -1.580 7 pm/N, respectively, with linearity above 99%. Overall, the simulation results are basically consistent with the experimental results, although some errors may arise due to 3D printing and FBG packaging. Therefore, different loading positions and forces can be distinguished by different center wavelength offset and offset direction of FBG.In order to verify the sensing performance of the fingerstall further, a hardness discrimination experiment was carried out primarily focused on perceiving objects of small hardness. And the standard ratio of wavelength to touch depth (ks) was proposed as a quantitative measure of hardness perception. Position 1 was selected as the experimental position for hardness perception, and four silicone blocks with different hardness (named as 0 HA, 10 HA, 20 HA and 40 HA) were selected as the experimental objects. Considering the time and safety factors, 5 mm was selected as the touching depth, and 60mm/min as the touching speed. The average responses of FBG to 0 HA, 10 HA, 20 HA and 40 HA are 58.57 pm, 242.3 pm, 580.95 pm, 1107.6 pm, respectively. The corresponding ks values are 11.717 pm/mm, 48.46 pm/mm, 116.19 pm/mm and 221.52 pm/mm, respectively. According to the data results, when the fingerstall touches the 40° silicone block, the ks is the largest, and the ks is the smallest at the 0° silicone block. When ks is larger, the greater the hardness of the object is. The sensing fingerstall has a good repetition, with repeatability errors of 3.82%, 0.97%, 0.51%, and 0.29%.In conclusion, a simple and wearable fingerstall has been designed. The tactile sensitivity of three different position has been studied, and a hardness discrimination experiment has been conducted. These research results can serve as a basis for bionic touch sensing of dexterous robotic hands.
Using different cores of a Multi-Core Fiber (MCF) as independent parallel spatial paths to transmit different information simultaneously offers cost, space and energy savings. The inter-core Differential Group Delay (DGD) is a key parameter of the multi-core fibers, and its accurate measurement is of great importance. In the multi-core fibers, errors in the preparation process or bending of the fiber caused by external forces can result in inter-core differential group delay in various cores. When light is transmitted in a multi-core fiber, the outgoing light from different cores will interfere with coherent superposition in space due to different group delays. According to this character, this paper proposes a spatial interferometric imaging-based method for measuring the inter-core differential group delay of multi-core fibers, and building an experimental measurement device. At the input of the multi-core fiber, the laser is shifted and injected by adjusting the three-dimensional adjustment table to excite different cores, and the output light of the excited cores will interfere in space. The computer controls the CCD to scan the wavelength of the input light in one dimension and records the spatial interferogram at different wavelengths in real time. The interferometric information of all pixels in the interferogram is used to construct a three-dimensional spatial interferometric spectrum, and the inter-core differential group delay of the multi-core fiber can be obtained by spectral analysis of the interferometric spectrum. The inter-core differential group delay of the weakly linked trench assisted seven core fiber is measured in this paper. We measured the inter-core differential group delay between the central and six outer cores respectively. When two cores are excited at the same time, a peak will appear at the delay difference between the two cores through the Fourier transform of the interference information. Subtracting the-inter core differential group delay between the central core and two adjacent outer cores yields the inter-core differential group delay between the two outer cores, which is only 10-4 dissimilar from the actually measured inter-core differential group delay of two adjacent outer cores. When three spikes are excited simultaneously, two adjacent spikes will interfere with each other. By Fourier transforming the interference information obtained by CCD containing the differential group delay, a plot of differential group delay versus interference intensity containing three spikes can be constructed. The values of the horizontal coordinates of the three spikes in the plot are the differential group delay between the three cores, and they correspond to the measurement results when only any two of the three cores are excited, respectively. Simultaneously, the bending dependence of the inter-core DGD of the multi-core iber is measured and researched. The inter-core differential group delay was measured for three bending radii of 5 mm, 75 mm, and 110 mm. The results reveal that the inter-core DGD decreases as the bending radius increases, which is consistent with the theory. This method's measurement instrument is simple, and the measuring precision can reach 10-4. It can measure the delay difference not only between two cores, but also between any nearby cores at the same time. In addition, the outgoing light from the end face of the multi-core fiber to be measured is focused through the objective lens and connected to the CCD and adjust the three-dimensional adjustment table, the excitation state of the cores in the multi-core fiber and the degree of coupling of optical energy can be observed on the CCD, which makes the selection and excitation of the cores to be measured easier. The spatial interference imaging approach suggested in this study has the characteristics of simple, fast, high accuracy. It can be widely used to measure inter-core DGD of the multi-core fiber in SDM systems, MWP signal processing and other application circumstances.
Magnetic fluids are widely used in the field of optical fiber magnetic field sensing as magnetic sensitive materials due to the adjustable refractive index. Optical fiber magnetic field sensor based on magnetic fluid has the advantages of high sensitivity and light weight, but the magnetic field measurement range of this kind of sensor is affected by the saturation magnetization of magnetic fluid, which limits the high magnetic field sensing range. Some studies have shown that magnetic hydrogels have the characteristics of high sensitivity to magnetic field, high saturation magnetization, high magnetic responsiveness and super magnetic similarity with magnetic fluids. In this paper, it is proposed to replace the base liquid of magnetic nanoparticles of magnetic fluid with hydrogel material to prepare magnetic hydrogel, hoping to seek a higher level of saturation magnetization, and make prospects for further improving the magnetic field measurement range of optical fiber sensors. A polyvinyl alcohol/ferric oxide (PVA/Fe3O4) magnetic hydrogel was prepared by the composite method, and used a fiber end face reflection-based measurement method to test the refractive index variation of the magnetic hydrogel under different magnetic fields, which proved that it had magnetic-induced refractive index variation characteristics and could be applied in the fiber magnetic field sensing structure based on refractive index variation. Based on this research, a cone-shaped fiber sensing structure based on magnetic hydrogel was designed. Single-mode optical fiber is pulled into a cone shape by melting and pulling method, and magnetic hydrogel is wrapped in the cone area and waist area of the cone-shaped optical fiber. The input light in the fiber will generate evanescent wave when it is in the conical transition region, which will excite the higher-order cladding mode in the fiber. After the fundamental mode and cladding mode are transmitted in the waist region of the tapered fiber, mode coupling and mode interference will occur when entering another tapered region. When the intensity of the magnetic field applied in the external environment changes, the refractive index of the magnetic hydrogel will change. In this experiment, we change the refractive index of the magnetic hydrogel by applying magnetic fields of different intensities to the sensing element of the tapered fiber wrapped by the magnetic hydrogel. The spectral change of the output light of the tapered fiber is detected by using the evanescent wave in the tapered region of the tapered fiber, which is sensitive to the refractive index of the external environment, to study the magnetic field sensing characteristics of this structure. The experimental results show that the sensitivity of wavelength shift is 86.42 pm/mT within the range of 6.4~22.6 mT, when the magnetic particle concentration is 2.1% at a constant temperature of 22 ℃. And also the sensitivity of wavelength shift is 51.42 pm/mT within the range of 5.5~30 mT, when the magnetic particle concentration is 2.9%. From the point of view of optical fiber magnetic sensing measurement, magnetic hydrogel has high application value and deserves further research. This paper also proposes that the detection sensitivity can be improved by selecting more appropriate dispersants to improve the dispersion uniformity of magnetic nanoparticles in hydrogels, and by improving the structure of optical fiber sensing elements. During the preparation of magnetic hydrogel, we can further improve the magnetic field sensing range and expand the application of magnetic hydrogel in the field of optical fiber magnetic field sensing by optimizing the freezing and thawing times and the concentration of magnetic nanoparticles.
Real-time health monitoring of the bridge is of great significance. Compared with traditional piezoelectric sensors, Fiber Bragg grating has the advantages of high sensitivity, strong wavelength division multiplexing ability, and strong anti-interference ability. It has been widely used in real-time monitoring of various large structures in recent years. Aiming at the selection of bridge acceleration sensor, this paper proposes a double fiber grating acceleration sensor based on arc cycloid hinge, and analyzes the resonant frequency and sensitivity of the sensor by establishing a mechanical model. The analysis results show that under the condition of different hinge thicknesses, the height of the proof mass and the length of the minor axis of the semi-ellipse have a great influence on the resonant frequency and sensitivity of the sensor. As h and e2 increase, the mass of the mass block increases, the sensitivity of the sensor increases, and the resonance frequency decreases. As the thickness t of the hinge becomes larger, the resonant frequency becomes larger, and the sensitivity of the sensor becomes smaller. When t is in the range of 1 mm to 3 mm, as c becomes larger, both the sensor sensitivity and the resonant frequency become smaller. According to the bridge in-situ calibration requirements, this paper uses MATLAB to optimize the parameters of the sensor structure. Then use ANSYS to conduct static stress analysis, modal analysis and harmonic response analysis. The modal analysis shows that the first 4 modal frequencies of the sensor model are 471.06 Hz, 2 878.1 Hz, 3 226.7 Hz, 9 208.4 Hz and 13 763 Hz. The static stress analysis shows that the strain produced by the sensor under the acceleration of gravity is 1.4 μm. The harmonic response analysis shows that the resonance frequency of the acceleration sensor model is 474 Hz. After the software simulation, the actual sensor is made and calibrated. When performing resonance frequency calibration, the signal generator sets the signal voltage to 1 V, starts the vibration test from 10 Hz, ends at 650 Hz, and records the wavelength change. The sensor has the largest wavelength variation near the vibration frequency of 460 Hz, and the wavelength variation is relatively stable at 10~250 Hz, that is, the resonance frequency is 460 Hz. When performing sensor sensitivity calibration, set the constant frequency of 30 Hz and 60 Hz on the vibration table as the test frequency of the simulated bridge site. During the 30 Hz test, the voltage value increases from 0.2 V to 0.9 V with a step size of 0.1 V. During the 60 Hz test, the voltage value increases from 0.2 V to 0.7 V with a step size of 0.1 V. When the frequency is 30 Hz, the sensitivity of the dual FBG of the sensor is 43.14 pm/g, and the fitting coefficient is 0.995 7; the sensitivity of the single FBG1 of the sensor is 21.74 pm/g, and the fitting coefficient is 0.997 7. When the frequency is 60 Hz, the sensitivity of dual FBG is 43.21 pm/g, and the fitting coefficient is 0.992 8; the sensitivity of single FBG1 sensor is 21.81 pm/g, and the fitting coefficient is 0.998 9. Set the vibration frequency of the vibration table to 50 Hz, and the input voltage of the signal generator to 0.3 V. The test direction of the acceleration sensor is installed perpendicular to the vibration direction of the hinge, and when the sensor performs vibration sensing perpendicular to the vibration direction of the hinge, the sensitivity of the sensor is 2.456 pm/g, which is much smaller than the sensitivity of the vibration direction of the hinge. The lateral interference degree of the sensor is about 5.7%, which proves that the acceleration sensor has a good lateral anti-interference ability. The calibration experiment results show that the arc cycloid hinge structure FBG acceleration sensor designed and manufactured in this paper has a smaller volume and a higher integration level compared with other FBG acceleration sensors under the premise that the parameters meet the bridge acceleration monitoring. Using a combined elliptical and rectangular mass structure, dual fiber gratings can be engraved on an optical fiber to facilitate wavelength data collection. The experiment proves that the acceleration sensor designed in this paper can be used for acceleration sensing on the bridge.
Laser chaotic communication system is widely used in the field of secure communication due to its unique advantages such as strong randomness, quasi-noise, and high bandwidth of chaotic signals. At present, laser chaotic synchronization communication generally depends on the laser internal nonlinear effect or photoelectric oscillator. Still, it is difficult to achieve high-quality synchronization communication because of the difficulty of hardware parameters matching between the transmitter and the receiver. Aiming at this shortcoming, some scholars have proposed to use the powerful nonlinear fitting ability of a neural network to model the receiver of a chaotic optical communication system, to realize high-quality synchronization communication. This paper proposes to use the long short-term memory neural network for the mathematical modeling of the chaotic optical transmitter. It successfully solves the problems of complex hardware systems and low synchronization coefficients in traditional chaotic optical communication and provides a reference for point-to-multipoint chaotic communication.This paper presents the design of a laser chaotic synchronization communication system based on long short-term memory neural network, and uses a cross-prediction algorithm to optimize the network model. In the off-line training stage, a large number of chaotic encrypted signals generated by the transmitter are used as input variables of the neural network, and the real chaotic carrier sequence is further selected by the cross-prediction algorithm as output variables of the neural network. To enable the long short-term memory neural network to accurately predict the output variables according to the input variables, each training iteration of the network will update its node state until the ideal loss value is reached. In the test stage, the node state of the neural network has been determined. When the input variable is received, the system will automatically map the predicted carrier sequence, and the received encrypted signal can be directly subtracted from the predicted carrier sequence to decrypt useful information. The scheme achieves a high synchronization coefficient and achieves high-quality chaotic synchronization communication.The simulation result consists of three parts. The first is the quality of decrypted information at the receiver end. After modeling and training of laser chaotic system by long short-term memory neural network, the system has good prediction effect and high-quality chaotic synchronization. The synchronization coefficient between the real target carrier and the predicted carrier is more than 99.9%, and the root means the square error is as low as 10-3. The noisy information is demodulated directly from the encrypted information minus the chaotic carrier predicted by the neural network, and the bit error rate is as low as 10-10. It is far lower than the hard decision threshold of the forward error correlation standard, which is 3.8×10-3. To verify the universality of the system, the simulation of optical feedback and photoelectric feedback synchronization communication system has the same level of communication quality. Secondly, the influence of the number of network nodes, the information coupling coefficient, and the signal-to-noise ratio on the chaotic synchronization communication performance is studied in the optical feedback chaotic synchronization communication system. The results show that when the coupling coefficient is 0.08, the signal-to-noise ratio is 30 dB unchanged, and the number of nodes is between 200 and 800, the system has good bit error rate performance, and the maximum is only 10-10. When the number of nodes is 300, the synchronization coefficient reaches the peak value of 0.999 93. When the number of nodes reaches 1 000~1 200, the neural network appears overfitting state, and the information appears with certain distortion. This paper further studies the effect of nodes in the range of 40~240 on system performance. In the case of a few nodes, the synchronization coefficients of the system are all above 0.999 8, the bit error rate is far lower than the hard decision threshold of forward error correlations standard, and the bit error rate is lower than 10-8 magnitude. When the number of network nodes is 240, the maximum synchronization coefficient is 0.999 96. For the coupling coefficient, the number of nodes is kept at 200 and the signal-to-noise ratio is unchanged at 30 dB. When the coupling coefficient is large and reaches 0.04~0.12, the bit error rate of the system can reach a relatively low level stably, all of which are lower than 10-6, and the system communication quality is good. At the same time, when the coupling coefficient reaches 0.11, the maximum synchronization coefficient of the system is 0.999 95. For the signal-to-noise ratio, keep the network nodes 200, the coupling coefficient 0.08 unchanged, the signal-to-noise ratio between 5~40 dB, and the system synchronization coefficient can reach above 0.999 8. When the signal-to-noise ratio reaches 15 dB, the bit error rate reaches the order of 10-6, far lower than the hard decision threshold of the forward error correlations standard. When the signal-to-noise ratio is 25 dB, the synchronization coefficient reaches the peak value, which is 0.999 966. Finally, to verify the actual availability of the system, the grayscale image of 256×256 is successfully transmitted in the optical feedback system. In addition, the system security is analyzed from three aspects: brute force search, plaintext attack, and ciphertext attack. The results show that the system can resist many attacks and has high security.The proposed laser chaotic synchronization communication based on long short-term memory neural network and the network structure optimization by cross-prediction algorithm achieves high-quality chaotic synchronization communication in both optical feedback and photoelectric oscillator system. This scheme successfully solves the problems of complex hardware systems and low synchronization coefficients in traditional chaotic optical communication. Then, the influence of long short-term memory neural network nodes, coupling coefficient, and signal-to-noise ratio on the communication performance of the system is studied. When there are more nodes or the coupling coefficient is low, the decryption information will appear with a certain distortion. Finally, the feasibility of this scheme is further verified by image transmission. As a whole, the synchronization coefficient of the system can be as high as 0.999 966, and the bit error rate is as low as 10-10, which realizes high-quality chaotic synchronization communication. The advantages of this scheme are as follows. First, long short-term memory neural network, with its long-term dependence on learning time series and strong robustness, enables this scheme to achieve high-quality synchronous communication in both optical feedback and photoelectric feedback systems and has certain universality. Second, in the chaotic synchronous communication system based on long short-term memory neural network in this paper, the system performance obtained by the state of a few nodes and multiple nodes is outstanding. Choosing the state of a few nodes can greatly reduce the time loss and save the time cost of training neural networks. Third, the long short-term memory neural network scheme proposed in this paper successfully solves the problem of hardware parameter matching between the two receivers in traditional chaotic optical communication and has the advantages of convenience and security, which provides a thought for the subsequent research of chaotic optical communication.
With the development of petroleum and natural gas resources, pipeline transportation has become one of the main transportation modes. Pipeline leakage caused by various external factors during transportation affects the safe running of the pipeline. The development of optical fiber sensing technology has brought a new solution for pipeline leakage monitoring. Most of the existing research are carried out by numerical simulation work, lacking experimental demonstration, and only studied the temperature field distribution in the soil area after pipeline leakage, without taking the impact of the difference in soil internal physical properties on the temperature change into account. In addition, the accuracy of the monitoring of small leaks still can't satisfy the monitoring requirements. And the conventional fiber arrangement is only suitable for monitoring the temperature along the line, when the surface of the object to be measured is large and the monitoring requires a higher spatial resolution, the one-dimensional laying method is difficult to achieve the required resolution, it needs to be optimized.Based on the advantages of high spatial resolution of optical frequency domain reflectometry technology, in this paper, we mainly focus on oil and gas pipeline monitoring wiring problem and build a sandbox model with a small proportion. On the basis of one-dimensional tiled line monitoring, a new way of laying optical fiber for sinusoidal surface monitoring is proposed. Using optical frequency domain reflectometry technology with high spatial resolution as a supporting tool, the superiority of sinusoidal layout is verified. The results show that the measurement effect of buried optical fiber with a sinusoidal layout is better than that of a conventional one-dimensional tiled layout because of its wider measurement range. We analyzed the effect of sinusoidal period and longitudinal length on temperature monitoring, these two factors have a greater effect on the monitored maximum temperature, which means the larger the sine period and vertical length, the lower the highest temperature detected. Therefore, if there is no precision requirement, the optical fiber can be arranged long and sparse. Otherwise, it needs to be even tighter. Through the combination of experiments and numerical simulations, it is obtained that the maximum thermal influence radius of the tiny heat source is 7 cm, and this result can further guide to optimize the arrangement of optical fiber. In addition, the relationship between soil thermal conductivity, soil bulk density, and water content is obtained by the variable method. The relationship between soil bulk density and soil thermal conductivity is basically linear, the relationship between soil thermal conductivity and water content is not a specific linear increasing or decreasing trend, it varies in a parabolic-like form with the influence of external factors. Water content also affects the vertical distribution of the soil temperature field. When the water content is below the threshold, as it increases the rate of soil heat transfer in the vertical direction becomes faster, the temperature around the heat source increases faster, the maximum temperature monitored becomes larger, the high temperature influence range becomes larger, and the degree of heat diffusion increases significantly. We have studied the effect of heat source temperature on soil heat transfer, and found that as the heat source temperature increases, the heat diffusion range becomes larger, the high temperature influence range becomes larger, and the distance of temperature transfer in the vertical direction becomes farther. The influence law of these factors on soil heat transfer provides a reference for pipeline leakage monitoring, which can further guide the fiber arrangement. Meanwhile, it also verifies the feasibility of optical frequency domain reflectometry distributed optical fiber technology can accurately measure soil temperature field and also provides guidance for other distributed temperature measurement technologies in the layout of buried optical fibers.
The Fiber Bragg Grating (FBG) vibration sensor based on the cantilever beam structure has strong lateral anti-interference ability, high stability, and simple structure. It is particularly suitable for one-dimensional acceleration measurement. And its working range and resolution are determined by its resonant frequency and sensitivity, however, because the resonant frequency and sensitivity of cantilever beam type sensors are mutually restricted, it is difficult for the current generation of cantilever beam type sensors to simultaneously meet the requirements of wide measurement bandwidth and high sensitivity. To satisfy this demand, a novel FBG accelerometer based on F-beam is developed. The FBG can be sensitized by the neutral layer far away from the cantilever beam, and suspended and fastened at both ends to successfully prevent the chirp effect of FBG.Firstly, the amplitude-frequency characteristics of the damped mass-spring system with one degree of freedom are studied. The flatness of the amplitude-frequency response curve varies with the change of the damping ratio. Without damping, the sensor's working bandwidth is narrow. By adding damping materials such as silicone oil, the sensor can have a larger working bandwidth. Generally, the damping ratio is 0.7. After that, the resonant frequency and sensitivity formulas of the sensor are derived, and its mathematical model is established according to these formulas. With the sensitivity formula as the objective function, and its size parameters and the resonant frequency formula as the constraint conditions, the sequential quadratic programming program is established by MATLAB to optimize the solution, and the sensor's size parameters that satisfy the operating band range and have high sensitivity are obtained. Imported 3D model of this sensor created by SOLIDWORKS into ANSYS software, where the material properties are adjusted, the mesh division is finished, and fixed constraints are given to the sensor base to produce the sensor's first-order and third-order modal vibration patterns. The modal analysis results verify the correctness of the theoretical analysis and show that the sensor has a good transverse anti-interference ability. And then, set the simulation conditions such as sweep frequency range to analyze the harmonic response of the sensor. By modifying the damping ratio, the simulated amplitude-frequency response curves under the conditions of two damping ratios are obtained to simulate the amplitude-frequency response of the sensor without silicone oil and filled with silicone oil. The simulated amplitude-frequency response curve at the maximum amplitude position is basically consistent with the theoretical curve. Finally, based on the theoretical and simulation results, two sensors were fabricated, one of which was directly encapsulated as sensor 1 and the other was encapsulated with silicone oil as sensor 2, and the amplitude and frequency response tests, sensitivity tests and transverse immunity tests were conducted on sensor 1 and sensor 2. In order to determine the sensor's amplitude-frequency response, the trigger signal's amplitude is fixed swept between 10 Hz and 240 Hz. Next, the sensor's minimum detection frequency is tested by continuously varying the excitation frequency between 0 Hz and 2 Hz, and the detection performance of the sensor under various excitation conditions is tested by setting various excitation conditions. In the sensitivity test experiment, fixed the excitation frequency and adjusted the acceleration to measure the sensor's sensitivity. In the lateral immunity test experiment, fixed the excitation frequency and acceleration, change the measurement direction to test the sensor's lateral immunity.The experimental indicates that the experimental results of sensor 1 and sensor 2 are basically consistent with the theoretical and the simulated amplitude-frequency curve. The resonant frequency of sensor 1 is about 168 Hz, the measurement bandwidth is 1.5~50 Hz, the sensitivity coefficient is 159.84 pm/g, the transverse immunity is 9.88%, and the error between the theoretical and actual values of resonant frequency and sensitivity is 0.93% and 3.29% respectively. The error may be caused by the low processing accuracy of the sensor in the production process and the immature fiber pre-stretching process. The measurement bandwidth of sensor 2 filled with silicone oil is 1.5~100 Hz, the sensitivity coefficient is 133.57 pm/g, and the lateral interference immunity is 8.1%. This two FBG sensors can well reflect the external sinusoidal excitation in their corresponding operating bands and have good detection performance. By filling silicone oil, the working frequency band can be effectively expanded, the sensitivity can be stabilized, the transverse interference immunity can be improved, and the measurement error can be reduced.
Visible light communication is a new communication method that uses the visible light band as a communication carrier and takes into account lighting and data transmission. It has the advantages of no electromagnetic interference, rich spectrum resources and so on. Combining MIMO with VLC system can improve the communication capacity and rate of the system. However, MIMO-VLC systems need channel estimation to obtain channel state information to ensure the reliability of system communication. Although the commonly used LS channel estimation algorithm has a low complexity, it requires a lot of pilot overhead, which leads to a reduction of spectrum efficiency. Compressed sensing is applied to channel estimation to reduce pilot overhead and improve channel estimation performance because it can sample signals at a rate lower than Nyquist sampling rate and has a higher reconfiguration progress. The commonly used compressed sensing reconstruction algorithm, OMP algorithm, needs to predict the sparsity of the channel, and the true sparsity of the channel is usually unpredictable, so it has limitations in practical application. The SAMP algorithm can adaptively reconstruct the channel characteristics when the channel sparsity is unknown, which solves the condition of predicting the channel sparsity, but also increases the number of iterations of the algorithm and reduces the efficiency. Aiming at the problem of slow running speed of the SAMP algorithm, this paper proposes a Prediction-sparsity Adaptive Matching Pursuit(SAMP) algorithm based on Discrete Fourier Transform(DFT). Firstly, the sparsity of the channel impulse response is preestimated by the sparsity prediction method of DFT. Taking the estimated sparsity as the initial step of the algorithm can quickly approach the real sparsity and improve the efficiency of the algorithm. Secondly, the SAMP algorithm is used to reconstruct the channel impulse response to improve the accuracy of channel estimation and ensure the reliability of system communication. According to the performance analysis of a MIMO-VLC system with 2 inputs and 2 outputs, the mean square deviation performance of the algorithm in the paper is significantly better than that of the LS algorithm. When the forward error correction code rate threshold (3.8×10-3) is satisfied and the pilot number is 16, the algorithm in this paper improves by 2 dB compared with the LS algorithm, and by 4.5 dB when the pilot number is 32. At the same time, the bit error rate performance of the algorithm in this paper is equivalent to that of the SAMP algorithm as a whole, which shows that the sparsity prediction method based on DFT will not reduce the reliability of system communication while improving the efficiency of the system. The system bit error rate increases with the increase of modulation order M, and the performance gain of the proposed algorithm is more obvious than that of the LS algorithm with the increase of modulation order. When the error rate reaches the FEC threshold and the modulation order is M=8, the performance of the algorithm is improved by 3.5 dB compared to the LS algorithm, and by 10 dB when the modulation order is M=64. This result shows that, when the modulation order is higher, the reduction of bit error rate is more obvious, which is conducive to the improvement of system communication efficiency. For the efficiency of CS algorithm, the DFT based sparsity prediction method significantly improves the running speed of the DFT-SAMP algorithm proposed in the paper. Compared with the SAMP algorithm, the efficiency of the algorithm in the paper increases by about 68% at 16 pilots and 69% at 32 pilots.
Seawater temperature and salinity are the most critical and fundamental physical parameters necessary for all oceanographic disciplines, which have important theoretical value and practical significance for studying ocean climate change, monitoring marine ecological environment, exploiting and utilizing marine resources, and ensuring military security, etc. The development of high-performance sensors for seawater parameter measurement has become one of the research hotspots. In recent years, optical fiber sensing methods have provided a new solution for high precision measurement of physical parameters with the advantages of anti-electromagnetic interference, corrosion resistance, small size, and real-time distributed measurement. At present, the widely studied optical fiber temperature and salt sensors mainly include optical fiber interference type sensors and fiber optic grating type sensors. Researchers at home and abroad have realized the design of optical fiber interference type temperature and salt sensors by micromachining the optical fiber, such as taper pulling, reverse taper pulling, side polishing, dislocation welding and core diameter mismatch welding, and achieved some research results. However, there are generally problems such as great fabrication difficulty and poor structural stability, which are difficult to meet the application requirements of marine engineering. In contrast, optical fiber grating type temperature and salinity sensor are mainly designed based on Fiber Bragg Grating (FBG), and a single FBG can form a sensor, which is simpler to manufacture, more stable in structure and more adaptable to the environment. However, the current fiber grating temperature and salinity sensors mostly adopt FBG with high reflectivity, which can only perform discrete point measurements and cannot realize distributed sensing. Besides, previous reports mostly used spectrometer demodulation, which cannot observe the real-time response of temperature and salinity. Therefore, a quasi-distributed temperature and salt sensor based on Drawing Tower Grating (DTG) is proposed in this paper. The sensor uses DTG coated with Polyimide(PI) as the salinity sensing element. The PI coating expands or contracts linearly in volume when in contact with solutions of varying salinity. The expansion or contraction response caused by the change in salinity is converted into an axial strain loaded on the PI-coated DTG, and the salinity can be measured by monitoring the drift of its central wavelength. During the experiment, the central wavelength of the fiber grating is demodulated in real time by the fiber grating array demodulator, and its data is collected and recorded by the computer in real time. In the temperature compensation coefficient measurement experiment, the optical fiber sensor was placed in the circulating temperature field from 25 ℃ to 30 ℃ and then back to 25 ℃. Compared with the electronic temperature sensor used for calibration, it can be found that both PI coated DTG and uncoated DTG can accurately measure the ambient temperature, and have good consistency and repeatability. Their temperature sensitivities are 10.24 pm/℃ and 10.02 pm/℃ respectively. In the experiment of simultaneous temperature and salt measurement, the fiber optic sensor was placed into a high concentration NaCl solution of 5 mol/L to make the PI coating lose water and shrink sufficiently. Then deionized water or low concentration NaCl solution was added to gradually dilute it to 4 mol/L, 3 mol/L, 2 mol/L, 1 mol/L, and 0.6 mol/L to observe its salinity response. In order to simulate the actual working environment of the sensor, the whole system was in a room temperature environment without temperature control operation, and the sensor was still able to measure the temperature of the solution accurately, and the average salinity sensitivity obtained after compensation was -5.58 pm/(mol/L). The experimental results show that the sensor can simultaneously measure the temperature and salinity of seawater in real-time and quasi-distributed, and also has the advantages of wide measurement range, high measurement accuracy and easy fabrication, which has certain prospects for application in marine engineering.
Due to the complex and ever-changing battlefield environment, fixed communication equipment is often damaged in combat zones, resulting in a massive breakdown of communication and thus affecting combat effectiveness. Using the advantages of the UAV's strong survivability and wide coverage, it is possible to carry communication equipment as an UAV aerial base station to provide communication services to ground users. The single UAV base station has disadvantages such as weak resistance to destruction and limited coverage and power, so the choice is made for multiple UAV base stations to work together to provide communication services to a large area and multiple users. In complex military environments such as strong electromagnetic interference and “electromagnetic silence”??, traditional wireless communication methods have poor confidentiality and are susceptible to interference and eavesdropping by the enemy, while wired communication requires cables to be laid in advance and is not easily used in complex, dynamic and changing military environments. Wireless ultraviolet communication, based on its all-round and terrain adaptable working characteristics, overcomes the shortcomings of wired communication requiring cable laying, as well as the strong absorption of ultraviolet light by the atmosphere, making ultraviolet communication with a low location detection rate performance, compared to wireless communication methods such as radio frequency communication and infrared light communication has better area covert transmission. It can be used as an independent network unit to access the modern communication network architecture, and can also be interconnected with wireless and wired communication networks to achieve resource information sharing. By establishing an airborne wireless UV non-direct vision communication model with the UAV coverage model, the coverage of the UAV is maximized while satisfying the premise of path loss. In order to ensure the quality of communication services for ground users, the received power of ground users is required to be greater than a certain threshold value, when the transmit power is certain, that is, the path loss when ground users communicate with the UAV must be less than the maximum acceptable path loss to ensure communication, the maximum acceptable path loss corresponds to a maximum coverage radius, and the path loss of all ground users communicating with the UAV in the radius area All of the ground users within this radius have a path loss less than this threshold. At the initial stage of deployment, the UAVs are randomly segmented over the target area, with the UAV projection on the ground as the centre and the coverage area as the radius, and users within its radius are considered to be covered and vice versa. By randomly deploying drones, a broad coverage blind spot, as well as coverage overlap, is created, resulting in a waste of resources for drones. Therefore how to provide communication coverage for ground users by optimizing the location of UAVs to meet requirements such as path loss and maximum coverage is an important problem faced when using UAVs as aerial base stations to build wireless communication networks. In solving the UAV deployment location optimization problem, this paper proposes a virtual force algorithm based on the relative distance to determine the positive and negative forces acting on the UAV based on the relative distance. The algorithm introduces three relative virtual forces, namely the relative virtual force between UAVs, the relative virtual force between UAVs and uncovered users, and the relative virtual force between UAVs and obstacles, and controls the UAV through the action of virtual forces UAV movement to the optimal position. This paper simulates the covert communication coverage of the UAV to ground users based on wireless ultraviolet light transmission technology, and analyses the performance of the algorithm under different scenarios as well as the effect of the ultraviolet light communication angle on the coverage of the algorithm. The simulation results show that the proposed algorithm effectively reduces the invalid coverage and overlapping coverage, and improves the coverage rate by 18.33%, 3.15% and 1.83% compared with the random deployment algorithm, greedy virtual force algorithm and mobile deployment algorithm, respectively, and the average distance travelled by the UAV in the proposed algorithm is the smallest, which indicates that the algorithm in this paper is more energy-saving compared with other algorithms. By choosing the appropriate reception elevation angle, transmission elevation angle, divergence angle and reception field of view angle, the coverage of the UAV can be effectively improved, and the service time can be extended by reducing energy consumption to meet the communication needs of the UAV and the user in a complex electromagnetic environment.
Optical fiber sensors are widely valued by scholars for their simplicity of manufacture, resistance to electromagnetic interference, chemical resistance and ease of distributed measurement. Interferometric fiber optic temperature sensors use the phase change of light to achieve sensing. Sagnac interferometer based temperature sensors are widely used in the sensing field due to their high sensitivity and ease of production. The change in the phase difference can lead to a shift in the interference spectrum, which can be analyzed as a function of temperature. It is an important research direction to design special fibers in Sagnac ring to improve the sensing performance of Sagnac-type temperature sensors. The flexible structural design and air-hole fill ability of photonic crystal fibers offer the possibility to achieve excellent properties of optical fibers. In order to improve the temperature range and sensitivity of Sagnac-type temperature sensors, a photonic crystal fiber design method with high birefringence and high temperature sensitivity properties is provided. The high birefringence of the fiber facilitates the demodulation of Saganc-type temperature sensors and the high temperature sensitivity of the fiber facilitates the sensing sensitivity of Sagnac-type temperature sensors. As the optical fiber itself has limited sensitivity to temperature, it can be made to have good temperature sensitivity by filling the air holes of the fiber with temperature sensitive liquid material. The electromagnetic field model of this photonic crystal fiber is developed in COMSOL and the fiber properties are analyzed and calculated. The effect of structure parameters on the birefringence and the temperature sensitivity of the fiber is analyzed using the finite element method, and the effect of the filling method and the type of filling liquid on the temperature sensitivity of the fiber is investigated on the basis of the determined structure. The optimal structure and filling method are determined. The results show that selective filling can achieve higher temperature sensitivity than full filling, and ethanol is the most suitable filling fluid compared to other temperature sensitive liquids. Under optimal conditions, the fiber achieves a temperature sensitivity of 2.050 7×10-5/℃ and a birefringence of 5.96×10-2 at 1 550 nm. The 2 mm length of this fiber is used in a Sagnac type temperature sensor to analyze the sensing characteristics by simulation, increasing in temperature from 0 ℃ to 75 ℃ in steps of 5 ℃ and using the trough of the transmission spectrum as a reference point to analyze the variation of the transmission spectrum with temperature. A polynomial fitting method is used to fit the wavelength and temperature in order to analyze the temperature sensitivity of the sensor, improve the accuracy of the fit and reduce the measurement error. The results show that the average sensitivity of the sensor can reach 11.28 nm/℃ and the maximum sensitivity is 15.94 nm/℃ in the range of 0~75 ℃, with an average temperature measurement error of 0.126 9 ℃. Compared to existing typical Sagnac temperature sensors, the Sagnac temperature sensor in this paper achieves a higher temperature sensitivity with a minimized fiber length, a larger temperature range and higher measurement accuracy. Therefore, the sensor has a promising application in the field of temperature measurement.
Space laser communication has the outstanding advantages of large transmission bandwidth, high transmission rate, and strong anti-interference ability, which is an important development direction of future communication technology. Since relay amplification cannot be realized in space laser communication links, a large transmission optical power is required in addition to ensuring a high modulation rate. Erbium-doped fiber amplifier achieves the amplification of 1.55 μm optical signal through the three-layer structure of erbium ions. The erbium-doped fiber is the core component of the erbium-doped fiber amplifier, and the erbium-doped fiber is a silicon fiber doped with a small number of erbium ions. In the space irradiation environment, high-energy particles impact the erbium-doped fiber, the core component of the erbium-doped fiber amplifier, resulting in a large number of carriers in the fiber, which combines with the original defects in the fiber to form new color-centered defects. The core defect leads to a dramatic increase in the loss of the fiber in the operating band, as well as a decrease in the gain performance of the erbium-doped fiber. As a rare earth element, La, like Er, is present in the interstitial positions of the quartz lattice structure. It can compete with Er ions for the interstitial positions and act as a dispersion of Er ions. It can achieve Al without affecting the maximum amount of Er ion doping. Low dose doping. La doping can disperse Er ions and suppress fluorescence quenching. There are few studies on the radiation effects of lanthanum-doped erbium-doped fibers. It is important to further understand the radiation-induced absorption mechanism of erbium-doped fibers to improve the performance of erbium-doped fibers in harsh environments. To verify the effect of La doping on the radiation resistance of erbium-doped fibers, two types of erbium-doped fibers, lanthanum-doped and non-lanthanum-doped, are selected in this paper, and the macroscopic radiation gain resistance performance and microstructural changes of the fibers are investigated. Radiation damage test study. The optical fiber was irradiated with a 60Co irradiation source at room temperature at a cumulative dose of 100 krad and a dose rate of 6.17 rad/s. The loss of the fiber was found to decrease along the wavelength direction in the range of 843~1 659 nm by electron probe tests and loss spectroscopy tests before and after irradiation, as well as online loss tests at specific wavelength points. However, the five fixed wavelength tests are not sufficient to fully express the loss variation of the fiber in each wavelength band under an irradiation environment. Offline loss tests were performed on both fibers before and after irradiation. The results showed that the increment before and after irradiation was 3 019 dB/km for S1 and 3 922 dB/km for S2. The loss increment of S2 after irradiation was significantly larger than that of S1 after irradiation. it was speculated that Al-OHC mainly caused the radiation-induced absorption. Absorption spectroscopy tests showed that La doping did not cause any change in the performance of Er ions in the fiber. the loss of the La-doped fiber at 1 200 nm was 0.030 67 dB/(km·krad), which was lower than 0.039 53 dB/(km·krad), and the gain of the La-doped fiber changed very little in the irradiated environment. The properties of the core matrix material did not change after irradiation by Raman testing, which proves that La doping does not cause changes in the glass lattice structure of the fiber. The paramagnetic defects of the fibers were further tested by electron paramagnetic resonance spectroscopy. The EPR signal intensities of the color-centered defects corresponding to Ge and Si did not differ much between the two types of light at 3 370 Gauss by fiber absorption spectroscopy and EPR tests. the peak at 3 330 Gauss is mainly due to the difference in Al content. the higher Al content in S2 produces a higher number of Al-OHC defects in the irradiated environment, and the corresponding EPR signal is stronger. the higher number of Al-OHC defects in S2 leads to a larger radiation-induced absorption in the 700~1 600 nm band, and the conclusion that the higher number of Al-OHC defects in S2 is consistent with the results of absorption spectroscopy tests before and after previous irradiation. The changes of Al-related paramagnetic defects in the fiber before and after irradiation were analyzed, indicating that the increase of Al content leads to more Al-OHC defects after irradiation, which in turn affects the gain performance of the fiber after irradiation. Further, by testing the gain performance of the two fibers before and after irradiation, it was found that the gain performance of the La-doped fiber changed less. It was verified that the loss and gain changes of the lanthanum- and erbium-doped fibers are smaller after irradiation, which indicates that lanthanum doping can improve the radiation resistance of the fibers. The doping of La can replace Al as the dispersant of Er ion to improve the radiation resistance of the fiber to a certain extent, and the doping of La does not negatively affect the gain performance of the fiber. This study can provide a reference for the radiation-hardening design of special optical fibers for subsequent space applications.
Relative Humidity (RH) is a physical parameter which reflects the degree of atmospheric dryness. It has wide applications in agriculture, biology, petrochemical fields, food-processing, medical treatment and Internet of Things (IOT) technology. Fiber-optic humidity sensors have attracted widespread attention from scholars due to their high-measurement accuracy, anti-electromagnetic interference, multiplexing, and distributed sensing. Especially, humidity sensors based on Fabry-Pérot Interferometers (FPIs) have been widely valued for their high repeatability, compact size and high-sensitivity. Various humidity sensors based on silica fiber-optic have been reported. In the majority of fiber-optic humidity sensors, however, the sensitivity of RH detection can be improved by coating hygroscopic materials. At the same time, because they are fragile and inflexible, silica optical fibers must be treated carefully. This increases the cost and complexity of the fabrication process. Therefore, it has great research significance and application value to study new materials and structures. In order to simplify the manufacture of sensor and obtain excellent humidity sensitivity, a composite humidity sensor based on Polymethyl Methacrylate (PMMA)-microsphere and Single-mode Fiber (SMF) is designed and fabricated. Since the Polymer Optical Fiber (POF) material is very easily heated to form a molten state. In this work, the proposed sensor can be fabricated with an electric soldering iron. When the soldering iron is heated to about 70℃, place it 1 cm below the POF to slightly attach the POF to the SMF. Slowly rotate the heating source (i. e., electric iron) to allow the POF to be evenly heated. A Fabry-Pérot (F-P) cavity is formed between PMMA-microsphere and the fiber endface. It should be noted that adjusting the heat source temperature and distance between fiber-optic and heat source, the temperature of heating can be controlled in the heating process. Compared with the alcohol lamp heating method, the electric soldering iron heating method can provide a stable heating source and high safety factor in the experiment. And then, the humidity sensing characteristics of the sensor are theoretically and experimentally studied. When the external ambient humidity rises, the volume of PMMA-microsphere can expand after absorbing moisture from the surrounding. It causes the length of the F-P microcavity to grow. Then, the peak (or valley) of the F-P reflection spectrum shift toward longer wavelengths (i.e., red-shift). Thus, the humidity sensing can be realized. To investigate the sensing performance of the designed sensor, a test system that includes a demodulator, a humidity box, and a personal computer is built. Firstly, humidity test experiments are performed when the RH increases from 30% to 80% at a step of 10%. The humidity experimental results show that the wavelength shifts approximately linearly with the humidity changing, and the linearity reaches 0.992 26. The sensitivity of the sensor is up to 173.36 pm/%RH in the humidity range of 30%~80%. Meanwhile, the sensor is placed in the temperature box (NBD-M1200-10IC, NOBODY. China). The reflection spectra of the sensor with different temperature are acquired by a spectral acquisition with the range of 1 520~1 580 nm. Experimental results demonstrate that the interference spectrum has a red-shifted about 3.15 nm in the temperature range from 35℃ to 65 ℃. The temperature sensitivity of the sensor is 105.07 pm/℃, and the linearity is 0.989 55. In the subsequent studies, we will consider cascading FBG in the proposed sensor to solve the problem that cross-sensitivity of temperature and humidity. Finally, the proposed sensor performs well, showing a superior stability and repeatability over the test cycles in the performance evaluation actualized. When relative humidity is 33%RH and 38%RH, the resonant wavelength of the reflection spectrum has a very small shift within 60 min. The maximum deviations are 0.08 nm and 0.09 nm, respectively. The results indicate that the proposed sensor can maintain good stability in a long-term working condition. In addition, the wavelength drift deviation of the three repeated experiments is small, and the sensitivities are 176.05 pm/%RH, 170.35 pm/%RH and 173.68 pm/%RH, respectively. The average humidity sensitivity of the three groups tests is 173.36 pm/%RH. The designed humidity sensor offers numerous advantages such as low cost, high-sensitivity, simple structure and easy fabrication, which has a wide application prospect in the field of biochemical, agriculture and environmental monitoring.
Optical resonators can limit light into a tiny space, enhancing the interaction between optics and materials. Optical microcavities supporting Whispering Gallery Modes (WGMs) have their advantages with miniaturization and easy integration. The WGMs in optical microcavities achieve optical confinement along the microcavities substance based on the principle of light total internal reflection, which can significantly strengthen light-matter interactions and increase quality factors. These advantages make them have great potential for high-sensitivity sensing applications. WGM optical microcavities have been prepared with various shapes, such as spheres, bottles, disks, rings, cylinders, hemispheres, and so on. In addition, optical microcavities have been also prepared with a variety of materials such as crystals, semiconductors, polymers and glass. Among them, polymer material has the advantages of being flexible, easy to process, and plasticity. It can be easy to be integrated on the optical fiber using the surface tension of the material to install natural formation. In addition, the polymer microbottle resonator is very well integrated. The result makes it possible for packaging the polymer microbottle resonator into a highly stable structure. In this paper, optical microbottle resonators prepared by silica material and Ultraviolet-Curable Adhesive (UCA) material are studied. The preparation methods for the two kinds of microbottle resonators are demonstrated. More specifically, silica microbottle resonators were prepared by the arc discharge method, and UCA polymer microbottle resonators were fabricated by the self-assembling technology. For silica microbottle resonators, the coating layer of a single-mode optical fiber was peeled off firstly and the end of the optical fiber was melted with high voltage discharge so that it formed a microsphere under the action of surface tension. Then, another optical fiber was moved close to the microsphere, and a microbottle resonator could be formed after multiple discharges around the area between the microsphere and the fiber. For UCA microbottle resonators, the polymer material UCA NOA61 is selected because the UCA material has a good light transmission and thermo-light coefficient, and can be cured under ultraviolet light. These properties make it easy to make a microbottle resonator. The self-assembling technology is used to fabricate UCA microbottle resonators. The UCA microbottle resonator was formed through the natural process. Firstly, a tapered fiber was made with the help of the heat-and-pull technique, then an appropriate amount of NOA61 droplets were transferred to the conical transition area above the fiber cone waist. As a result, NOA61 droplets would be self-assembled on the fiber cone because of its own gravity and surface tension to form a flat elongated structure. Subsequently, the NOA61 droplets attached to the tapered fiber were solidified by an ultraviolet lamp. Finally, the samples were placed into a temperature box to heat it and solidify them completely. In the test system, a broadband source in the communication band was a light source to excite the WGMs in the microbottle resonator. The output was detected by an optical spectrum analyzer. In addition, a tapered fiber, fabricated by using the heat-and-pull technique, is chosen as a waveguide to excite WGMs. The basic properties of the two kinds of different materials microbottle resonator were analyzed by a coupled tapered optical fiber. The quality factor of the silica microbottle resonator was 4.683×104 and the quality factor of the UCA microbottle resonator was 3.353×104, correspondingly. The quality factors of the two kinds of microbottle resonators are very close. Temperature sensing applications based on the microbottle resonators were tested. The experimental results show that the sensitivity of the silica microbottle resonator is 11.13 pm/℃ when the temperature rises and 10.25 pm/℃ when the temperature drops. The sensitivity of the UCA microbottle resonator is 111.89 pm/℃ when the temperature rises and 102.02 pm/°C when the temperature drops. Both of them maintain a good consistency when rising and falling. In particular, the temperature sensitivity based on the UCA microbottle resonator is 10 times higher than that of the Silica microbottle resonator. The demonstrated sensors based on our microbottle resonator have the advantages of small size, low price, good plasticity and repeatability, and high sensitivity, and have potential applications in the field of temperature sensing.
With the rapid development of optical fiber communication and optical fiber sensing, the Fiber Bragg Grating (FBG) sensor has become one of the fastest growing and most widely used optical fiber sensors. Owing to the FBG sensor has many advantages in terms of compact size, low cost, wavelength-encoded, anti-electromagnetic interference, easy multiplexing and so on, it has attracted great interests in the field of sensing and been widely used in medical healthcare, pressure detection, battery safety condition and building structural health monitoring. FBG sensors demodulate parameters such as strain, vibration and temperature by detecting the changes of FBG reflection wavelength, with the aid of core technology of laser source and demodulation method. However, the current Tunable Distributed Feedback (DFB) lasers for optical communication applications often have narrow tuning range and low tuning speed, and require additional auxiliary wavelength reference when directly used for FBG sensor demodulation system, which makes the system complex and affects its practicability. Therefore, there is still no tunable laser source with simple structure and low cost that is specifically suitable for FBG sensing systems.In this paper, a wavelength correction-free FBG sensing system was proposed to identify the wavelength changes and demodulate the temperature changes of the FBG sensor without wavelength correction of the wavelength-swept DFB laser based on Reconstruction Equivalent Chirp (REC) technique. As one of the most significant wavelength tunable laser sources, the DFB semiconductor laser has the characteristics of high reliability, low noise, fast response, high output power, good repeatability, and simple tuning scheme. Compared with many other tunable lasers with different realization schemes, the fabrication process of DFB laser is the best choice for both cost and applicability. The DFB laser used in this paper was designed and fabricated using a special process method-REC technique. By sampling the seed grating with uniform period, the low-cost submicron holographic exposure technique of laser chip fabrication can achieve the same wavelength accuracy as the high-cost nanometer electron beam lithography technique while retaining its intrinsic characteristics. When a tunable DFB laser is selected as the laser source for an FBG temperature sensing system, the temperature information can be demodulated by reading the voltage signal of the optical signal reflected by the FBG through the Photodetector (PD). In the wavelength-swept-based FBG sensing demodulation system, the only data that can be obtained directly from the Oscilloscope (OSC) is the optical intensity of the FBG reflection spectra and the corresponding time in the sweeping cycle of the DFB laser. In order to accurately analyze the external environment changes measured by the FBG sensor from these data, it is necessary to calibrate the relationship between wavelength and time during the wavelength-swept process of DFB laser in advance. Firstly, the instantaneous wavelength is calibrated by the calibration method based on Mach-Zehnder Interferometer (MZI), and the relationship between wavelength and time is obtained. Then in the FBG temperature sensing system based on tunable laser, the FBG sensor is placed in the thermostatic bath with a constant temperature accuracy of 0.1 ℃. In the experiment, the tunable REC-DFB semiconductor laser was used as the wavelength-swept laser source. After the optical signal passed through the circulator, it acted on the FBG sensor for temperature sensing, and then the FBG reflected optical signal was received by the PD and recorded by the OSC. By using the characteristic that the reflected optical signal reaches maximum when the laser wavelength coincides with the reflection central wavelength of the measured FBG, and then using the relationship between wavelength and time in the laser sweeping cycle that has been calibrated, the wavelength corresponding to the maximum output signal of the FBG demodulation system is extracted. Finally, by detecting the wavelength offset of the reflected FBG, the measured changes of external temperature can be linearly demodulated even when the laser wavelength changes are nonlinear.Experimental results show that the proposed wavelength-correction-free FBG sensing system can perform accurate temperature detection in the range of 200 ℃, and the coefficient of determination (R2) of linear fitting relationship between wavelength and temperature is 0.995 6. The linearity of the measured wavelength and temperature is good, with the 1-R2 of only 0.004 4. The laser source of sensing system can be tuned up to 2.5 nm with 1 kHz sawtooth wave modulation, and no additional wavelength correction device is required for the measurement process.
Different human posture may express different current states and needs of human body. At present, human posture recognition can be roughly divided into visual technology and sensor technology. However, the image extracted by the camera is susceptible to illumination and background interference, and the recognition accuracy and robustness are not ideal under complex conditions. Traditional sensors (capacitive, voltage, etc.) are vulnerable to electromagnetic interference, so they are not suitable for human posture recognition applications. In recent years, Fiber Bragg Grating (FBG) sensors have been widely used in structural monitoring because of their light weight, anti-electromagnetic interference, wavelength division multiplexing and other advantages. In order to further improve the recognition accuracy of human posture in bio-medicine and kinematics, this paper designs a smart insole based on FBG sensor for human posture recognition, and combines K-fold cross-validation support vector regression algorithm to improve the recognition accuracy. COMSOL simulation software is used to analyze the force on the sole of the foot, and the four main force parts of the big toe, the first metatarsal bone, the third metatarsal bone and the heel are determined. Using the method of wavelength division multiplexing, four FBG sensors with different center wavelengths are arranged in these four positions, and PDMS material packaging is used to design smart insoles. And each FBG sensor is need to be calibrated, the experimental results show that the FBG sensor can maintain a stable measurement performance during the pressure process at constant room temperature, and the fitting linear relationship indicates that the change of its center wavelength is proportional to the load, and verifies that the wavelength offset of FBG has a high linearity and sensitivity to the pressure. In this work, a total of 25 participants are recruited to accomplish eight human postures, including standing, sitting, standing on one foot, folding forward, leaning forward, leaning back, half squat and full squat, respectively. The changes in the center wavelength of the string FBG sensor are recorded. In order to reduce errors, each posture is repeated twice and the average value of its center wavelength offset is taken to construct the data set. The K-fold cross-validation Support Vector Regression (KCV-SVR) model is introduced for data processing, meanwhile, the K value is set to 5, wavelength offset is taken as input, and different postures are taken as output. After K-fold cross-validation, the optimal value of SVR penalty factor and radial basis function parameter are obtained automatically, which are 0.5 and 8 respectively. The experimental results show that the Root Mean Square Error (RMSE) of SVR regression model is 0.510 6, the Mean Absolute Error (MAE) is 0.132 3, and the coefficient of determination R2 is 0.967 7. RMSE, MAE and R2 of KCV-SVR regression model decreased by 0.460 4, 66.3% and 3.3%, respectively. By comparing the prediction error of SVR and KCV-SVR regression model, the maximum error of SVR is 0.309 6, the minimum error is 0.012, and the average error is 0.088 4. However, the maximum error of KCV-SVR is 0.301 6, the minimum error is 0.001 5, and the average error is 0.057 6. It can be seen that the prediction result of KCV-SVR regression model is better than that of SVR regression model. KCV-SVR regression model basically realizes the effective recognition of human postures, and provides a new idea for the recognition of human postures based on flexible FBG sensor.
Humidity is one of the important parameters in the field of sensing, and the monitoring and control of humidity is used in many applications, such as agriculture, cultural relics protection, environmental safety, and pharmaceutical engineering. Since traditional electronic humidity sensors are mainly based on capacitance or resistivity measurements, which are more susceptible to electromagnetic interference. Moreover, under high humidity environment, water vapor will corrode the circuit board, which is a great test for the long-term stability of electronic humidity sensors. Compared with traditional humidity sensors, fiber optic humidity sensors have the advantages of corrosion resistance, light weight, can be operated remotely, not subject to electromagnetic interference, etc., and have been widely studied in recent years. Among them, U-shaped microfiber has the advantages of small size, low fabrication cost, high sensitivity, etc., which has a broad prospect in the field of humidity measurement. In this paper, a U-shaped microfiber humidity sensor based on Polyvinyl alcohol (PVA) coating is proposed, and the single-mode optical fiber is pulled into micron-sized microfiber by fiber optic melting and cone pulling machine, and the microfiber with different diameters can be prepared by changing the flame temperature and cone pulling speed, etc. PVA is a strong hydrophilic material, and its refractive index changes with the humidity of the surrounding environment. Due to the high adherence of PVA on the surface of silica, it can be used for the measurement of humidity in a wide range of applications. PVA has high adhesion on the surface of silica, so it can be easily coated on the surface of optical fiber. Because of these special properties of PVA, combining PVA with ultrafine optical fiber can be used to measure humidity. Using the drop coating method, the PVA solution is uniformly coated on the surface of the optical fiber, and the coated ultrafine optical fiber is passed through a homemade mold, which is made of three capillary glass tubes, and the ultrafine optical fiber is fixed into a U-shape by means of UV photoresist and ultraviolet lamp. The paper prepared three humidity sensors, which were used to study the effect of coating thickness on the humidity of the sensors. It was found that the humidity sensitivity of the U-shaped sensors was extremely low when they were not coated with PVA; after coating with PVA, the sensitivity of the sensors increased with the increase of the coating thickness. At the same time, the thicker the coating the greater the loss of the sensor, and it will lead to a longer fabrication time of the sensor, which will cause the PVA droplets on the dropper to evaporate, thus affecting the uniformity of the coating. Therefore, there is a need to select a suitable coating thickness. In order to investigate the effect of the diameter of the waist zone on the humidity of the sensor, two different diameters of the sensor are prepared in this paper. The experimental results found that the smaller the diameter of the waist region, the higher the sensitivity of the sensor, which is due to the fact that the smaller the waist diameter of the ultrafine optical fiber coupled with the external environment the stronger the swift field, which strengthens the interaction between them. The experimental results show that the sensor prepared in this paper has a high humidity sensitivity of 146.1 pm/%RH in the detection range of 34%RH~90%RH. In the humidity measurement experiments, the fluctuation of the temperature will have a certain effect on the experimental results. This is because temperature changes the physicochemical properties of PVA film and U-shaped microfiber, such as changes in refractive index and expansion. Therefore, it is an important work to explore the effect of temperature on the crosstalk of the sensor. Temperature experiments show that the temperature sensitivity of the sensor is 15.8/℃ in the range of 40 ℃~80 ℃, and its crosstalk sensitivity is 0.108%RH/℃, which is much lower than the humidity sensitivity, so the temperature has less effect on this sensor. The sensor designed in this paper has a simple preparation process, high sensitivity, easy to carry, low cost, less influence of temperature crosstalk, which has a wide range of applications in the field of humidity detection.
The rapid development of wireless communication technology has made the non-regeneratable Radio Frequency (RF) spectrum resources extremely scarce. The In-band Full-duplex (IBFD) wireless communication scheme utilizes the same frequency to transmit and receive signals at the same time slot, overcoming the shortcomings of traditional half-duplex communication schemes that can only transmit and receive signals in different frequency carriers or different time slots, and doubling the utilization rate of the RF spectrum. However, RF self-interference is the primary problem that must be resolved for the application of IBFD scheme. Microwave photonic RF Self-interference Cancellation (SIC) technology is attracting more and more attention by virtue of the advantages of the large signal processing bandwidth and high amplitude and time regulation accuracy. In this paper, considering the influence of environmental variation on the RF SIC performance, we focus on the control algorithm for the microwave photonic RF SCI system.Firstly, a theoretical model of microwave photonic RF SIC link is established, and the expressions of RF SIC value for the single-frequency signal and the signal with a certain bandwidth are derived. Then, the parameters that affect the RF SIC performance are analyzed, including the amplitude mismatch and delay mismatch. The analysis results provide a basic reference for the design of the microwave photonic RF SIC scheme, the optimization of the regulation units in optical domain and the construction of the experimental system.Secondly, an adaptive feedback control system for microwave photonic RF SIC system is designed and constructed, which is composed of the optical domain regulation units, RF signal down conversion units, data acquisition units and data processing units. The Field Programmable Gate Array (FPGA) is applied for digital signal processing, adaptive algorithm iteration and control instruction output to the regulation units in optical domain via serial communication unit. Also the clock conversion is conducted in FPGA to provide different clock for different digital unit.Thirdly, an adaptive algorithm for microwave photonic RF SIC system in FPGA is implemented. The algorithm is divided into two steps. One step is a cross-correlation algorithm to obtain the amplitude and delay mismatch between the interference signal and the reference signal, which provides the initial value for the other step of Particle Swarm Optimization (PSO) algorithm. The PSO algorithm realizes a further RF SIC through the circle of the acquisition of the residual self-interference power by Analog-to-digital Converter (ADC), the optimization iteration for control instruction to the regulation units in optical domain. During the circle, the tuning accuracy of regulation units in optical domain and the sampling accuracy of ADC are considered comprehensively to optimize the iteration logic loop, which are applicable for the really established system.Finally, a microwave photonic RF SIC system with direct modulation is established, for which the feedback control algorithm based on FPGA is demonstrated. The cancellation depth of 35 dB for the 2.4 GHz center frequency and 40 MHz bandwidth is realized by the microwave photonic RF SIC system. The measured results verify the feasibility and efficiency of the FPGA based control algorithm.
In the digital chaotic secure communication network, the dynamic packet switching technology of digital chaotic signal plays an important role in improving the switching and transmission capacity of the optical chaotic network. In the optical chaotic packet switching node, the processing of optical chaotic signals involves multiplexing, demultiplexing, switching, regeneration, storage and so on. However, the premise of realizing the optical chaotic signal processing mentioned above is to have optical chaotic logic gates (such as logic NOT, AND, NAND, OR, NOR, XOR, XNOR) and digital chaotic bit computing devices (chaotic combinational logic operation devices and chaotic sequential logic operation devices) with low power, low loss and high speed. Therefore, in order to realize the digital chaotic signal dynamic packet switching technology, it is necessary to explore the low-loss and high-speed digital chaotic logic operation. Compared with traditional logic devices (such as digital circuit logic devices, optoelectronic logic devices, all-optical logic devices), chaotic logic operation devices have the characteristics of more security, more flexibility, lower power cost and so on. However, at present, most of the schemes implement the static chaotic logic operation, and the development of reconfigurable chaotic logic operation, chaotic combinational and sequential logic operation is still lagging behind. The external interference will not only affect the chaotic signal, but also may cause the system parameters to drift, change the output polarization state of the laser, resulting in the instability of the chaotic logic operation, and even errors. Therefore, it is of great value and significance to study the reconfigurable and storable chaotic logic operation with strong robustness and the ability of error detection and correction.In the implementation scheme of reconfigurable and storable chaotic logic operation, the normalized injection current of D-VCSEL is modulated as a logic input, and the applied electric field on PPLN1 is modulated as a logic control signal. The logic outputs Z1 and Z2 are demodulated by a threshold mechanism for synchronization errors between the polarizations (x-PCD1 and y-PCD1) from the D-VCSEL and those (x-PCR and y-PCR) from R-VCSEL. The logic outputs Z3 and Z4 are obtained by demodulating the x-PCD1 and the x-PCR, respectively, based on threshold mechanism. The system can reconstruct and store the chaotic logic operations by transforming the logic operation relationship between the logic control signal and the logic input. It is found that when the noise intensity is 1×109, the logic outputs Z1 and Z2 demodulated by the complete chaotic synchronization mechanism have more errors, but Z3 and Z4 have no errors. In order to improve the anti-noise performance of Z1 and Z2, the threshold mechanism is used to demodulate the synchronization error. When the noise intensity reaches 1.77×109, the logic output will not generate bit errors. When the noise intensity reaches 1.84×109, the logic outputs Z1 and Z2 generate bit errors, but the logic outputs Z3 and Z4 remain correct. If the noise intensity reaches 1.89×109, the logic outputs Z1, Z2, Z3 and Z4 all have bit errors, and the logic operation fails. Therefore, the anti-noise performance of Z3 and Z4 is better than that of Z1and Z2. And if the noise intensity is in the range of 1.84×109, the logic outputs Z1 and Z2 can be checked with Z3 and Z4 to realize error detection and correction processing.
As a core component of a fiber laser system, the fiber combiner not only directly determines the pump and output power of fiber lasers, but also serves as an important guarantee for the safe operation of the all-fiber laser system in a high-power environment. At the same time, the fiber combiner is simple and stable in structure, and not easy to be interfered with by the outside environment. It can realize the expansion of power and spectrum without a lot of optical components and free space optical devices. The fiber combiner that can transmit lights in the mid-infrared band has attracted much attention because of its wide application in national defense, military, scientific research and business. In addition, due to its significant design and preparation difficulties, only a few institutions currently master the relevant technology. As a very important part of mid-infrared photonic devices, it has become a research hotspot in this field all over the world.In this paper, in theory, the optical field distribution of multimode fiber combiner is analyzed and the power loss of several main modes in the fiber core is calculated. The mechanism of loss in the tapering process of fiber combiner is elucidated. The basic criteria of adiabatic tapering and brightness conservation are analyzed in detail, which lays a foundation for the design of high transmission efficiency and good beam quality combiners. Experimentally, starting with the preparation process and key technologies of mid-infrared sulfide fiber combiner device, the 7×1 sulfide fiber combiner device with high performance has been successfully developed after solving the key problems of the ordered arrangement, melting tapering, end cutting, homogeneity fusion and end angle polishing. The key performance indexes are tested and analyzed. In terms of testing and characterization, the transmission efficiency, beam quality, structural stability and power damage threshold of the 7×1 mid-infrared sulfide fiber combiner are tested by using mid-infrared light source, detector and beam quality analyzer. The average transmission efficiency of nearly 80% (@4.778 μm) is obtained for different ports of the 7×1 sulfide fiber combiners. When the diameter of the output fiber core is about 350 μm, the best M2x/y value of the 7×1 sulfide fiber combiner is 19.63/22.48. The tensile tension at the fusion point is more than 300 g, which is better than similar structures. When the input laser power (@1.976 μm) exceeds 10 W, the maximum output power of the 7×1 sulfide fiber combiner can reach 4.32 W.This paper provides some ideas for the processing of mid-infrared sulfide fiber and the fabrication of fiber combiner devices. On the one hand, mid-infrared fiber as the main body of mid-infrared fiber combiner, its performance largely determines the final performance of the fiber combiner. In recent years, mid-infrared single-mode fiber, multi-mode fiber and mid-infrared doped fiber with low loss have been prepared. However, compared with commercial quartz fiber, the preparation technology of high performance mid-infrared fiber is still immature, which is the main reason limiting the development of mid-infrared fiber combiners and other mid-infrared fiber devices. On the other hand, how to optimize the soft glass fiber processing platform and eliminate the influence of preparation process on device performance as much as possible is also a problem to be solved in the future.
The Intensity Modulation/Direct Detection (IM/DD) system is simple and easy to implement, which is most widely used in Free Space Optical (FSO) communication. Its typical modulation schemes include On-off Keying (OOK), L-level Pulse Position Modulation (LPPM), Differential Pulse Position Modulation (DPPM) and Multiple Pulse Position Modulation (MPPM), Digital Pulse Interval Modulation (DPIM), etc. OOK is the simplest modulation scheme to implement, but the power utilization is too low and the anti-interference ability is poor. LPPM has superior power utilization at the expense of a large amount of bandwidth. Also the transmission capacity is difficult to meet the requirements of FSO communication, and strict symbol synchronization is required during demodulation, which increases the complexity of the system. DPPM improves the bandwidth utilization relatively and does not require symbol synchronization. But redundant“0”time slots are still generated in the coding process, which makes this modulation scheme not high enough in transmission capacity and bandwidth utilization. MPPM further improves the bandwidth utilization, nevertheless the error performance is poor. And strict symbol synchronization is required which increases the complexity of the system. In order to improve the performance of the traditional PPM schemes, a Novel Differential Pulse Position Modulation (NDPPM) scheme is proposed in this paper, combining with the advantages of DPPM without symbol synchronization and high bandwidth utilization of MPPM.A novel differential pulse position modulation scheme named NDPPM is proposed in this paper. The mapping relationship and symbol structure of NDPPM are studied, the modulation performance is analyzed and compared with other modulation schemes. The average time slot error rate and packet error rate models of FSO communication system under Gamma-Gamma turbulent channel are derived. According to the derived models above, the error performance simulation is carried out. The influence of turbulence intensity, transmission distance and modulation order on the error performance of NDPPM system is analyzed, and the error performance of NDPPM and several other modulation schemes is compared and analyzed.NDPPM does not require symbol synchronization like DPPM, and its transmission capacity is about 4 times that of LPPM and more than 2 times that of DPPM and MPPM for a larger n. The bandwidth requirement of NDPPM is second only to MPPM, which is 1/4 of LPPM and half of DPPM for a larger n, but the transmission capacity, power utilization and error performance are better than that of MPPM. Also compared with MPPM, NDPPM does not require symbol synchronization, which can simplify the complexity of system implementation. Through the simulation of NDPPM system and channel parameter, it can be seen that the increase of turbulence intensity or transmission distance will lead to the decrease of NDPPM system performance, but under different turbulence intensity, the change of transmission distance has different effects on packet error rate performance. Under weak and medium turbulence conditions, the change of transmission distance has a great influence on the performance of packet error rate, while under strong turbulence condition, the influence of the change of transmission distance on packet error rate performance becomes smaller .The reason is that when the turbulence is strong, the turbulence is the main factor affecting the performance of packet error rate, while in weak and medium turbulence, the change of transmission distance is the main factor affecting the performance of packet error rate. The performance of NDPPM system can be improved by increasing the modulation order or Signal-to-Noise Ratio (SNR). If when the packet error rate is 10-6, every 1 increasing in modulation order, 2 dB in SNR can be saved at least.Through the analysis of NDPPM code pattern, its modulation performance is derived. Compared with the traditional PPM schemes, NDPPM has the highest transmission capacity, and its bandwidth requirement is lower, second only to MPPM. Also NDPPM does not require symbol synchronization, which can simplify the complexity of system implementation. The error performance of NDPPM is significantly better than that of MPPM and OOK. When Cn2=9.0×10-15 m-2/3, n=7 and the packet error rate is 10-6, its performance is superior to MPPM and OOK at 4 dB and 10 dB , respectively. And the performance advantage becomes more apparent as the modulation order increases. The error performance of NDPPM is not as good as that of DPPM, nevertheless, higher modulation order can be adopted to obtain better performance, e.g. when Cn2=4.0×10-14 m-2/3 and μ0=10 dB, the packet error rate of NDPPM with n=7 is one order of magnitude lower than that of DPPM with n=5. The simulation results of NDPPM system show that the increase of turbulence intensity or transmission distance can lead to the decrease of its system performance, while the influence of the change of transmission distance on packet error rate performance is different under different turbulence intensity. The performance of NDPPM system can be improved by increasing the modulation orders or SNR. Considering the complexity of system implementation and performance comprehensively, NDPPM has some advantages and applications in FSO communication. Also the appropriate modulation scheme should be adopted according to the actual demand in the application.
The advancement in marine resource utilization by humans has spurred the need for performance metrics for underwater communication technology. Traditional underwater acoustic communication has reached its limitations due to its slow speed and significant delay, while submarine optical cables also present challenges in deployment and maintenance. Thus, underwater wireless optical communication technology, notable for its speed, high capacity, energy efficiency, and minimal delay, has emerged as an efficient solution to the problems of underwater high-speed wireless communication. However, the presence of underwater turbulence or aquatic organisms may lead to uncertainty in the direction of the communication emission light sources, hindering optical path alignment in the communication system and compounding the challenges of underwater wireless optical communication chain development. To address this, some strategies for underwater wireless optical communication employ mechanical fixation or manual adjustments for optical path alignment, which unfortunately limit the flexibility of the system. In light of these challenges, this paper presents the design of an underwater wireless optical communication system with optical alignment capability, providing a theoretical and practical groundwork for future underwater wireless optical communication networking.The paper proposes an underwater wireless optical dynamic communication system based on a servo system, and uses a two-dimensional waterproof photoelectric turntable as the loading platform. The platform integrates various components such as a CMOS camera, a Fresnel optical antenna, an APD detector module, and employs an external STM32F407 master controller to capture and align the signal spot at the transmission end of the system. The initial experiment tested the spot alignment accuracy of the servo system and analyzed communication rate, bit error rate, and detector sensitivity upon completing the communication optical path alignment.Following system power-up, light spots are simulated at different underwater positions by adjusting the azimuth and pitch of transmitting end turntable A. At this stage, a capture alignment command is issued to receiving turntable B, controlling it to align with the light spot. With an optical power of 30 mw at the transmitting end, the spot alignment takes 8.2 seconds when the azimuth miss distance is 815 and elevation miss distance is 697, with an azimuth alignment error of 0.17 mrad and an elevation alignment error of 0.42 mrad. After the system's optical alignment is completed, the communication performance is tested. With an error rate maintained at 10-6, the minimum signal amplitude output at communication rates of 10 Mbps, 20 Mbps, 30 Mbps, 40 Mbps, and 50 Mbps is measured. The detector sensitivity at these communication rates are -31.87 dBm, -29.03 dBm, -28.56 dBm, -27.49 dBm, and -26.74 d Bm, respectively, enabling the detection of weak light signals. The system can capture the emission spot at different positions within its field of view and sustain a stable communication link to perform communication functions.By integrating a servo system with the underwater wireless optical communication system, an underwater wireless optical dynamic communication system is designed capable of high precision optical path alignment. The system benefits from a broad field of view capture, simple structure, and high communication rate. Compared to traditional ATP laser communication systems using coarse and fine tracking modes, this system simplifies its structure based on communication distance and the underwater environment, and uses a large aperture Fresnel optical antenna to optocouple the signal into the APD detector, thereby reducing optical power attenuation. Experimental tests show that the servo system and the communication system work as expected, achieving spot capture, optical path alignment, and data communication. The results validate that the servo system can address the complex alignment of communication optical paths in underwater environments. In future work, the aim is to enhance the servo system's performance and develop its capability to track mobile communication transmitters. In addition, the communication performance of the system is optimized, fully utilizing the three optical windows of the platform to achieve duplex communication in underwater environments.
Strain monitoring is important for structural health monitoring of bridges, dams, high-rise buildings and so on. The metrological performance of strain sensing systems is an important guarantee for the accuracy of structural monitoring data. Common sensors for strain monitoring include Fiber Bragg grating strain sensors, resistive strain gauges, and vibrating wire strain gauges. Fibre Bragg grating strain sensors have the advantages of strong anti-electromagnetic interference, high precision, good durability, and distributed measurement. They have a large number of applications in the field of engineering construction and instrumentation. Fibre Bragg grating strain sensors usually work under dynamic conditions during bridge health monitoring. If the sensor measurement data is incorrect, it will have a negative impact on subsequent control, monitoring and fault diagnosis systems. Therefore, it is necessary to perform on-site calibration of the Fiber Bragg grating strain sensor in actual use to ensure the accuracy of the measurement results. At present, there are mainly two methods for on-site calibration of Fiber Bragg grating strain sensors: static calibration and dynamic calibration. In the actual bridge inspection, static calibration must block traffic operation for a period of time, which will have a certain impact on social and economic life, while dynamic calibration has little effect on traffic operation, so it is necessary to perform dynamic calibration of Fiber Bragg grating strain sensors. Aiming at the accuracy of fiber grating strain sensor in measuring dynamic strain value of bridge structure, a calibration method of Fibre Bragg grating strain sensor under dynamic excitation is proposed. The equal-strength cantilever beam is used to simulate the bridge structure, and weights of different weights are suspended instantaneously at the end of the equal-strength cantilever beam to simulate the dynamic excitation generated by the vehicle on the bridge. The resistance strain gauge has a simple structure, high output accuracy and good stability and is suitable for use as a standard sensor for the transmission of strain values. Therefore, a high-precision resistance strain gauge is selected as the reference sensor. The resistance strain gauge is used as the reference sensor, and the Fiber Bragg grating strain sensor is used as the sensor to be calibrated, and the measurement data sequences of the reference sensor and the sensor to be calibrated are compared. The premise of dynamic calibration is that the changes of the magnitude waveform of the sensor to be calibrated and the reference sensor under the same excitation source are basically the same. If the waveform of the two magnitude sequences is poorly matched, the dynamic calibration of the sensor cannot be performed. Because the sampling start time of the reference sensor and the sensor to be calibrated are not synchronized, and their own characteristics are different, the change waveform of the two measured value sequences will appear time misalignment. Therefore, in view of the time dislocation problem in the measurement data sequence, a cross-correlation algorithm is used to match the measurement data of the reference sensor and the sensor to be calibrated. Then, when the Fiber Bragg grating strain sensor has the initial value, the measurement value calibration is studied. When the Fiber Bragg grating strain sensor and the resistance strain gauge have the initial value, the ratio of the wave peak value of the two is used as the sensitivity calibration coefficient of the Fiber Bragg grating strain sensor. The experimental results show that the method effectively solves the time dislocation problem of the measurement data sequence, the data matching rate is more than 98%, and the dynamic calibration of the fiber grating strain sensor is realized. The use of different weights as the excitation source does not effect the calibration results. The sensitivity calibration coefficients of Fiber Bragg grating strain sensors at 1 529 nm and 1 547 nm wavelengths are basically consistent with the static calibration results.
Distributed optical fiber sensor has been intensively researched owing to its various advantages, such as long monitoring distance, better sensing accuracy and spatial resolution. It has been widely applied in power cables, oil pipelines, transportation. Among the numerous kinds of distributed optical fiber sensors, Brillouin Optical Time-Domain Analyzer (BOTDA) attracts much attention due to its precise measurement for the temperature and strain in ultra-long sensing range. BOTDA is based on the effect of Simulated Brillouin Scattering (SBS), frequency difference between Brillouin scattering light and incident light is defined as Brillouin Frequency Shift (BFS), which is a linear function of temperature and strain. BFS is usually determined by finding the central frequency, which is with the maximum amplitude of the local Brillouin gain spectrum. However, in order to obtain Brillouin gain spectrum in long-distance monitoring applications, BOTDA needs to scan in frequency range around the Brillouin frequency of the optical fiber, so that a number of time-domain traces associated with each frequency are measured to ensure measurement accuracy, which will lead to tradeoffs among measurement accuracy and real-time performance. To enhance both the processing time and sensing accuracy, in recent years, machine learning and deep neural network have been widely proposed and introduced to BOTDA to extract temperature distribution from the measured Brillouin gain spectrum along the sensing fiber. The temperature extraction can be considered as a nonlinear regression problem and the regression model is constructed by learning from the spectrum samples using learning algorithms. Unlike other methods with the shallow architectures, deep neural network is composed of multiple processing layers that can learn representations of data with multiple levels of abstraction. Among many methodological variants of deep learning, Recurrent Neural Network (RNN) has achieved impressive performance in various challenging areas. By adding the time hidden layer into the architecture, RNN acquired better accuracy for sequential data due to the consideration of sequence characteristics. However, RNN has the problem of gradient disappearance. Therefore, we adopt a deep network called Long Short Term Memory (LSTM) for the temperature extraction of Brillouin gain spectrum. LSTM is a variation of RNN architecture that is overcome the problems of gradient disappearance and explosion, is particularly suitable to input sequences. In this paper, the data set is generated by using Lorentz function for LSTM network training, and the mapping relationship between LSTM network and temperature was established. A 40km BOTDA setup for temperature sensing is built to verify the performance of the trained LSTM. BOTDA that operates over a long sensing fiber is prone to be affected by the detrimental non-Local Effects (NLE), since NLE can distort Brillouin gain spectrum, therefore correctly retrieving BFS is very challenging. The experimental setup that we used to acquire data has distortion phenomenon in long distance temperature monitoring. We firstly use the spectral line subtraction method to correct the distorted Brillouin gain spectrum, the corrected Brillouin gain spectrum appears Lorentz shape, then leverage LSTM to learn the feature of the corrected Brillouin gain spectrum, finally, by feeding the Lorentz spectrum sequentially into the well-trained LSTM model, the temperature information along the sensing fiber of Brillouin spectrum can be quickly determined. The performance of LSTM is investigated both in simulation and experiment under different cases of frequency scanning steps, compared with classical ELM algorithm and curve fitting methods, the LSTM algorithm shows that the minimum root mean square error is 0.11℃. Besides, LSTM network has good robustness to frequency step change, even under the circumstance of large frequency step, the method still has good measurement accuracy, which improves the real-time performance of Brillouin optical time-domain temperature sensing system.
Microwave signal generator is the key component of modern Radio-Frequency (RF) systems, such as navigation, radar, communications, and electronic warfare. Optoelectronic Oscillator (OEO), a new type of microwave oscillator, is able to directly generate a high-frequency microwave signal with low phase noise. Therefore, OEO is considered a promising solution for high-performance microwave signal generation. Phase noise is one of the vital parameters to evaluate the performance of microwave signal sources, including OEO. To date, a number of Phase Noise Measurement (PNM) methods have been designed and carried out to accurately measure the phase noise of microwave signal sources, including direct spectrum method, phase detector method, and frequency discriminator method. Among these methods, the frequency-discriminator-based PNM method is more attractive since it can eliminate the requirement of a low phase noise reference oscillator. However, the measurement sensitivity of this method is related to the time delay, which is limited by the large loss of electrical cables. Fortunately, a photonic delay line PNM method has been proposed with a high measurement sensitivity, where a section of optical fiber is applied to provide a long-time delay with negligible loss. In recent years, a lot of efforts have been devoted to improving the overall performance of the photonic delay line PNM system. For instance, a microwave photonic phase shifter and microwave photonic mixer have been employed to replace the electrical phase shifter and mixer of a traditional photonic delay line PNM system to extend the operation bandwidth. In addition, digital phase demodulation is adopted to eliminate the calibration procedure. Nevertheless, a problem with most previous photonic delay line-based schemes is that too many discrete electrical and optical components are used, which makes them bulky and leads to considerable coupling loss. Furthermore, the functionality of current photonic delay line PNM systems is relatively homogeneous, with only phase noise measurement, and few solutions with dual- or multi-functionality have been reported. However, during the development, optimization and operation of OEO-based RF systems, especially signal generation systems, it is necessary to develop a low-cost, simple and compact phase noise measurement solution to evaluate the quality of the signal source in time and make corresponding parameter adjustments to optimize its performance. In this paper, we propose a photonics-based dual-functional system that can achieve both microwave signal generation and phase noise measurement using an Electro-absorption Modulated Laser (EML). In this system, an EML-based optoelectronic oscillator module and a phase noise measurement module based on a photonic delay line method are implemented simultaneously. By replacing the laser source and intensity modulator with a single EML, the proposed photonics-based dual-functional system achieves a low cost and simple structure, as well as maintaining high performance. In the proof-of-concept experiment, a 9.952-GHz signal generated by the EML-based OEO has a side mode suppression ratio of 66 dB and a phase noise of -116.53 dBc/Hz@10 kHz, which is over 25 dB lower than that of Anritsu-MG3692B. In addition, the PNM module has a phase noise floor reaching -133.71 dBc/Hz@10 kHz, which is lower than that of the commercial signal source analyzer R&S FSV40. The experimental results show that the dual-functional system is capable of simultaneously generating a high-performance microwave signal and providing a high-sensitivity phase noise measurement solution for both internal and external microwave signals. This solution will facilitate the development, optimization and operation of OEO-based RF systems, especially signal generation systems, by evaluating the quality of the signal source in time and making the corresponding parameter adjustments to optimize its performance. Therefore, the proposed scheme is suitable for occasions where a high-performance microwave signal is required, and the phase noise performance of different microwave signals within the system can be monitored at the same time.
Optical clocks have reached an instability level of 10-19. Remote clock comparisons require more stable frequency transfer systems. Remote time and frequency transfer are currently performed using RF link or laser link. The signal pathway between the clock and RF/laser transceiver also requires high stability. In some applications, ambient temperature change is the main noise source. However, it is difficult to control optical fiber temperature precisely under certain circumstances such as on a space station.A noise suppression method is developed and a frequency transfer system is designed using Michelson interferometer based on a 3×3 fiber coupler. A 3×3 fiber coupler rather than a 2×2 fiber coupler is chosen to build a Michelson interferometer because the direction of the fiber length change can be judged effectively when a 3×3 fiber coupler is used. An optical comb is used as a frequency source, with a wavelength of 1 550 nm, an output power of 40 mW, and fr (repetition frequency) at 200 MHz. A 1 310 nm CW-laser is used as a measurement scale.The two laser beams are input into the same optical fiber via a WDM. Another WDM split the two laser beams at the end of the optical fiber. The optical comb laser beam can be directly input into the target device. The measurement laser beam is reflected by a Faraday rotation mirror back to the fiber coupler. The optical fiber is the measuring arm of the Michelson interferometer. Photodiodes detect the interference fringe.An embedded system is employed in counting the interference fringe shift amount, calculating the compensation value, tuning the fiber length, and compensating the time delay variation in real time. A compensation device was built, and experiments were carried out.The experimental system is mainly composed of a compensation device, 30 m optical fiber, wavelength division multiplexers, Faraday rotation mirrors, and other auxiliary devices. The temperature of the 30 m optical fiber is controlled by an external TEC. For each test, the temperature of 30 m optical fiber is decreased from 26 ℃ to 5 ℃ and then increased to 18 ℃. The temperature variation range is 21 ℃.When the compensation is off, the time delay changes for approximately 20 ps for each 10 ℃ change. When the compensation is on, the time delay has mostly no change. The change is more remarkable than 2 fs at only a few points, which is still lesser than 6 fs.In another experiment, the optical comb laser is split into two beams and input into two optical fibers. Each laser beam is detected by a photodiode. The frequency signals from photodiodes are filtered by bandpass filters and input to a 5 125 A phase noise test set as input signal and reference signal respectively. The frequency instability of fr is measured in the following three cases. In the first case, two short optical fibers of a similar length are used. The frequency instability characterizes the background noise of the test system. In the second case, one optical fiber is a short fiber, and the other is a 30 m fiber. The compensation device is disabled and connected in front of the 30 m fiber. The frequency instability of fr increases by about an order of magnitude compared with the first case. In the third case, one optical fiber is a short fiber and the other is a 30 m fiber. The compensation device is connected in front of the 30 m fiber and enabled. There is no obvious change in the frequency instability of fr between the first and the third case.This work is expected to provide an effective solution for the noise suppression of the transmission pathway from the optical clock to the RF/laser transceiver under space conditions.
With the continuously increasing demand for optical network bandwidth, high-speed data transmission and large-capacity optical fiber communication technologies are being developed to meet the goal. In Mode Division Multiplexing (MDM) systems, the orthogonal fiber modes are made advantage of carrying different user information in the form of Multiple-input and Multiple-output (MIMO) channels. In the case, both the system spectral efficiency and transmission capacity can be effectively improved by means of MIMO digital signal processing technology. At the same time, to save the system cost and reduce complexity of implementation, more attention is being paid to low-complexity MIMO technology, or even MIMO-free MDM high-speed signal transmission, with applications to short-distance application scenarios such as optical access networks, data centers, and supercomputer interconnections.On the other hand, the mode multiplexer/demultiplexer, as a key component for MDM communication systems, are required for a low mode-dependent loss and mode crosstalk as possible. Their performance parameters can directly affect the system's bit error rate and the difficulty of algorithm compensation at the receiver. There are several kinds of mode multiplexers, including those of free space type, optical waveguide, fiber coupler, and photonic lantern. Free-space multiplexers have some advantages in mode purity and mode crosstalk, but are limited by complex optical platforms. The multiplexing/demultiplexing scheme based on the waveguide structure has large connection loss with few-mode fibers since the width of the optical waveguides is much smaller than the core radius of the few-mode fibers. The other two multiplexer/demultiplexer are in the form of fiberization and capable of seamless connection to few-mode fiber systems. Among them, the commercially available Mode-selective Photonic Lantern (MSPL) mode multiplexer, as a passive optical device, has the advantages of simple structure, low insertion loss, and multi-port input. The MSPLs are also compatible with other technologies such as wavelength division multiplexing, polarization multiplexing.In the paper, we focus on the high-speed signal transmission capability of MIMO-free Mode Division Multiplexing (MDM) system constructed by the 3-port MSPLs, where each mode channel carries a Dual-polarization Quadrature Phase Shift Keying (DP-QPSK) signal at a bit rate of 100 Gb/s. The state of the Few-mode Polarization Controller (FMPC) used in the experiment can be accurately characterized by the Signal-to-crosstalk Ratios (SXRs) of LP01 and LP11 mode channels. The relationship between the Bit Error Rate (BER) and the SXRs is measured by adjusting the FMPC. The experimental results show that, no bit error after forward error correction can be achieved when the SXR of each mode channel is more than about 8 dB. When the SXRs of the two channels are respectively 14.25 dB and 13.81 dB, the corresponding degradation of received optical power relative to the back-to-back transceiver system are 1.40 dB and 4.76 dB at the BER threshold of 10-2, respectively. The effects of fiber attenuation, polarization mode dispersion and mode crosstalk on the high speed transmission system are analyzed, and the transmission distance limited by crosstalk is estimated to be about 30 km for the few-mode fiber system of interest. Compared with the technologies reported so far, the research in this paper has the highest channel rate, and indicates that the MIMO-free crosstalk limited distance is about 30 km. The 2×2 high-speed MIMO-free mode division multiplexing experiment is carried out before. 100 Gb/s DP-QPSK has higher requirements on the SXR of the channel. Using the same experimental system, appropriately reducing the channel data rate can achieve 3×3 mode division multiplexing system.
Surface Plasmon Resonance (SPR) is a prominent optical phenomenon that arises as the extent of energy transferring from photons to surface plasmon waves under appropriate conditions. In the past few years, this optical effect, owing to its high sensitivity, real-time detection, and anti-interference has already been extensively investigated and applied in medical treatment, environment monitoring, biomedical sensing and so on. Based on the principle of SPR, a novel D-shaped gold surface plasmon resonance photonic crystal fiber with one open-ring is proposed for detecting low refractive index materials has been investigated in detail. The proposed photonic crystal fiber of the simulation model is composed of three layers of air holes. The radii of air holes in the first-layer and third-layer are r1 and r3, respectively. While the second-layer air ring consists of air holes with two different radii, r2 and rs. The refractive index of air is fixed at nair = 1 and the radius of the cladding is R. A thin gold film with thickness tg is deposited on the inner surface of the micro-opening analyte channel on the upper side, the radius and the central location of the channel are rs and 2.5×Λ-1.25×rs, respectively. The fiber material is fused silica and the RI is determined by the Sellmeier equation, the relative dielectric constant of gold can be demonstrated by the Drude-Lorentz model. This paper uses the finite element method and sets the boundary conditions of the perfect matching layer for simulation. In order to investigate how the sensing performance of the proposed PCF-SPR sensor is affected by the parameters of the optical fiber, the effect of various parameters of the fiber such as air radii (r1, r2, r3, rs), air hole spacing (Λ) and the gold film (tg) on the SPR loss spectrum have been studied separately. The simulation results show that the confinement loss decreases as r1 increases. This can be attributed to the fact that more energy is confined to the core when r1increases, which affects the coupling between the core and plasmonic modes. At the same time, the confinement loss also decreases with the increase of r2, and the corresponding blue shift occurs with the resonance peaks moving toward a shorter wavelength over the process. The reason is that the increase of r2 will increase the refractive index difference between the plasmonic mode and core mode, which will affect the coupling between them. Therefore, with the increase of r2, the shorter wavelength can excite the plasmonic mode, resulting in the phenomenon of wavelength blue shift in the loss spectrum. Since the air holes of the third layer are located at the outermost part of the fiber, the change of r3 has little impact on the confinement loss, which can greatly reduce fabrication difficulty of the sensor. The pitches between the air holes are also an important factor in confinement loss, the change of Λ will influence the refractive index of core mode and plasmonic mode, which in turn affects the phase matching condition and energy coupling between them. The thickness of gold film plays a vital role in the sensing performance. If the gold film is too thick, the electric field can not penetrate the gold film, which will reduce the sensitivity of the proposed sensor. While if the gold film is too thin, the plasmonic wave will be strongly suppressed due to radiation damping. Therefore, the thickness of gold film can significantly affect the coupling between the core mode and the plasmonic mode. After optimizing the various parameters affecting the sensing performance of the sensor, we analyse the analytes with different refractive indices. Simulation results show that the sensor operates in the near-infrared and mid-infrared region with the wavelength range of 2 020~3 036 nm in the refractive index range of the analyte of 1.18~1.30. When the refractive index of the analyte is in the range of 1.23 to 1.30, the sensor operates in the band of 2 135~3 036 nm, and the average value of spectral sensitivity is up to 11 650 nm/RIU. When the refractive index of the analyte is between 1.29 and 1.30, the sensor operates in the mid-infrared band of 2 648~3 036 nm, and the maximum spectral sensitivity and resolution are 38 800 nm/RIU and 2.37×10-6 RIU, respectively. The proposed sensor shows great significance in detecting low refractive indexes in near- and mid-infrared waveband, and has potential applications in biomedical sensing, water environment and humidity detection and so on.
Optical Orthogonal Frequency Division Multiplexing (OOFDM) technology is a new optical transmission technology. It is the product of the combination of Orthogonal Frequency Division Multiplexing (OFDM) technology and optical fiber communication technology, and has all the advantages of these two technologies. In recent years, OOFDM has become one of the research hotspots in the field of optical communication due to its unique advantages, especially the Coherent Optical Orthogonal Frequency Division Multiplexing (CO-OFDM) system. The CO-OFDM system has the advantages of high spectral efficiency, receiving sensitivity, and robustness, which make it become a research hotspot for realizing high-capacity, high-speed and long-distance optical fiber communication. The CO-OFDM signal is superimposed by modulated subcarrier signals. When the phases of the multiple subcarrier signals are the same, the superimposed signal power can be much greater than the average power, which can result in high Peak-to-average Power Ratio (PAPR). High PAPR not only causes nonlinear distortion when the system passes through optical amplifiers, DAC, ADC and other devices, but also leads to a decrease Bit Error Rate (BER).To solve the PAPR problem of CO-OFDM in OOFDM, the characteristics of related algorithms are studied. The clipping algorithm is the nonlinear processing of the signal, it will cause degradation performance such as high BER, low transmission rate, short transmission distance. An improved ICF algorithm is studied, which can improve the impact of the clipping algorithm on the system BER because of the iterative process. DCT transform is an orthogonal transform, which has a good effect of decorrelation and concentration the energy of the signal. DCT can change the correlation of the input sequence and further reduce the PAPR of the signal. A new structure of the DCT cascade improved clipping algorithm is proposed.From the time domain waveform diagram, the peak value of the original signal is 4.485, and the peak value of the DCT-ICF algorithm is 3.273. Compared with the original signal, the proposed algorithm effectively reduces the peak value of the time domain waveform by 1.212. From the perspective of the Complementary Cumulative Distribution Function (CCDF), which measures the PAPR distribution of the signal. When the CCDF is 10-4, the PAPR0 value of the original signal is 11.48 dB, the DCT algorithm is 8.022 dB, the ICF (CR=4, iter=4) algorithm is 6.93 dB, and the DCT-ICF (CR=4, iter=4) scheme is 6.795 dB. Compared with the original algorithm, the optimized amplitude of the DCT algorithm is 3.458 dB, the ICF (CR=4, iter=4) algorithm is 4.55 dB, and the proposed algorithm is 4.685 dB. When the BER is 10-3, the Optical Signal to Noise Ratio (OSNR) of the DCT-ICF (CR=3, iter=4) algorithm is 20.76 dB, DCT-ICF (CR=4, iter=4) algorithm is 20.96 dB, and DCT-ICF (CR=5, iter=4) algorithm is 21.47 dB, it can achieve long-distance transmission. And the BER can increase with the increase of the SMF transmission distance. The proposed algorithm has good BER performance. In addition to that, the total amount of computational amount of the proposed algorithm is 19 970.In this paper, the DCT algorithm and clipping algorithm are introduced in detail, and their principles are analyzed. According to their characteristics, the DCT-ICF algorithm is proposed. In terms of Power Spectral Density (PSD), the ICF algorithm improves the nonlinear distortion of the clipping algorithm in the system, and then improves the system performance. From the time domain waveform, the proposed algorithm reduces the signal peak significantly. According to the CCDF simulation curve, the proposed algorithm can effectively suppress the PAPR. From the transformation curve of BER with OSNR and the transformation curve of BER with SMF length, it can be seen that the proposed algorithm can maintain good performance during the transmission process. In consideration of the computational complexity, the proposed algorithm has a lower computational effort. In conclusion, the algorithm is feasible after considering various factors.
Fiber-optic Fabry-Pérot sensors have a wide range of applications, including aerospace, large-scale construction, oil collection, and many other fields. In many cases, dynamic parameters, such as dynamic pressure, vibration, acoustics, and ultrasonics are required to be measured. In order to measure these parameters, a variety of fiber-optic Fabry-Pérot sensors are produced. In some fields, the multi-cavity fiber-optic Fabry-Pérot sensor is inevitable for some advantages. For example, in the field of aerospace engine testing, dynamic pressure is a key parameter that often needs to be measured, and the micro-electro-mechanical system external Fabry-Pérot interferometer pressure sensors with multiple Fabry-Pérot cavities are often designed for aerospace engine pressure measurement due to their consistency and airtightness. Moreover, multi-cavity Fabry-Pérot sensors are good candidates for multi-parameter measurements. The different Fabry-Pérot cavities with different lengths are used to measure different parameters to achieve multi-parameter measurement. Therefore, multi-cavity Fabry-Pérot sensors are becoming increasingly important in engineering applications. However, extracting dynamic signals in multi-cavity Fabry-Pérot sensors is a challenge. In this paper, an improved passive three-wavelength phase demodulation technology based on a broadband light source is proposed for dynamic interrogation of the shortest cavity in a multi-cavity Fabry-Pérot sensor. According to the principle of low coherence interference, when the optical path difference introduced by the Fabry-Pérot interferometer is less than the coherent length received by the photodetectors, interference occurs. In contrast, when the optical path difference introduced by the Fabry-Pérot interferometers is longer than five times the coherence length, the interference phenomenon becomes insignificant and it can be considered that the interference disappears. Therefore, a flat-top amplified spontaneous emission light source and three broadband fiber filters were used to ensure the interference only occurs in the short cavity. The quadrature signals are obtained by three filtered optical signals with arbitrary cavity length using an improved phase calibration algorithm. The established signal calibration algorithm allows the demodulation technology for arbitrary short cavity lengths and arbitrary central wavelength. The demodulation technology can work without the direct-current voltages, so the demodulation system can reduce the fiber-optic disturbance noise. The arctangent algorithm is established to extract vibration signals by the quadrature signals. Compared with the previous phase calibration algorithm, the phase calibration algorithm proposed in the paper is more concise. The experimental system was consisted of a reflective bracket, a light source, a multi-cavity Fabry-Pérot interferometer, a fiber-optic coupler, three fiber filters, three photodiodes, an analog-to-digital conversion and a personal computer. The light from the light source passed through the fiber-optic coupler to the multi-cavity Fabry-Pérot interferometer. A multi-cavity Fabry-Pérot interferometer consists of a gradient-index lens and a 300-μm-thick double-polished quartz glass fixed on a piezoelectric transducer. The light reflected from the interferometer passed through the coupler and through the filters to the photodiodes. Three interferometric signals at each center wavelength were obtained using three photodiodes. The voltage signals were collected by analog-to-digital conversion and transmitted to a personal computer. The feasibility of the demodulation algorithm was verified by simulations and experiments. The experimental results show that the vibration signals with a frequency of 1 kHz and peak-to-peak amplitude of 2.6 μm is successfully extracted with different Fabry-Pérot cavity length, which proves that the three-wavelength demodulation algorithm can be used for optical fiber multi-cavity Fabry-Pérot sensor with arbitrary short cavity length. The demodulation speed is 500 kHz and the demodulation resolution is 0.25 nm. The demodulation technology makes it possible to extract dynamic signals in a multi-cavity Fabry-Pérot sensor. If the spectrometer is used at the same time, the dynamic signal measured by the short cavity and the static signal measured by the long cavity can be interrogated at the same time. This demodulation technology has the advantages of a compact system, low cost, fast speed and high robustness, illustrating its bright potential for multi-cavity Fabry-Pérot sensors.
The resonant micro-optical gyroscope has advantages in miniaturization and integration compared with other gyroscopes. The back-reflection noise is one of the main optical noises restricting the sensitivity of the resonant micro-optical gyroscope which mainly comes from the coupling points between the waveguide ring resonator and the tail fiber. The model of the back-reflection noise of the resonant micro-optical gyroscope based on the reflection-type waveguide ring resonator is established. The influences of the intensity item and the interference item of the back-reflection noise under the reciprocal system and the nonreciprocal system are analyzed, respectively. When the resonant micro-optical gyroscope system is reciprocal, the intensity item of the back-reflection noise has the same impact on the frequency deviation of the clockwise and the counter clockwise lightwave which counteracts, so it has no impact on measuring the rotation rate. When the system is nonreciprocal, the intensity item of the back-reflection noise introduces the noise of the magnitude of 10°/s. The interference item of the back-reflection noise introduces the noise of the magnitude of 257°/s and 261°/s respectively when the system is reciprocal and nonreciprocal. The suppression effects of the back-reflection noise in the resonant micro-optical gyroscope with different modulation techniques are compared. The separation modulation technique can suppress the intensity item of the back-reflection noise when using different modulation frequency and the interference item of the back-reflection noise can be suppressed below the shot-noise limited sensitivity when the carrier suppression reaches 120 dB which is achievable using four phase modulators. The reciprocal modulation technique can improve the reciprocity of the resonant micro-optical gyroscope and can suppress the residual intensity modulation noise of the phase modulator and the frequency noise of the laser effectively. When using the reciprocal modulation technique, the interference item of the back-reflection noise can be suppressed by carrier suppression but the intensity item of the back-reflection noise can not be suppressed which brings the noise of the magnitude of 10°/s according to the simulation result. So it is necessary to add the optical switch or the pulse modulator in the resonant micro-optical gyroscope system to suppress the intensity item when using the reciprocal modulation technique. The optical switch or the pulse modulator can separate the clockwise and the counter clockwise lightwave in time and avoid the energy coupling between the signal light and the back-reflection light which is equivalent to reducing the back-reflection coefficient. In theory, the intensity item and the interference item of the back-reflection noise can be suppressed totally but the suppression effect is limited by the channel crosstalk of the optical switch or the pulse modulator. According to the simulation result, the intensity item of the back-reflection noise can be suppressed below the shot-noise limited sensitivity when the crosstalk of the optical switch or the pulse modulator is 45 dB. To suppress the interference item of the back-reflection noise below the shot-noise limited sensitivity the crosstalk of the optical switch or the pulse modulator should be better than 115 dB which is difficult to achieve. The above analyses provide the theoretical basis for the establishment of the resonant micro-optical gyroscope system. The resonant micro-optical gyroscopes using the separation modulation technique and the reciprocal modulation technique are established, respectively. The outputs of the two system are tested in 1 800 s. The test results show that the gyro output is stable under the separation modulation system because the intensity item and the interference item of the back-reflection noise are both suppressed. The noise of the magnitude of 10°/s is introduced in the system using the reciprocal modulation technique because the intensity item of the back-reflection noise is not suppressed which is coincident with the simulation result.
Imaging fiber plays an important role in medicine, industry, aerospace and other fields because of its excellent flexibility, especially in the application of optical fiber endoscope in medicine. Optical fiber image transmission system is usually composed of imaging objective, imaging fiber and image sensor.At present, the number of pixels in cameras can reach millions or even tens of millions, but the number of pixels in optical fibers is usually only a few hundred thousand.Therefore, the resolution of the system is limited by the resolution of the imaging fiber itself, and the imaging resolution of the whole system basically depends on the number of pixels that the imaging fiber can transmit. At present, the imaging fiber bundles on the market have either high resolution but small total cross-sectional area, or large cross sectional size but fiber diameter up to ten microns. This phenomenon results in insufficient pixels and small image area of high resolution image fiber, while large cross section can not reach high resolution due to technological limitations. To solve the problems, this paper proposes a multi-aperture high-resolution imaging technology based on imaging fiber array, which uses the imaging fiber array and image Mosaic technology to break through the bottleneck of improving pixel number. The number of pixels in the system can be increased by using high resolution and small cross section imaging fiber arrays. Combined with the characteristics of overlapping imaging of microlens array, the problem of information loss caused by direct imaging of imaging fiber array can be solved and the integrity of optical fiber array imaging can be realized.This method is expected to increase the number of pixels in optical fiber image transmission system to millions of order of magnitude and improve the resolution of the system. The imaging fiber is designed to be arranged 6×8, and the microlens array is designed based on the imaging fiber array. There are two groups of aspherical lenses made of PMMA material, and the imaging fiber array and the two groups of microlens arrays have uniform positions. Add a telecentric objective lens in front of the microlens array as the main lens of the image transmission system to solve the problem of complete overlap of adjacent subgraphs caused by direct imaging of the microlens array. The focal length of the lens is 10.1 mm, the aperture coefficient is 6.3, and the field Angle is 88°. The simulation results show that both the main lens and the microlens array can meet the performance requirements of the imaging fiber, and the object information can be successfully transmitted to the imaging fiber. The modulation transfer function value of the system can reach more than 0.5 at 50 lp/mm, without weakening the quality of the primary image, and meet the resolution requirements of the imaging fiber. Experimental results show that the system contains 400 000 effective pixels and the system resolution is 40 lp/mm.The image is clear and complete, which proves that the design of the imaging system has a good feasibility, and has an important practical reference significance for improving the resolution of the optical fiber image transmission system.
Because the signal light is very weak and the communication link is easily interfered, space optical communication and space quantum communication require extremely high precision for the tracking system of the terminal payload. The accuracy of the fine tracking subsystem determines the tracking accuracy of the entire terminal system. Therefore, in the fine tracking stage, the system must meet the requirements of high precision and large bandwidth. However, the accuracy of traditional fine tracking systems is easily affected by external disturbances. To achieve higher tracking accuracy and stronger interference suppression capability, based on the traditional precise tracking system of typical optical communication terminals, a design method of an additional integrated module is proposed, and this module is cascaded after the PID controller of the traditional control system. Based on performance indicators such as control bandwidth, interference suppression capability, and stability, the non-dominated sorting genetic algorithm II is used to obtain the global optimal controller parameters, and a precise tracking system with intelligent parameter search is realized, which can achieve the tracking accuracy of sub-micro radian scale. Based on the measured angular interference data of a typical optical communication satellite terminal in orbit, the simulation compares the new system and the traditional system. The results show that on the basis of maintaining the stability of the closed-loop system, the new system can increase the error suppression bandwidth by 33.7% and improve the interference suppression ability of the full frequency band by 19.5%, of which the interference error suppression performance within 10 Hz is improved by more than 95%. For the four frequency bands of 0~1 Hz, 1~10 Hz, 10~50 Hz, and 50~100 Hz, the accuracy of the new system is improved by 99.5%, 95.7%, 71.3%, and 29.9% respectively compared with the traditional system. A physical verification system is built in the laboratory environment, and it is verified that the tracking accuracy and interference suppression performance of the system are greatly improved compared with the traditional system, especially below 10 Hz, the improvement rate is more than 20 times. When the interference frequency is 5 Hz and 10 Hz, the interference rejection ratio of the system reaches -53.57 dB and -46.31 dB, respectively. The experimental results and simulation results are consistent with a good fit. The system can be used in space optical communication scenarios with longer distances and higher precision requirements, which is of great significance to the development of the future space optical communication field.
Supercontinuum refers to the phenomenon that the spectrum of a high power pulse transmitted in a nonlinear medium is broadened by a variety of nonlinear effects. The generation of supercontinuum can greatly broaden the spectrum of optical signals, usually to the range of tens to hundreds of nanometers. In addition, supercontinuum light has the advantages of super brightness, wide band, high stability and high spatial coherence. Therefore, it has great application value in many fields, such as optical frequency calculation, optical communication, optical coherence tomography and biomedical science. In 1976, the supercontinuum in fiber was observed for the first time in nanosecond pulses generated by dye lasers, but the band coverage of the supercontinuum is narrow and the pump power required is high. The photonic crystal fiber made up for these deficiencies. Photonic crystal fiber, also known as microstructured fiber, is a special fiber whose cross section has two-dimensional periodic refractive index variation and extends indefinitely along the fiber axis. Photonic crystal fibers exhibit many special optical properties due to the variation of refractive index contrast between core and cladding. Photonic crystal fiber plays an important role in optical fiber communication, optical fiber sensing, meteorology, medical imaging and supercontinuum generation due to its flexible structure and special optical properties. Photonic crystal fiber can obtain the position of zero dispersion point at the desired wavelength by adjusting the structure, thus generating supercontinuum spectrum based on various nonlinear effects. For the generation of supercontinuum in photonic crystal fibers, the excitation conditions and the optimization and adjustment of its flatness and spectrum width have always been the focus of research. Especially in engineering applications, supercontinuum has many specific requirements. Although some progress has been made in the study of visible to near-infrared supercontinuum in photonic crystal fibers, the reported spectrum broadening is still limited and the required fiber length is relatively long. In order to improve the spectral width and make it have higher application value, in this work, based on the simulation calculation to explore the influence of optical fiber air hole structure size on dispersion, the optimized optical fiber structure geometric parameters are obtained. After independently designing the optical fiber, a kind of solid-core photonic crystal fiber with high nonlinearity is obtained by using the stack method. The full vector finite element method was used to simulate the photonic crystal fiber, and the zero dispersion point of the photonic crystal fiber was obtained at 880 nm. At the pump wavelength, the photonic crystal fiber has a nonlinear coefficient of 33.67 km-1?W-1 and an effective mode field area of 4.72 μm2. Through the establishment of a supercontinuum experimental device, a 1 030 nm, 150 fs linear polarization ultrafast fiber source is coupled into the photonic crystal fiber, and the coupling efficiency is 52.7% at low power. The generation process of supercontinuum from visible to near infrared region is studied under different pump power and different optical fiber length. It can be seen from the analysis that when the average pumping power increases in 0.5 m long photonic crystal fiber, the output spectrum broadening increases accordingly. At the maximum average pump power of 1 320 mW, a supercontinuum spectrum with a broadening range from 475 nm to 1 870 nm is obtained, and the flatness of the spectrum is improved compared with that at low pump power. By studying the effect of fiber length on the supercontinuum spectrum, it can be found that the supercontinuum is further broadened and flatness is improved with the increase of fiber length under the condition of constant average pumping power. Finally, the broadband supercontinuum output from 450 nm to 1 900 nm was achieved in the 1.5 m long photonic crystal fiber, and the spectrum has good flatness and coherence. Such broadband light sources have potential applications in optical coherence tomography, spectroscopy, communications, early cancer detection and food quality control.
Dynamic pressure measurements under high-temperature and other harsh environments, such as the pressure monitoring in aerospace engines, on-line health monitoring and control of molten salt reactors and gas-cooled reactors in nuclear applications, in-cylinder pressure monitoring in the automotive internal combustion engines, have a wide range of application requirements. The sensors used for dynamic pressure measurement include electronic pressure sensors and fiber-optics sensors. Among them, electronic sensors are highly dependent on temperature or close proximity electronics, limiting their high-temperature capabilities. In order to effectively protect electronic pressure sensors used in harsh environments, the engineering solutions, such as impulse lines, have been used to isolate sensors sensitive to heat and corrosion. But the impulse lines can also dampen the pressure signals, which will make it difficult to achieve in situ dynamic pressure monitoring, and increase the likelihood of blockages or bubbles impacting pressure measurements. Compared with electronic pressure sensors, fiber-optic pressure sensors have attracted widespread attention due to their high-temperature resistance, high sensitivity, anti-electromagnetic interference, corrosion resistance, simple structure, and small size. At present, most of reported fiber-optic pressure sensors are used for static pressure measurement and various types of fiber-optic FP pressure sensors have been fabricated using the MEMS, chemical corrosion, arc-discharge, laser processing techniques. Among them, the pressure sensors fabricated by chemical corrosion, arc discharge and laser processing technology are usually produced in a single piece, which results in poor consistency between sensors. By contrast, the MEMS technique can be applied in mass production and the materials used to fabricate fiber-optic pressure sensors by MEMS technology mainly include silicon, Pyrex glass, sapphire. Due to the limitation of temperature resistance of the material itself, the pressure sensors made by silicon-glass bonding will have a lower operating temperature. Pressure sensors made of sapphire can withstand high temperatures, but adhesives are usually used to connect the sensitive head and the signal transmission fiber. And if different materials are used to fabricate the fiber-optic sensor or use adhesive to realize the connection between the sensor head and optical fiber, the mismatch of the Coefficients of Thermal Expansion (CTE) between different materials will easily reduce the sensor stability and result in large temperature cross-sensitivity in the high temperature environment. In this paper, we propose a MEMS-based all-silica fiber-optic Fabry-Perot dynamic pressure sensor used the silica wafer with ultralow CTE and softening point as high as about 1 750 ℃. The sensor heads are batch-fabricated with silica wafers using MEMS technique and three-layer silica direct bonding technology, which ensures consistency in the sensor heads and cost effectiveness and have the desired pressure measurement range and sensitivity by flexibly designing the related parameters. The all-silica adhesive-free integration between the sensor head, hollow silica tube and the optical fiber is achieved using CO2 laser fusion. The sensor exhibits an ultralow thermal drift (about 0.069 nm/℃) and good thermal stability owing to the low CTE of silica and the all-silica adhesive-free design, which can effectively avoid the sensor damage induced by the CTE mismatch of different materials at high temperatures and increase the lifetime of the sensor in high temperature environments. To investigate the high-temperature static pressure performance of the all-silica pressure sensor, a static test system was set up and the system includes a high temperature and pressure testing platform, a demodulator, and a personal computer. High-temperature static pressure experimental results show that the proposed all-silica fiber-optic pressure sensor can function under the temperature range from 23 to 800 ℃ with the nonlinearity of approximately 1.13% at 800 ℃ and exhibited a good linear response to pressure at high temperatures, and the pressure sensitivity at room temperature and 800 ℃ was 810.84 nm/MPa and 755.52 nm/MPa, respectively. At the same time, a dynamic test system was set up and the system includes the standard piezoelectric sensor, the sinusoidal pressure generator, and a personal computer. Room-temperature dynamic pressure experimental results show that the proposed all-silica fiber-optic pressure sensor can function under the 2 kHz dynamic pressure environment and exhibited good dynamic pressure response characteristics. Furthermore, the frequency response of the all-silica fiber-optic pressure sensor is in good agreement with the standard piezoelectric sensor. We believe that the proposed all-silica fiber-optic FP dynamic pressure sensor will find broader and more promising applications in dynamic pressure measurement fields at a high temperature and extreme environments due to its low cost, small size, batch-production, and ultralow temperature coefficient.
Strain measurement is an important problem in bridge health assessment. Fiber Bragg grating is a kind of optical fiber passive components with wavelength modulation effect, and has small volume, high corrosion resistance, good electrical insulation measurement precision, good stability, short response time, and the advantages of distributed measurement. Along with the development of technology, optical fiber Bragg grating sensor for structural health monitoring, space exploration, power system measurement, medical equipment improvement, and other fields has been widely concerned. However, in the process of monitoring bridge strain, the sensitivity of FBG strain sensor will decrease due to long-time use. The resistance strain gauge has the advantages of high precision, good consistency and good repeatability, and is commonly used as the strain sensor. Therefore, in practical process, the resistance strain gauge can usually be used as the reference sensor and the FIBER Bragg grating strain sensor as the sensor to be calibrated. Therefore, an in-situ calibration method is proposed for the accuracy of fiber Bragg grating strain sensor in measuring bridge structural strain. The resistance strain gauge was used as the reference sensor, and the FIBER Bragg grating strain sensor was used as the sensor to be calibrated. The temperature was kept unchanged to avoid the wavelength drift of the fiber Bragg grating strain sensor caused by temperature change. The cantilever beam with equal strength is used to simulate the bridge structure, and the weight is used as the excitation source to simulate the loading and unloading process of the bridge load. By referring to the strain measurement of the response of the sensor and the sensor to be calibrated, the two sensors are installed in a close position to measure and compare. In the actual bridge measurement process, due to uncontrollable environmental factors affecting the measurement results, abnormal jump points appear in the strain output waveform, which can affect the calibration coefficient generated by the actual data matching results, so it is necessary to remove abnormal jump points. In addition, due to the difference in characteristics of different sensors, there is usually data dislocation between the measured value sequence of the calibrated sensor and the reference value sequence, which makes it impossible to judge the consistency of the strain response of the sensor. Therefore, based on the data sequence matching method of dynamic time warping algorithm, the strain data of resistance strain gauge and fiber grating strain sensor are matched and analyzed, and the data matching rate is above 96%. The results show that the method can effectively solve the problem of whether the strain response of the sensor is consistent, and can realize the in-situ calibration of the FIBER Bragg grating strain sensor, and the in-situ calibration results are basically the same as the static calibration results.
With the continuous development of basic theories and high performance devices, distributed optical fiber acoustic sensing technology has gradually transitioned from the qualitative detection stage to the quantitative detection stage and has been widely used in various applications. Demodulation of disturbance signal is indispensable for quantitative detection, so many demodulation methods intended for optical fiber sensing systems have been derived. The PGC (Phase Generation Carrier) technology has been widely used in optical fiber acoustic sensing systems due to its outstanding advantages of simple structure, high sensitivity, wide dynamic range, good linearity, and strong timeliness. According to the demodulation method type, the PGC technology can be mainly divided into the PGC-DCM (Differential and Cross Multiplying) algorithm and the PGC-Arctan algorithm. In terms of demodulation performance, the PGC-DCM algorithm has relatively high requirements on a circuit and is susceptible to interference from hardware facilities, especially system light intensity disturbances. In contrast, the PGC-Arctan algorithm is less sensitive to light intensity disturbances and has relatively low circuit requirements, but it has high requirements for the modulation depth of the system and faces the problem of phase unwinding. To address this limitation of modulation depth, this study proposes a method that introduces the differential self-division operation in the traditional PGC-Arctan algorithm framework to eliminate the influence of the modulation depth. Theoretical analysis shows that by introducing the differential self-division operation, the first-order and second-order Bessel function values of C can be eliminated, thereby eliminating the influence of the modulation depth drift on demodulation. Then, the original vibration signal is obtained through the square root operation and the arctangent operation. Using the simulation platform to analyze and compare the proposed algorithm and other common algorithms from two aspects: different modulation depths and different SNR. Calculate the amplitude error and the total harmonic distortion of the four algorithms with different modulation depths and signal-to-noise ratios respectively. The amplitude error measures the linear distortion of the demodulated signal, and the total harmonic distortion measures the nonlinear distortion of the demodulated signal. The simulation results show that the amplitude error of the proposed algorithm is less than 0.150% and the total harmonic distortion is less than 0.100%. And with the continuous reduction of the SNR, the linear distortion and nonlinear distortion of the demodulated signal do not fluctuate greatly, indicating that the proposed algorithm has a certain anti-noise performance. An experimental platform is built to verify the simulation results and further study the performance of the proposed algorithm. Firstly, the four algorithms are analyzed and compared under different modulation depths. Comparing the minimum value of the amplitude error and the total harmonic distortion, the minimum amplitude error of the PGC-DSVV (differential self-division) algorithm is 0.105%, which is 0.525%, 0.858%, and 2.900% lower than the other three algorithms, respectively. The minimum total harmonic distortion of the PGC-DSVV algorithm is 0.068%, which is 0.101%, 0.662%, and 0.595% lower than the other three algorithms, respectively. After that, the dynamic range of the proposed algorithm is analyzed. The experimental results show that demodulated signal bandwidth and dynamic range of the PGC-DSVV algorithm are larger than other algorithms. The dynamic range of PGC-DSVV is 62.5 dB when the frequency is 200 Hz, which is 6.1 dB, 11.7 dB, and 14.5 dB higher than the other three algorithms, respectively. And the dynamic range is 31.5 dB when the frequency increases to 5 kHz. The comprehensive analysis results verify that the proposed algorithm can effectively eliminate the modulation depth influence, indicating its good stability and noise resistance. Therefore, the proposed algorithm has a good application prospect in high-performance optical fiber acoustic sensing systems.
With the continuous development of international communication and industrial testing field,optical fiber cables are often in harsh working environments with high temperature, high humidity and high pressure, and will be used in deep-sea signal transmission, urban communication network construction, petrochemical smelting, national defense and military industries. Its application environment puts forward higher and higher requirements for optical fiber strength. The strength of conventional silica fiber can not meet the requirements of harsh environments, which restricts the further expansion of its market application range. Theoretically, using the bond length and surface energy between fused silica atoms, it can be calculated that the theoretical maximum breaking force of standard single-mode fiber is 203 N, while the average breaking force of commercial single-mode fiber is 47.6 N, which is far less than the theoretical maximum breaking force. The main reason is that during the optical fiber manufacturing process, it can inevitably experience the thermal and cold changes of high-temperature fused silica to accumulate internal stress inside the optical fiber. All will cause micro-cracks on the surface of the fiber, reducing the strength of the fiber. Therefore, suppressing the micro-cracks on the surface of the optical fiber and effectively improving the strength of the silica fiber have become the key exploratory areas by researchers. This article uses the passive single-mode quartz preform provided by Zhongtian Technology Co., Ltd(diameter 35 mm, core NA 0.14). The experiment is designed by an online active temperature-controlled annealing furnace to reduce the temperature difference between the surface temperature and room temperature when the fiber is released from the furnace, eliminate the internal stress of the fiber and inhibit the generation of micro-cracks on the surface and inside of the fiber. The newly installed online active temperature control annealing furnace has a length of 600 mm, and the furnace body has built-in three-stage heating wire, which can realize the temperature adjustment of 0~600 ℃ inside the furnace body. Acrylate was used as the coating material, and the fiber was drawled online by UV curing. The fiber cladding diameter was 125±1 μm, the coating diameter was 245±5 μm, and the coating/cladding concentricity error was less than 10.0 μm. The breaking force of the optical fiber is the reference standard to measure the strength of the optical fiber. According to international standard, the average breaking force of optical fiber is tested by universal tensile testing machine. The running speed of the tensile testing machine was 50 mm/min, and 15 samples were selected for each set of tests, and the length of each sample was 1 m. Different preform pretreatment processes,drawing speeds and active temperature control annealing processes are measured in experiment. The surface morphology of preforms and fibers with different treating conditions were characterized by reflective optical microscope (OLYMPUS, BX53M) and Scanning Electron Microscopy(SEM,ZEISS-EVO-18). The influencing factors of optical fiber breaking force were analyzed and studied. The result shows that average breaking force of the fiber behaves a downward trend with the increasing of drawing speed. Through the analysis of the fracture curve of the optical fiber Weibull function under different process conditions, with the optimization of the process conditions, the tensile force of the optical fiber increases but the sample consistency deteriorates. The micro-cracks on the surface of optical fibers and preforms can be effectively suppressed and average fiber breaking force was increased from 36.69 N without any treatment to 68.28 N, and the breaking force increased by 86%, through flame polishing and gradient cooling treatment on preforms, optimizing the active temperature control annealing process and decreasing the drawing speed. Relevant experimental surfaces carried out flame polishing pretreatment on the preform and optimization of the annealing process during the drawing process, while reducing the fiber drawing speed, which can effectively improve the average breaking force of the fiber. The research has opened up a wider application space for high-strength optical fibers in harsh environments such as oil exploration, submarine optical cable laying, and climate monitoring.
Hydrogen is explosive and hydrogen sensors are used in hydrogen monitoring work. The hydrogen sensor films used in previous hydrogen monitoring work were WO3-based and Mg-based hydrogen sensor films, which only available in an aerobic environment. Hydrogen sensing films for monitoring hydrogen concentration in an oxygen-free environment remain to be further investigated. Tantalum is stable in nature and has a high solubility for hydrogen in oxygen-free environment. In this paper, 40 nm Ta0.88Pd0.12~10 nm Pd~6 nm Pt~40 nm PTFE multilayer films were deposited on the end face of single mode optical fiber for hydrogen concentration monitoring for the absence of oxygen. The reflectivity of the deposited film under different hydrogen concentration was probed by the sensing demodulator. The sensing performance were investigated by a series of hydrogen sensing experiments. Firstly, the sensing film are designed for hydrogen sensing in oxygen-free environment. The Ta0.88Pd0.12 thin film is used as basal layer for sensing. Palladium film can improve the selectivity of hydrogen sensing film. Tantalum and palladium absorb hydrogen and become TaHx and PdHx. This phenomenon will result in a decrease in the reflectivity of the film, so that hydrogen concentration can be monitored by the change of reflected light intensity. Platinum film has good catalytic effect and excellent oxidation resistance, so it is employed as a protective layer. PTFE is hydrophobic and can hinder the adsorption of water molecules on the surface of the hydrogen sensing film. Moreover, it has good stability under various ambient environment, which can reduce the negative influence of temperature and humidity. The hydrogen sensing probe was fabricated by magnetron sputtering aforementioned multilayer films. The microscopic morphology of hydrogen sensing film was characterized by scanning electron microscope. Elements of hydrogen sensing thin film were analyzed by energy dispersive spectrometer. The phases of hydrogen sensing film were analyzed by X-ray diffractometer. Secondly, a fiber optic hydrogen sensing system based on Ta-based hydrogen sensing film was constructed, including amplified spontaneous emission light source, attenuator, coupler, spectral acquisition module, reference fiber grating and the fabricated sensing probe. The spectral response of the reference fiber grating with high-reflection was acquired by a compact spectral acquisition module with the range of 1 520~1 570 nm. The Reflection peak intensity (I1) and background intensity (I2) were obtained simultaneously. Reflection peak intensity (I1) of the high-reflection fiber grating is hardly affected by the reflectivity of hydrogen sensing film and is used as the reference signal. The ratio of I1 over I2 is traced as main measuring parameter to enhance the signal noise ratio of sensing system and to suppress the other noise induced by light source fluctuations, insertion loss, and fiber bending. Finally, we investigated the hydrogen sensing performance of the fabricated sensing probe. The probes are characterized in different hydrogen concentration provided by a gas mixer including two gas flow meters with N2 as carrier gas. A series of experiments are carried out to verify the sensitivity and repeatability of the fiber optic hydrogen sensing system with the proposed Ta-based probe. Three on/off cycles under a hydrogen concentration of 3 000 ppm are conducted. When the sensor is put in nitrogen, the value of I1/I2 is on a lower level. When the hydrogen with a concentration of 3000 ppm is turned on, the value of I1/I2 rises to a higher value each time. The results have shown the sensor has a good repeatability and recovery during hydrogen on/off cycles. Multiple experiments under gradient hydrogen concentration with a lower range of 100 ppm~1 000 ppm and a higher range of 1 000 ppm~20 000 ppm show that the different hydrogen sensitivity for different hydrogen concentration ranges. When the hydrogen concentration is in the range of 100 ppm~1 000 ppm, the sensitivity of sensor probe is the largest. The theoretical resolution is 20 ppm in the range of 100 ppm~1 000 ppm hydrogen concentration. This is because the hydrogen sensing film can easily reach saturation in the absorption of hydrogen at high concentrations of hydrogen. As the hydrogen concentration increases over 1 000 ppm, the reaction rate of the sensing film with hydrogen becomes slower. The result implies the sensor probe presents better sensitivity towards lower hydrogen concentration. In conclusion, the sensor probe proposed in this paper has the potential to monitor hydrogen concentrations in an oxygen-free environment and is suitable for monitoring the change of low concentration hydrogen gas.
Optically pumped spin-polarized vertical-cavity surface-emitting lasers might provide properties superior to electrically pumped vertical-cavity surface-emitting lasers, such as faster modulation dynamics, lager modulation bandwidth, lower threshold, and stronger polarization determination. New applications of optically pumped spin-vertical-cavity surface-emitting lasers are foreseen in high-speed optical communication, optical information processing, data storage, quantum computing and biochemical sensing. Various forms of ultrafast instability are observed in optically pumped spin-vertical-cavity surface-emitting lasers, including periodic oscillations, polarization switching, and chaos dynamics. Due to weak material and cavity anisotropy, the output of electrically pumped vertical-cavity surface-emitting laser usually includes two orthogonal polarization components, which is beneficial to realize dual channel optical communication. Therefore, the dual-channel secure communications based on electrically pumped vertical-cavity surface-emitting lasers have received extensive attention in recent years. In most of these examples, the drive-response electrically-pumped vertical-cavity surface-emitting lasers system was always used for secure communications. In such a system, chaotic x-polarization component and y-polarization component were yielded via the introduction of the external perturbations, typically feedback in the drive vertical-cavity surface-emitting laser. The response vertical-cavity surface-emitting laser yielded similar chaotic x-polarization component and y-polarization component when its parameters were identical to those of the drive vertical-cavity surface-emitting laser. Moreover, chaos synchronization between each pair of polarization components played a key role in security and encrypted message recovery. However, the realization of high-quality chaos synchronizations relies on the assumptions that the drive and response electrically pumped vertical-cavity surface-emitting lasers are completely symmetrical in structure, and their parameters match perfectly. In addition, previous works showed that due to the existence of two polarization components in drive and response vertical-cavity surface-emitting lasers, the structural symmetry of these two lasers is broken, which leads to the degradation of the quality of chaos synchronization. Under this asymmetric structure, high-quality chaos synchronization can be received by limiting a certain delay difference between the self-feedback delay of the drive vertical-cavity surface-emitting laser and the channel delay. The assumptions and limits as described above do not hold in practice. Realization of high-quality chaos synchronization will meet much more challenges in practice due to the inevitable imperfect match between the driving and response vertical-cavity surface-emitting lasers, and the variation of the delay difference at any time.Optically-pumped spin-vertical-cavity surface-emitting laser has better controllability for polarization switch, which is conducive to the realization of two parallel reservoir computers. Moreover, it can yield ultrafast chaotic dynamic without feedback or subject to short feedback delay, thus forming very short spacing between two virtual nodes under sufficient nodes, denoting two reservoir computers using two chaotic polarization components of optically-pumped spin-vertical-cavity surface-emitting laser can deal with two high-speed chaotic time-series in parallel data. In this work, we utilize two parallel reservoir computers using the two polarization components of an optically pumped spin-vertical-cavity surface-emitting laser with both optical feedback and optical injection, to model the chaotic dynamics of the output two polarization components from another optically pumped spin-vertical-cavity surface-emitting lasers as a transmitter. High-quality chaotic synchronization between a transmitting polarization component and its corresponding trained reservoir can be realized by training vertical-cavity surface-emitting laser-based reservoir. Under such a synchronization condition, we demonstrate the successful dual-channel secure communications with 16QAM messages under guaranteeing their securities. We further discuss the bit error ratio performances for two decoded messages under different parameters. We demonstrate that all bit error ratio via different parameters keep at 0. Our findings show that a delay-based optical reservoir computing provides an effective method for the practical application of optical secure communication.
The ocean is abundant in chemical and power resources, it is the space for human survival and development. Underwater Wireless Communication (UWC) technology realizes the wireless transmission of ocean exploration information, and has received extensive attention from researchers in recent years. Currently, radio frequency communication technology and acoustic communication technology are two comparatively mature technologies in seawater communications, in which the problems conceal. In the radio frequency communication, the large volume of transceiver, the high cost and energy consumption, and the rapidly attenuated radio wave underwater would make it impossible to achieve long-distance transmission and high-speed underwater communication. As for the acoustic communication technology, its large-size equipment, high power consumption, low transmission rate, limited available bandwidth, and severe multipath effects during transmission, would cause speed limits of the data transmission and the increase in bit error rate. With the advantages of no electromagnetic radiation, fast speed, strong mobility, good safety, high bandwidth and green environmental protection, the underwater wireless optical communication has become a new choice for underwater sensor data transmission and acquisition of marine monitoring information, thus playing a paramount role in the detection in underwater environments and development of marine resources. Specifically, the lower loss of blue-green light caused by seawater absorption and scattering can help to reach the underwater transmission rate as Gbit/s. Therefore, blue-green light is used to carry information to realize long-distance underwater transmission. However, as the light beam propagating in seawater, not only will it be affected by the attenuation effect of the absorption and scattering of the seawater impurities, but also easily affected by ocean turbulence caused by fluctuations in refractive index. Ocean turbulence will lead to a series of problems, including light intensity flicker, beam drift, beam expansion, wavefront distortion and other effects, turning to the consequence of the optical signal fading, degrading the communication quality and deteriorating its performance.In previous studies, turbulence was regarded as isotropic, in which the vortex structure (that is, the spatial frequency of turbulence) was symmetric in different directions, and isotropic turbulence was a simple and idealized model. The ocean turbulence occurring naturally is often anisotropic. Ocean turbulence is composed of eddy structures with different sizes and frequencies (that is, eddy structures are asymmetric in different directions). Therefore, this paper considers the asymmetry of ocean vortex motion (that is, the case where the horizontal scale of the vortex is much larger than the vertical scale), as well as uses ocean turbulence parameters and anisotropy factors to express the equivalent structural parameters of ocean turbulence, applying the equivalent structural parameters of ocean turbulence expressed by ocean turbulence parameters and anisotropy factors, the extended Huygens-Fresnel principle and the asymptotic Rytov theory are also used to derive the average received optical power and radiation of a finite-size detector. Meanwhile, the paper also conducts research on the degree of flux variance, and the packet error rate performance of the double-headed pulse interval modulated Gaussian beam in anisotropic ocean turbulence under the Gamma-Gamma turbulence channel model. DHPIM has its built-in symbol synchronization and slot synchronization functions. Compared with PPM and DPIM, DHPIM has shorter symbol length, higher transmission rate, larger transmission capacity, higher bandwidth requirements and better ability to resist multipath dispersion. In the simulation analysis of ocean turbulence parameters (temperature variance dissipation rate, turbulence energy consumption) under different anisotropic ocean turbulence Spread rate, the ratio of the contribution of temperature and salinity fluctuations to the power spectrum), bit resolution and transmission rate, photodetector responsivity and the impact of link distance on the packet error rate, the results come out and indicate that: no matter what kind of ocean turbulence parameters, the performance of the wireless optical communication system is all proportional to the anisotropy factors in the x-direction and y-direction. For different anisotropy factors, when the temperature variance dissipation rate χT decreases, the ratio of temperature and salinity contribution to the power spectrum ω decreases, or the turbulent energy dissipation rate εincreases, the intensity of ocean turbulence can be weakened, and the system decreases accordingly. For the same bit error rate level in the isotropic ocean turbulence, the wireless optical communication system in the anisotropic ocean turbulence can achieve a longer transmission distance. Simultaneously, approaches such as reducing the bit rate, increasing the responsivity of the photodetector, applying a smaller modulation order while appropriately using a larger diameter aperture to receive the average signal fluctuations, would resist the impact of ocean turbulence and reduce the system's packet error rate effectively . This study provides a certain reference value for improving the performance of underwater wireless optical communication systems in an anisotropic ocean turbulent environment.
With the development of science and technology, people have higher and higher requirements for the capacity of network communication. Conventional single-mode single-core optical fibers have gradually approached the Shannon transmission limit of 100 Tbit/s. mULTICORE FIBER (MCF) based on Space Division Multiplexing (SDM) technology has good application prospects for the problem of upcoming capacity shrinkage. However, a specific issue related to MCFs is Inter-Core Crosstalk (ICXT) which can degrade signal quality and limit SDM performance significantly. In real MCF, due to the random perturbations and additional birefringence fluctuations, the ICXT changes randomly in the longitudinal direction. Therefore, approaches to estimate the ICXT are required to analyze the performance of an MCF transmission system. Coupled Mode Theory (CMT) and Coupled Power Theory (CPT) are common methods for studying the coupling characteristics of weakly coupled MCFs. In this paper, the CMT and CPT are optimized considering the influence of stochastic bending and twisting perturbations in the actual fiber laying process, and the ICXT estimation expression is obtained. For the CMT, due to the consideration of bending and twisting perturbations, the equivalent propagation constant in the Coupled Mode Equation (CME) is not constant, but the relationship related to the transmission distance, bending radius, and twisting rate. Therefore, it is impossible to obtain analytical expressions by directly solving the CMEs. The MCF can be divided into N segments by the segmentation method. In each segment, the distance is small enough so that its equivalent propagation constant and the incident power can be approximately regarded as a constant in weakly coupled MCFs. The final ICXT result is obtained by superimposing N sections of ICXT which is obtained by solving the CME in a short section. We call this model the optimized coupled mode theory model (OCMM). For the CPT, the local power coupled coefficient can be obtained by defining the equivalent propagation constant which contains the influence of bending and twisting perturbations. And the optimized average power coupled coefficient can be obtained by averaging the twisting rate. Finally, the crosstalk estimation expression can be obtained by substituting the optimized power coupled coefficient into the coupled power equation and solving it. We call this model the optimized Coupled Power Theory Model (OCPM). Simulations are carried out to compare the above two optimized models with discrete change models and experimental data. The simulation and experimental data are in good agreement. The OCMM results that ICXT as a function of the MCF length show an upward trend of oscillation along the results of the Discrete Change Model (DCM), in which the crosstalk cumulatively increases near the phase matching point and is almost unchanged near the non-phase matching point. Therefore, the OCMM can better reflect the fluctuation of the physical structure of the fiber, while the OCPM shows linear average crosstalk. In addition, the ICXT as a function of the bending radius in the actual homogeneous and actual inhomogeneous MCF is simulated, and the results of the two models are in good agreement. However, in the actual homogeneous MCF, the simulation results of OCPM and OCMM will be somewhat different. This is because OCPM is a simulated average crosstalk, so OCMM is more accurate in this case. In the actual non-homogeneous MCF, the simulation results of OCPM and OCMM are in good agreement. In this case, the calculation speed of OCPM will be faster. In summary, this paper derives optimized crosstalk estimation models based on CMT and CPT, respectively, and verifies the correctness and accuracy of the theoretical model through comparative studies of simulation and experiment. We can select suitable theoretical models for different actual situations.
The traditional polarization-maintaining fiber alignment method is divided into longitudinal and lateral observation methods. However, both of them have severe defects. For example, the longitudinal observation method limits the application scenarios due to the destruction of the polarization fiber in the detection section. In contrast, the widely used lateral observation method requires high standards for experimental equipment and complex manual operations and alignment algorithms. Regarding the panda polarization maintaining fiber as the research object, instead of the traditional polarization-maintaining fiber alignment thinking of applying a high standard light source and adjusting the imaging surface. Based on applying the matching liquid mixed with glycerin and deionized water, the ideal refraction environment can be created due to the reduced effects of multiple spherical aberrations. Hence, high accuracy visualization can be achieved by projection from the incoherent parallel light sources. Then, applying the rotating motor with a fiber clamp setup and imaging system to form an experimental device, a long-distance section of the polarization-maintaining fiber can be aligned. Additionally, the LabVIEW platform can demonstrate the results of direct observation and real-time extraction of light intensity distribution information, which contains angle information of the polarization axis from the polarization-maintaining fiber. This information can also be called the characteristic value of the high accuracy alignment method, i.e., the three segment diffraction fringe spaces d2, d3, d4, which are sensitive to the polarization axis angle θ inside the polarization-maintaining fiber. According to the internal structure of the polarization-maintaining fiber and the geometric characteristics of the eigenvalue, the geometric relationship between the eigenvalue and the angle of the polarization axis θ of the polarization-maintaining fiber can be deduced theoretically. Furthermore, through computer processing of the experimental data and theoretical analysis of the light intensity distribution, the relationship between the characteristic areas and the angle of the polarization axis was obtained. Thus, real-time alignment between the polarization axis of the polarization fiber and the observation surface could be realized. In the case of the image processing algorithm, the collected information is primarily binarized and converted into gray-scale digital signals, and then the collected data are normalized to improve the signal-to-noise ratio and reconstruct the image edge by the two-dimensional wavelet algorithm. Due to these algorithm techniques, the accuracy of the alignment method has been improved significantly, the deviation ratio is better than 1.32%, and the accuracy of the alignment method is up to ±0.3°. Moreover, polarization-maintaining fibers might have several process defects, and the primary influencing factors are material purity and environmental impacts during the drawing manufacturing process. All of these defects lead to three typical asymmetries of the panda eyes distribution: the radii of the two panda eyes are inconsistent, the distance between the center of the two panda eyes is inconsistent, and the line of the center of the two panda eyes is not collinear with the line of the center of the fiber core. According to the analysis of geometric theory, the high-precision positioning method is also compatible with the errors caused by the asymmetry of the panda eyes distribution.
The optical fiber sensing system is widely used in many fields, such as long-distance oil and gas pipelines, tunnel safety detection, large structural safety detection, perimeter security detection and so on. Optical fiber sensing signal identification plays a key role in real-time monitoring, abnormal alarm and other aspects. Its working performance directly determines the performance of the real-time, accuracy and stability of the optical fiber sensing detection system. Therefore, fast and accurate identification and classification is of great significance to ensure the safety of various fields and reduce the cost loss caused by equipment damage.To improve the real-time and accuracy of optical fiber vibration signal pattern recognition, a feature extraction algorithm based on compensation distance estimation is proposed. The algorithm draws on the human auditory perception mechanism and extract the MEL frequency cepstrum coefficients from the optical fiber sensing vibration signals. The algrithm uses the compensation distance estimation technology to formulate the feature selection strategy,and finally realizes feature evaluation and optimization. The MFCC feature extraction algrithm can extract the feature of the vibration signal acquired by the optical fiber sensing system, and then identifies the interference signal according to the modal prediction.However, the extracted feature vector by the MFCC feature extraction algorithm has the problems of high dimension and vector redundancy . When these feature vectors are trained and recognized by the classifier, it will increase the time cost and reduce the recognition accuracy. Therefore, effectively reduce the dimension of the MFCC eigenvector is the key to improving the real-time performance and accuracy of optical fiber sensing vibration signals.This paper proposes a feature extraction method based on compensated distance estimation. The CDET algorithm jointly evaluates the intra-class and inter-class discreteness of eigenvectors. The feature evaluation is performed on different dimensions of the feature matrix, and the redundant vector of low score is deleted from the original feature vector matrix, thereby realizing feature dimension reduction. Solve the influence of redundant vectors on classification, avoid the problem of complex operation caused by too many extracted feature dimensions, and improve real-time performance.The steps of the algorithm are to calculate the average distance between samples of the same condition and different conditions , then average the within-class and between-class distances, and then calculate the within-class variance and between-class variance factors. Then calculate the compensation coefficient between the two variance factors, calculate the ratio of the inter-class distance to the intra-class distance, multiply the compensation coefficient to obtain the distance evaluation standard, and select a better feature dimension according to the distance evaluation standard. The experimental results show that the feature extraction algorithm of vibration signal based on compensation distance estimation technology can effectively reduce the redundant vectors that affect the classification accuracy in optical fiber sensing system. It solves the problems of feature representation and operation complexity for the vibration signal, and further improves the effectiveness and real-time performance of vibration signal pattern recognition of the optical fiber sensor system. Compared with Principal Component Analysis (PCA), our algorithm has the same performance in the low-dimensional case. With the increase of dimension, the performance of the algorithm in this paper is better in recognition accuracy. In terms of anti-noise performance, in the presence of noise, in order to improve the feature identification, the dimension of the feature vector extracted from the MFCC feature increases. The PCA method is difficult to distinguish the features caused by noise, and the algorithm in this paper can reduce the influence of superimposed noise by pruning redundant vectors, and can extract feature vectors with high feature recognition. Therefore, the proposed algorithm also has certain anti-noise performance.
With the development of modern communication technology, traditional electronic instantaneous frequency measurement systems face bottlenecks such as bandwidth and speed, and can no longer meet the needs of modern electronic warfare. This paper proposes a scheme of multiple microwave signal instantaneous frequency measurement. The light source is provided by a single laser and divided into two branches by a splitter. The upper branch microwave signal is modulated onto the optical carrier by a Mach-Zehnder modulator, then used as the pump light, and the lower branch is used to generate an optical frequency comb with high flatness and the same interval. The pump is split into multiple channels by a demultiplexer, and each frequency comb of the Optical Frequency Comb(OFC) is sent into the corresponding channel through the optical circulator. In each channel, the pumb and the frequency comb are sent into dispersion-shifted fiber simultaneously. The Brillouin scattering can be stimulated if the frequency of the pumb is a Brillouin frequency shift higher than the frequency of the comb, and brillouin gain occurs in this channel. Thus frequency-space mapping is achieved. The frequency of the microwave signal can be judged by monitoring the change in the intensity of the output optical signal of the corresponding channel. In the simulation experiment, the instantaneous frequency measurement of the single-frequency signal and the multi-frequency signal in the range of 0~25 GHz is carried out. The simulation results show that the output power of the channel with the microwave signal to be measured is significantly increased compared with other channels. The method can measure single-frequency or multi-frequency microwave signals from 0.1 GHz to 25 GHz with a resolution of 0.1 GHz and a measurement error of ±0.05 GHz. In addition, the influence of the offset point drift of the Mach-Zindel modulator on accuracy of the measurement is analyzed. The simulation results show that the scheme can suppress the influence of the offset point drift on the measurement results to a certain extent, which proves the feasibility and reliability of the scheme. Using the photon assisted method, the microwave frequency can be measured in large frequency range in real time, which has a wide measurement range and strong anti-electromagnetic interference ability, and has a broad application prospect in electronic warfare system.
For the field of ocean depth detection, a small volume diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure is made. The length and diameter of the fiber Bragg grating pressure sensor are approximately 40 mm and 20 mm, respectively. The diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure uses an ultrashort fiber Bragg grating string. There are two fiber Bragg gratings on the ultrashort fiber Bragg grating string. The length of the two fiber Bragg gratings is 1 mm, and the interval between them is 20 mm. One fiber Bragg grating is used to measure pressure, and the other fiber Bragg grating is only affected by temperature, which can eliminate the influence of temperature on the pressure measuring fiber Bragg grating. The optical fiber is encapsulated in a metal tube a short distance away from the measuring pressure fiber Bragg grating. The end of the metal tube is fixed on the elastic metal diaphragm by a laser welding process. In this way, the optical fiber and metal diaphragm are not fixed by epoxy adhesive directly, which can avoid the influence of aging and creep of epoxy adhesive on the performance of the diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure. In the measuring range of 0.6 MPa, the theoretical pressure sensitivity of the diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure is -1.214 nm/MPa, and the pressure sensitivity of the diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure obtained by the finite element analysis method is -1.364 nm/MPa. After the diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure is fabricated, the pressure and temperature characteristics of the sensor are tested. With the help of a fiber Bragg grating only affected by temperature, the influence of temperature on the pressure measuring fiber Bragg grating is eliminated through calculation. The actual average pressure sensitivity of the diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure is -1.728 nm/MPa. Moreover, the linearity of the boost and buck curves of the diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure is more than 99.9%, and the boost and buck curves of the diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure also coincide well. In addition, the best way of the tail fiber seal, the reason of the different temperature response characteristics of the double fiber grating, the method of improving the linearity and coincidence degree of the pressure curve, the reason and solution of affecting the stability of the sensor, and the reason of improving the sensitivity of the measured pressure are discussed. First, when the thickness of the metal tube that encapsulates the optical fiber is relatively thin, comparative experiments show that the method of sealing the fiber tail with epoxy glue is better than laser welding. By sealing the fiber tail with epoxy glue, the wavelength shift of the fiber Bragg grating can reach 2 nm. It can be seen that sealing the tail of the optical fiber with epoxy glue more easily maintains the prestress applied to the optical fiber. Second, a temperature response test experiment of a diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure is carried out. The experimental results show that the temperature response of the fiber Bragg grating and pressure response of the fiber Bragg grating are slightly different. Combined with the simulation analysis, it is found that the main reason for the difference in the temperature response trend of dual fiber Bragg gratings is the defect of the structural design. Third, to improve the linearity and coincidence of the boost and buck curves of the sensor, temperature and pressure aging processes are added to the sensor manufacturing process. The experimental results show that these methods are effective. Next, considering the influence of an optical fiber tail wobble on the stability of a diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure, it is suggested to use apodisated linearly chirped fiber gratings or set up a region to isolate external forces to solve the problem. Finally, the problem that the actual pressure sensitivity of a diaphragm-type fiber Bragg grating pressure sensor with a temperature compensation structure is higher than the theoretical value is discussed from several angles. The main reasons are that the effective length of the fiber Bragg grating decreases due to the flow of epoxy glue and the pressure sensitivity of the fiber Bragg grating increases due to the increase in prestress.
Traditional detection approaches usually employ Piezoelectric Transducers (PZTs) as the ultrasonic source and receiver. However, these current-driven transducers have some inherent drawbacks (susceptibility to electromagnetic interference, narrowband frequency response, and not resistant to high temperature and corrosion). Fiber-optic sensors have attracted significant attention in ultrasonic detecting owing to their outstanding advantages, such as: small size, easy reuse, wideband frequency response, and immunity to electromagnetic interference. The majority of fiber-optic ultrasonic sensors has based on fiber Bragg gratings and Fabry-perot interferometers. However, frequency response range of ultrasonic sensors based on FBGs is relatively narrow. The Fabry-perot interferometers ultrasonic sensors generally consist of a diaphragm and a fiber-optic end-face as two reflectors. Nevertheless, the complex preparation of diaphragm materials, poor chemical stability, and heat resistance, limit the application of sensor. In this study, a compact fiber-optic ultrasonic sensor based on a Tapered Seven-core Fiber (TSCF) is proposed and experimentally demonstrated. This proposed sensor has the advantages of easy fabrication, compact structure, and high sensitivity. The sensor comprises a TSCF sandwiched between two Single-mode Fibers (SMFs), forming a cascade structure of SMF-TSCF-SMF. The SCF (YOFC, MC1010-A, China) is used to make ultrasonic sensors. A commercial fiber fusion splicer (Fujikura, FSM-80C) is used to fabricate the SMF-TSCF-SMF structure. Thereafter, the optical fiber fused biconical taper system (FBTZolix) is used to taper the SCF into the TSCF with diameters of 11 μm, 19 μm and 29 μm. A certain prestress is applied to keep the SCF tight and straight during the fused tapering process. High order modes are easily excited owing to the core mismatch of SMF and Seven-core Fiber (SCF). The excited multiple modes continue to propagate along the SCF and then arrive at the tapered region. These transmission spectra exhibited multiple interference peaks. This is because complex optical modes are excited and are involved in mode interference. Therefore, these transmission spectra are not in a standard sinusoidal pattern, but become more irregular. Due to the sharply reduced taper diameter (as thin as several micrometers), the core distances are largely decreased and the evanescent fields are extended simultaneously. Thus, it is sufficient to induce diverse inter-modal coupling at the abrupt taper, including the mode coupling among cores, and coupling and recoupling of the cladding-to-core modes. Highly sensitive mode interferences are obtained. For the TSCF, the ultrasonic wavelength is much longer than the taper diameter and shorter than the fiber length. The fiber taper is axially constrained, that is, the axial elongation of the fiber taper can be neglected. The core and cladding diameters in the tapered region become thinner, and the TSCF has an obvious effect on evanescent waves. When the sensor is immersed in water, the Ultrasonic Wave (UW) signal periodically changes the refractive index of the surrounding liquid and modulates the transmission spectrum according to the evanescent-field interaction between the liquid and the transmitting light. Meanwhile, due to the effect of evanescent field, the light energy transmitted in the fiber can penetrate into the surrounding medium, resulting in energy reduction. Thus, the TSCF sensor with a diameter taper of 19 μm is used as the receiving source of ultrasonic signals. Driven by a function generator, the PZT (SIUI, 1Z20SJ50DJ) separately emits a 1 MHz continuous wave with a voltage amplitude of 10 V as the ultrasonic source. The edge filtering method is used to demodulate the ultrasonic signal received by the TSCF sensor. A tunable laser (Santec-710) with a 100 kHz linewidth and 0.1 pm tunable resolution was used as the light source. The output power of the tunable laser was 20 mW. The photodetector (New Focus, Model 2117) with a bandwidth of 10 MHz converts the optical signal into a voltage signal, which is finally monitored by an oscilloscope (RIGOL, DS2302A). The bandpass filter built into the photodetector has a frequency range of 500 kHz to 3 MHz, which is used to shield the surrounding noise. UW detection is processed in water at room temperature, which provides an almost constant temperature environment around the sensor. The sensor directly faces the emitting end of PZT with a separation of 2.5 cm. The continuous signals exhibit good uniformity and stability in the time domain. The peak-to-peak voltage of TSCF is about 0.4 V.
Recently, with the large-scale popularization of Light Emitting Diode (LED), Visible Light Communication (VLC) with LED as the emission light source has developed rapidly. This technology has the advantages of rich spectrum resources, no electromagnetic radiation, high confidentiality and deep coupling with lighting. As one of the important applications of visible light communication, visible light indoor positioning technology has attracted extensive attention of researchers. Among many indoor location methods, the location method based on Received Signal Strength (RSS) is the easiest to implement without additional hardware equipment. It is widely used in the field of indoor location. In the existing literature, the research of visible light positioning technology based on RSS mainly focuses on the multiple light sources model, however, due to the influence of channel attenuation on the received signal, the positioning accuracy is not high when the target receiver is located in the corner area. To solve this problem, an adaptive visible light location algorithm based on region division is proposed. Based on the analysis of single light source and multiple light sources localization algorithms, a two light sources localization algorithm based on symmetrical structure receiver is designed to make up for the localization error in the edge region. The mirror solution generated in the localization of two light sources is eliminated by using the receiver with this special structure. According to the error distribution characteristics of the above three algorithms in the positioning plane, the fairness function is constructed, and combined with the Lambert model, the positioning region is divided into multiple sub regions. In the positioning stage, the receiver area is roughly judged according to the characteristics of the received signal to achieve rough positioning, and then the positioning algorithm with better performance is adaptively selected to achieve accurate positioning. The simulation results show that at 5 m×5 m×3 m indoor environment, the average positioning error of the algorithm is about 2.5 cm, which is improved by 46%, 24% and 55% respectively compared with single light source, two light sources and multiple light sources positioning algorithms. Further, at 1.5 m×1.5 m×2 m indoor environment, an actual visible light positioning system is built, four synchronous label code signals are generated by Field-Programmable Gate Array(FPGA), and the LED light source is driven by the amplification circuit. The receiver uses Photoelectrical Detector (PD) to obtain the signal, the Microcontroller Unit (MCU) decodes it, and then realizes the positioning through the corresponding positioning algorithm. The experimental results show that 94% of the test points achieve the positioning accuracy of 5 cm. The positioning errors at the four corners of the positioning area are 3.2 cm, 3.4 cm, 3.5 cm and 2.8 cm respectively. Compared with the traditional multiple light sources location algorithm, the location error of edge and corner region is greatly improved. This research provides a new method for visible light positioning system, which can significantly improve the positioning accuracy at the cost of low complexity. It has potential research value in the field of visible light communication and positioning.
A cobweb topology sensor network was designed to meet the high requirements of large structure health monitoring on the reuse capacity and maintenance cost of fiber Bragg grating sensor network.In this structured network, Wavelength Division Multiplex(WDM) is used to increase the multiplexing capacity of the network, and the model based on gated cyclic unit is optimized to demodulate overlapping wavelengths.The new sensor network designed has high network reliability and network reuse capacity. Part of the structure of cobweb network is selected for experiment, and four kinds of fault conditions are designed for comparison. Through the four kinds of fault conditions, the signal can be effectively transmitted, which proves that the cobweb network has high reliability.In this paper, the four cases are summarized and summarized in the form of a table. The table shows that in these four cases, the network can still be used normally and has a high reliability.By improving the network structure of the demodulation model, the recognition accuracy of the model is increased, and the well-trained model is used to demodulate the spectra with different overlapping degrees. In 89.9% cases, the root mean square of the model is less than 1 PM, which proves that the improved model can effectively demodulate the overlapping spectra, and greatly increases the network reuse capacity.The experimental results of demodulation are presented in the form of tables and pictures. It can be seen that the central wavelength of each sensor can be well identified and the physical variables can be obtained under the condition of different degrees of spectral overlap.The new sensor network can increase the reliability and reuse capacity effectively.
Surface Plasmon Resonance(SPR) is a physical phenomenon, when the frequency and wave number of incident light coincide with the frequency of free electrons vibrating on the metal surface, then the electrons (i.e., plasma) on the metal surface absorb light energy and resonate, and its resonance wavelength changes with the refractive index of the precious metal surface, so SPR has a wide range of application needs in medical detection, environmental monitoring and other fields. Based on the principle of SPR, this work investigates the refractive index sensing characteristics of D-type highly birefringent photonic crystal fibers in detail. The current reports on PCF SPR sensors are mainly based on the establishment of theoretical models and numerical simulations. It is difficult to experimentally prepare PCF SPR sensors. According to the high birefringence photonic crystal fiber used in the experiment, the photonic crystal fiber of the simulation model is composed of five layers of air holes. The first layer of the cladding contains 2.2 μm, the diameter of the large air hole is 4.5 μm, and the polishing depth is represented by h, that is, the distance from the core of the photonic crystal fiber to the polishing surface, and the angle between the slow axis of the high birefringence photonic crystal fiber and the polishing surface is defined as the polishing direction θ. The gold film is coated on the flat polishing surface of the optical fiber to facilitate contact with the object to be measured. According to previous theoretical and experimental research, we set the thickness (t) of the gold film to 45 nm. The refractive index of the photonic crystal fiber background material and the refractive index of gold used in the simulation are given by the experimental data of linear interpolation. In order to obtain the waveguide mode of the side-polished high-birefringent photonic crystal fiber, this paper uses the finite element method commercial software COMSOL Multiphysics and sets the boundary conditions of the perfect matching layer for simulation. The refractive index of the analyte is set in the range of 1.330 to 1.400. Through finite element method modeling and simulation, the influence of polishing angle on the sensitivity of birefringence and refractive index sensing are studied in this paper. The simulation results show that when the height of the polishing surface from the fiber core is less than 1.5 times the duty cycle, the closer the polishing surface is to the core, the smaller the birefringence. As the polishing angle increases, the birefringence first increases and then decreases, and the refractive index sensing sensitivity decreases accordingly. When the polishing angle is 0 degrees and the refractive index ranges from 1.330 to 1.400, the average refractive index sensitivity of the device is as high as 3 457.14 nm/RIU. In addition, we have prepared a D-type high birefringence photonic crystal fiber SPR sensor. There is a big difference between the theoretical and experimental sensitivity values. The main reasons are: 1) The polishing surface is uneven (defects caused by air holes), which makes it difficult to completely remove the debris generated during the polishing process, which will affect the sensing performance. Performance; 2) After preparing the D-type fiber sample, the fiber is not coated in time. The D-type fiber is exposed to the air for a long time, and the dust in the air will further affect the performance of the device; 3) Although every time Before the test, the sensor will be cleaned repeatedly with ethanol, but it is difficult to completely remove the refractive index matching liquid left in the PCF air hole, which will affect the accuracy of the subsequent measurement results. For example, we tested the refractive index of 1.33 for the first time and cleaned it with alcohol. Drop 1.34 optical fiber matching liquid, because the previous liquid remains, the real value is difficult to achieve 1.34, and the calculation is still calculated according to 1.34; 4) The theoretical maximum value is under the condition that the polishing plane is parallel to the connection line of the two large air holes. The D-type high birefringence photonic crystal fiber SPR sensor was further used to test the concentration of glucose dissolved. The concentration of glucose solution increases from 0 g/dL to 10 g/dL in steps of 2 g/dL. As the glucose concentration increases, the D-type high birefringence photonic crystal fiber SPR sensor The peak wavelength of the pit of the transmission spectrum will be red-shifted. In the 0 g/dL glucose solution, the SPR resonance wavelength appeared at 578.96 nm, and when the glucose solution concentration was 10 g/dL, the SPR resonance wavelength drifted to 587.49 nm. According to the relationship between the glucose concentration and the peak wavelength of the pit of the transmission spectrum, the average sensitivity is 1.89 nm/(g/dL). The research results show that the D-shaped photonic crystal fiber SPR sensor can be applied to the fields of biology, chemistry and environmental monitoring.
Fiber optic SPR sensors have wide application prospects in the field of biosensing due to the advantages of label-free, fast response in real-time, and good biocompatibility. Traditional multimode fiber optic SPR sensors are limited by low sensitivity, which leads to their insufficient performance in the detection of low-concentration analytes. Therefore, improving the sensitivity of fiber optic SPR sensors and applying them to trace detection of biomolecules have received increasing attention from researchers. There are two main ways to improve sensitivity. One is to increase the strength of the evanescent field by changing the structure of fiber (e.g. U-shaped, D-shaped, tapered, etc.), and the other is to use the excellent photoelectric properties of new materials (e.g. high refractive index oxides, ceramic materials, two-dimensional materials, etc.) to improve the sensitivity of the sensor. Similar to graphene, Transition Metal Dichalcogenides (TMDCs) have also attracted much attention. Among them, Tungsten disulfide (WS2) shows many unique optoelectronic properties such as high complex RI ratio (the ratio of the real part to the imaginary part of the RI), direct band gap and large surface-to-volume ratio. It is shown that the application potential of WS2 in SPR sensors. However, there are relatively few experimental studies of WS2 in SPR sensors, and they mainly focus on prism-based SPR sensors.Based on the above-mentioned methods, the sensitivity of the fiber optic SPR sensor is improved by combining the tapering of the fiber and the coating of WS2 nanomaterials. In this paper, a tapered fiber optic SPR sensor based on the structure of WS2-Au is proposed. The relationship between the dielectric constant of WS2 and the wavelength is obtained by the first principles using the generalized-gradient approximation. The effects of taper ratio and the thickness of WS2 on the sensitivity of sensors are studied through theoretical simulations used the transfer matrix method. Then, four kinds of sensors (600 μm fiber SPR sensor, tapered fiber SPR sensor, tapered fiber Au-WS2 SPR sensor and tapered fiber WS2-Au SPR sensor) are manufactured. Among them, the WS2 nanosheets are modified on the fiber surface using electrostatic self-assembly technology, and the Au film is obtained by vacuum magnetron sputtering coating technology. An experimental setup is built to measure their Refractive Index (RI) sensitivity. The experimental RI sensitivity of the proposed tapered fiber WS2-Au SPR sensor can reach 4 158.171 nm/RIU, which is 125.8% higher than that of the multimode fiber SPR sensor and 50.1% higher than that of the tapered fiber Au-WS2 SPR sensor. Experimental results show a good agreement with the numerical simulations. It is demonstrated that the introduction of the WS2 layer can improve the sensitivity of the fiber optic SPR sensor and enhance the reliability of the sensor. In summary, the developed sensor can provide a high-sensitivity, low-cost, simple and environment-friendly platform for the biochemical detection.
To realize the electrical beam scanning of the phased-array radar, it is necessary to implement true-time delay compensation for the transmitted and received signals in each transmission channel according to the beam direction. Utilizing microwave photonic technology, the microwave signal can be modulated to the optical carrier for transmission and processing. Compared to the frequency of the laser carrier, the relative bandwidth of the microwave signal is extremely narrow. The true-time delay lines based on optical fibers or on-chip waveguides have a large microwave bandwidth, small in-band amplitude and phase fluctuations, and small propagating loss and are immune to electromagnetic interference. The increment of optical lengths of different delay states can be precisely controlled to be far less than a full microwave wavelength. Thus, broadband beamforming can be realized using only subwavelength stepped optical delay lines, which can greatly reduce the beam directional dispersion of broadband microwave signals. The discrete delay values of stepped delay lines lead to discrete directional directions of antenna beams, which results in a certain deviation between the actual and designed directions of the beam. The influence of the minimum delay change on the equivalent phase distribution of the microwave front is analyzed, and a theoretical model of the relationship between delay steps and directional deviations of radar beams is established. The theoretical analysis of beam scanning based on subwavelength stepped optical delay lines shows that the beam directional deviation is proportional to the minimum delay step and inversely proportional to the array element spacing, the square of the number of elements, and the cosine of the beam direction. Through numerical simulation for the X-band wideband radar, the beam direction at each frequency point is almost the same in the frequency range of 8~12 GHz, which indicates that the directional dispersion has been effectively suppressed. It can be observed that some singularities will appear at specific azimuths and delay steps, where the directional deviation will reach extreme values due to the discrete increment of delay. The azimuthal deviation of the singularity gradually increases as the azimuth and the delay step increase. When the delay step is not larger than 3 ps, the azimuthal deviation does not exceed ±0.13°, which is less than ±1/35 of the narrowest beam width and thus can be almost neglected. Peakpower loss and sidelobe suppression are also simulated. When the delay step is less than 3 ps, the beam peak power drops no more than 0.051 dB, and the in-band fluctuation is less than 0.028 dB. The maximum broadband relative sidelobe power is less than -12.5 dB, and the maximum fluctuation is less than 0.24 dB. Based on the scheme of a subwavelength stepped optical delay line without an electrical phase shifter, 9-bit optical delay lines with a minimum delay step of 3 ps are prepared, and the maximum delay exceeds 1.53 ns. The optical delay lines are manufactured by cascaded 2×2 magneto-optical switches and optical fibers. The distributions of the spatial electric field are measured on the nearfield platform and converted to farfield patterns by spherical wave compensation at different designed azimuths and frequencies. When the designed beams point at 0°, 30° and 60°, the measured maximum directional deviations are 0.24°, 0.28° and 0.77°, and the in-band directional dispersions are 0.21°, 0.28° and 0.98°, respectively. Compared to the beam squint based on the scheme of the wavelength stepped optical delay line and electrical phase shifter, the beam directional dispersion is effectively suppressed. Furthermore, under the microwave frequency range of 8~12 GHz and the azimuthal scanning range of ±60°, the experimental results demonstrate that the peak power loss can be reduced to less than 0.89 dB and that the sidelobe suppression ratio can exceed 11.06.
Based on distributed fiber optic sensors, fiber grating sensors and fiber interferometers have been widely used in tensile force measurement. In contrast, fiber interferometers have been widely studied due to their high sensitivity, such as Mach-Zehnder Interferometers (MZI), Fabry-Perot Interferometers (FPI), and Sagnac Interferometers (SI). On this basis, in order to improve the tensile force sensitivity of fiber interferometers, cascade fiber interferometers based on Vernier effect are proposed, such as cascaded dual MZI, cascaded dual FPI and cascaded MZI-FPI structures. However, the matching of the optical path difference and insertion loss of the cascade structure has always been a difficult problem to solve, which can affect the spectral quality of the Vernier effect. Therefore, the sensing accuracy and resolution of the tensile force measurements are limited.In this paper, a parallel FPI all-fiber tensile force sensor based on the Vernier effect is proposed, which is composed of a Sensing FPI (SFPI) and a Reference FPI (RFPI) in parallel. The structure is prepared only by an arc discharge technology, which ensures the uniformity and repeatability of the structure preparation. Among them, the SFPI is a closed air cavity. The tapered air microcavity is fabricated by precisely controlling the discharge position of HCF by an optical fiber fusion splicer. Then, the tapered air microcavity is discharged multiple times with a small current to improve the reflectivity of the microcavity. RFPI is an open air cavity. A section of HCF and Single-Mode Fiber (SMF) with an inner diameter of 80 μm is directly fused, and the length of the HCF cavity is precisely controlled by a precision cutting platform, so that the FSR of the interference spectrum is consistent with the SFPI. Subsequently, a section of HCF with an inner diameter of 10 μm is splicing on the end face.The parallel structure only consists of SMF and HCF, and the thermal expansion coefficient and thermo-optic coefficient of silica and air are very small, which reduces the crosstalk effect of temperature on tensile force. Through theoretical analysis, it is found that when the optical path difference between SFPI and RFPI is close to when not equal, a Vernier effect can be formed, and the smaller the optical path difference ratio between the two, the greater the sensitivity magnification. In order to form a high-quality Vernier envelope, a fiber attenuator is added between the RFPI and the fiber coupler to adjust the insertion loss of the RFPI to match the energy of the SFPI. The influence of the fiber attenuator on the Vernier envelope quality is verified by numerical analysis and experiments. The experimental results show that after adding the fiber attenuator, the contrast ratio of the Vernier envelope is increased from 0.05 to 0.2, and the magnification is four times.In the experiment, SFPI with a cavity length of 67 μm is prepared for tensile force test. In order to verify the hysteresis of the tensile force sensor, the experiments of increasing and decreasing the tensile force are carried out, respectively, with the sensitivities of 4.022 nm/N and 3.986 nm/N. In order to further increase the tensile force sensitivity of the sensor, two RFPIs with cavity lengths of 80 μm and 63 μm are prepared in this experiment, and formed parallel structure 1 and parallel structure 2 with SFPI, respectively. Tensile force experiments are carried out on two groups of parallel structures. With the increase of tensile force, the reflection spectrum of structure 1 undergoes a clear blue-shift. The corresponding sensitivity is -19.31 nm/N with the linearity of 0.992, which is 4.8 times larger than the sensitivity of a single SFPI. With the increase of tensile force, the reflection spectrum of structure 2 undergoes an obvious red-shift. The corresponding tensile force sensitivity is 63.5 nm/N with the linearity of 0.993, which is 15.8 times larger than the sensitivity of a single SFPI. The simulation analysis shows that when the SFPI cavity length is greater than the RFPI cavity length, with the increase of the tensile force, the drift direction of the envelope is consistent with the drift direction of the single SFPI interference spectrum.To explore the temperature crosstalk of the sensors, the temperature experiment of single SFPI and parallel structure 1 is carried out, and the temperature measurement range is 100°C ~600 °C. The experimental results show that the temperature sensitivity of single SFPI and parallel structure 1 are 3.93 pm/°C and -18.91 pm/°C, respectively, the sensitivity is amplified by about 4.8 times, and the temperature crosstalk is only 9.8×10-4 N/°C.To verify the stability of the proposed sensor, structure 1 is tested under different tensile force conditions. The experimental results show that the maximum drift of the Vernier envelope at 0.79 N is 0.02 nm, which proves that the sensor has good stability.In this paper, a high-sensitivity fiber-optic tensile force sensor based on the Vernier effect is proposed, which consists of a parallel FPI structure. By matching the energies of SFPI and RFPI through fiber attenuators, the Vernier envelope quality is optimized. The tensile force sensitivity can be improved from 4.022 nm/N to 63.5 nm/N, and the amplification factor is 15.8 with the linearity of 0.993. The simulation results show that the experimental results are basically consistent with the theory. At the same time, the temperature crosstalk of the sensor in the range of 100°C ~600 °C is only 9.8×10-4 N/°C.
The adaptive optical system without wavefront detection has the advantages of simple structure and easy application, and it is now turning into a research hotspot in the field of optical communication. With the rapid development of artificial intelligence technology in recent years, deep learning has been introduced into wavefront detection-free adaptive optics systems to correct wavefront aberrations. This paper proposes an adaptive optical wavefront recovery method based on the residual attention network in order to prevent the degradation of neural network. To prevent the degradation phenomenon of neural network, the residual network is first used as the backbone network, and its hopping layer connection property is utilised to enable the network model to learn deeper features. The input light intensity map is transformed into a feature map by a 7×7 downsampling convolution operation in the residual network, followed by a maximum pooling operation with a filter size of 3×3 to reduce the computational parameters and prevent overfitting phenomenon. Then, to increase the feature extraction capability of the network without significantly increasing the computational effort, this paper constructs a multi-scale residual hybrid attention network structure based on the residual network, using a null convolution operation to convert the light intensity image into a feature map for backward propagation. In the attention layer, features are extracted by convolution kernels of different scales in a distributed manner, and the dual-stream network structures of 3×3 and 5×5 sizes are chosen to extract the feature maps. The attention mechanism is used to improve the recognition rate of the network for broken light spot features and to achieve the effect of enhancing the network to express light intensity image features. Each hybrid attention module contains two convolution operations and one hybrid attention computation operation. The dimensionality of the feature map remains unchanged after the attention layer, and each channel is assigned a different weight coefficient. Finally, a network loss function combining the realistic evaluation metrics of wave crest and trough values as well as root mean square values of wavefront is designed, and the Zernike coefficients of the actual wavefront aberration are ensured in the training to match the final result. In order to simulate the transmission of vortex beams at different turbulence intensities, the Zernike coefficients and corresponding light intensity maps are randomly generated at different turbulence intensities in accordance with Kolmogorov turbulence theory. Parameters such as the gradient descent algorithm, batch size and number of iterations of the network are set reasonably, and the simulations are carried out using the keras deep learning library. The final results show that the residual attention network can reconstruct the turbulent phase quickly and accurately, and the recovered residual aberrations have peaks and troughs between 0.05 and 0.3 rad and root mean square between 0.01 and 0.07 rad. The experimental results show that the Zernike coefficients predicted by the residual attention model are similar to the actual coefficients and the phases reconstructed by the coefficients are highly similar to the actual phases compared with other network models. The effectiveness of the hybrid attention network in the task of reconstructing wavefront phases is then also verified, with the highest accuracy achieved with less increase in time complexity. The high accuracy, real-time performance and flexibility of the residual attention network provide practical applications for deep learning in adaptive optical systems.
There has been a lot of interest in the installation of high-speed short-reach optical interconnect systems recently because of the growth of 5G and the Internet of Things (IoT), which have caused the data traffic between and within data centres to expand quickly. In data centres, optical transmission systems frequently use optical Intensity Modulation and Direct Detection (IM/DD) to save cost and power consumption. However, loss of optical phase from square law detection and fiber dispersion cause a nonlinear distortion in the optical IM/DD system. Moreover, the nonlinear responses of modulator and driver/amplifier also cause serious nonlinear distortions at the same time, which seriously reduce the optical IM/DD system's transmission range and capacity.Various equalization algorithms have been proposed to eliminate them. A classical equalization scheme is the combination of feedforward and decision feedback equalizer, but the nonlinear distortions can not be effectively equalized. Volterra Nonlinear Equalizer (VNE) can correct for nonlinear distortions, nevertheless, higher-order VNE items in strongly nonlinear settings result in a significant increase in complexity. On the other hand, nonlinear equalizers based on neural networks were also widely investigated in optical communication recently, which includes feedforward neural network, radial basis function neural networks, convolutional neural network and recurrent neural network. In contrast to the feedforward equalizer and VNE, feedforward neural network equalizer exhibits stronger equalization performances, but also brings a higher complexity in order to compensate for strong nonlinear impairments in optical IM/DD system. Moreover, equalizers based on auto-regressive recurrent neural network have higher complexity, however, better performance thanks to the involvement of additional feedback neurons. These equalisers, however, only employ one or two hidden-layers. In optical IM/DD systems, the influence of the number of hidden-layers as well as the number of neurons in every hidden layer on the performance of the equalizer remains unknown. Also, the optimal structure of neural network equalizer is worth exploring. Thus, we constructed a 112-Gbps 20-km four-level pulse-amplitude modulation optical IM/DD transmission simulation platform to investigate the influence of the number of hidden-layers and the number of neurons in every hidden layer on Recurrent Neural Network Equalizer (RNNE) performance. Also, to seek the most efficient equalization scheme with better complexity and Bit Error Rate (BER) performance. The effects of the number of hidden layers and the number of hidden neurons on the performance of RNNE are studied quantitatively to determine the ideal structure for RNNE. Initially, the performance of the RNNE with different numbers of neuron in the second hidden layer has been compared when the number of neurons in the first hidden layer is fixed. The results show that when RNNE has a comparable number of neurons in each hidden layer, the BER and complexity performance is optimized. Then, as for the RNNE with multiple hidden layers, we quantitatively examined the influence of the number of hidden-layer on the BER and complexity of RNNE. According to the results, the two-hidden-layer RNNE outperform RNNE with three-hidden-layer. The complexity of two-hidden-layer RNNE is 23.3% less complex than a single-hidden-layer RNNE. With similar algorithm complexities, the power budget of the two-hidden-layer RNNE is approximately 1 dB higher as compared to the single-hidden-layer RNNE at 7%-OH FEC threshold. This optimization strategy provides a reference for the selection of the number of hidden-layer number as well as the number of hidden neuron while using RNNE to compensate for nonlinear distortions in the optical IM/DD system.
Since Orbital Angular Momentum (OAM) mode can form an infinite N qubit basis, it provides a new coding scheme for communication links. However, ocean turbulence and other external disturbances can cause phase disturbance of OAM mode, which generates the cross talk between the energy states of OAM modes. The self-focusing property of POV is beneficial to the transmission of OAM modes. The transmission quality of POV beams in oceanic turbulence is most related to the beam radius, while it is nearly free from the wavelength, topological charge, and radius-thickness ratio. Because of these special characteristics, POV has attracted wide attention of researchers. Moreover, the absorption of seawater reduces the information carrying capacity of POV photons and weakens the reliability of the system. Therefore, it is necessary to study the absorption of seawater and the transmission characteristics of POV. In addition, the modulation of POV also increases the information capacity carried by the POV's OAM mode. In this paper, the ABEP and information capacity of underwater optical transmission system based on POV carrier and M-PSK modulation are studied under weak absorption turbulence. The wavelength dependence of fading channel is especially considered. The fading channel consists of two parts: signal energy loss caused by seawater turbulence and seawater absorption. Under the condition of paraxial transmission and Rytov approximation, the closed expressions of mean signal-to-noise ratio and signal error probability for M-PSK OAM channel are derived. Considering that the number of OAM signal level channels in common use is far less than the number of OAM topological charge (energy level) of POV vortex carrier, the concept that POV vortex carrier communication link is a symmetric link composed of multiple OAM level channels is proposed. Using the derived expression of ABEP of M-PSK OAM channel and the concept of symmetric link of multi-OAM channel, a novel closed-form expression of ABER and average capacity under various modulation schemes is proposed. Finally, through numerical analysis of the model, new results are draw. The results show that when transmitting over short distances, the wavelength of the photon of the OAM signal of POV has a greater impact on the information capacity of the transmission system than on the absorption of seawater, while when transmitting over long distances, the effect of seawater absorption on the information capacity of the optical transmission system is greater than the wavelength. When the ring radius of the perfect optical vortex increases, the information capacity of the system also increases, and ABEP decreases, but with the increase of the ring radius of POV there is a stable region of ABEP decrease and information capacity increase related to turbulence intensity. The increase in the signal-to-noise ratio of the receiving and transmitting optical systems results in an increase in the information capacity of the system and a decrease in ABEP. The increase of the inner-scale and the decrease of the outer-scale also lead to the increase the information capacity of the system and the decrease of ABEP. In addition, the OAM signal photons of the POV under QPSK modulation are less affected in absorbing weak turbulence links than other orders of modulation. Therefore, on the basis of selecting the appropriate modulation scheme, the ring radius of the POV and the signal-to-noise ratio of the transceiver optical system, which can effectively enhance the information capacity of the OAM signal photon of the POV.
As an important part of modern communication system, satellite communication undertakes important tasks such as communication, earth observation, navigation and positioning in military and civil fields. The traditional spaceborne optoelectronic load realizes the data signal and power transmission between the relative rotating bodies through the slip rings. With the continuous development of optical fiber technology and related components, laser communication with optical fiber as the transmission medium has gradually replaced the traditional signal transmission with wires. The fiber optic rotary joints have the characteristics of a wide communication frequency band, strong anti-electromagnetic interference ability, strong confidentiality ability, fast transmission rate, low loss, etc. Its performance largely determines the service life of the satellite. Low loss and high reliability are important indicators of single-channel fiber optic rotary joints. This paper takes the single-channel fiber optic rotary joints as the research object. In order to achieve its low loss and high reliability goals, it is necessary to explore the factors affecting the insertion loss. The gap between the single-mode fiber and the gradient-index lens and the position error between the two gradient-index lens collimators are all important factors that affect the insertion loss of the fiber optic rotary connector. The Gaussian beam coupling has attracted the attention of universities and research institutions from all over the world. But the previous analysis ignored the influence of the position error between the fiber and the gradient-index lens on the coupling efficiency. There is no corresponding compensation method for the above-mentioned errors, which is crucial for improving performance parameters and reducing the difficulty of processing and assembly. This paper takes the single-channel fiber optic rotary joints as the research object. In order to achieve the goals of low loss and high reliability, it is necessary to explore the factors affecting the insertion loss. The fiber optic rotary connector studied in this paper uses two gradient-index lens collimators as the main optics. Theoretically, the propagation model of Gaussian beam in the construction of gradient-index lens is established, and the optical characterization parameters of the gradient-index lens are obtained by mathematical analysis method of light transmission matrix. In order to describe the propagation of the Gaussian beam in the gradient-index lens, the (x, y, z) and (x', y', z') coordinate systems are established, and the electric field vector equations are established for the lenses at the receiving end and the transmitting end. Based on this equation, the influence of lateral offsets on the coupling efficiency of the system is discussed. Using the geometrical optics analysis method, the energy distribution equation under the separation misalignment is established, and the influence of the separation misalignment on the coupling efficiency of the system is analyzed.This paper design the single-channel fiber optic rotary joints with low loss as the key parameter by ZEMAX, and the optical model of the single-channel fiber optic rotary joints is established, and the optical parameters of the gradient-index lens are preliminarily determined. For the convenience of processing and assembly, the two gradient-index lenses are designed with the same parameters. First, without changing the working distance, set the distances to 0, 0.05 mm, 0.10 mm, 0.15 mm, 0.20 mm, and 0.25 mm between the optical fiber at the transmitting end and the gradient-index lens. In order to obtain the insertion loss at different positions, the value of the fiber at the receiving end and the gradient-index lens is changed. It can be seen from the analysis that the same insertion loss as the initial value can be obtained by adjusting the position of the optical fiber. This method can reduce the influence of the error between the optical fiber and the gradient-index lens.Secondly, by changing the lateral offsets and separation misalignment of the two gradient-index lenses, the effects of lateral offsets and separation misalignment on the insertion loss of the system are obtained. It should be noted that due to the particularity of the gradient-index lens, the lateral offsets cannot be so large that the Gaussian beam cannot be coupled into the fiber. The axial distance is controlled within 0~14 mm, and the radial distance is controlled within 0~0.25 mm. It can be seen from the simulation that the lateral offsets have a great influence on the insertion loss of the system, and it is necessary to strictly ensure the accuracy in processing and assembly.In view of the above errors, the insertion loss is reduced to 0.2 dB by the displacement method, which provides a reference for the optimal design of the single-channel fiber optic rotary joints. For the separation misalignment and lateral offsets between two gradient-index lenses, a beam steering technology based on wedge prism and flat glass is proposed. This method mainly uses two wedge prisms to achieve beam steering, the flat glass adjusts the transmission optical axis and the receiving optical axis to be on the same axis as possible. The insertion loss of systems can be reduced to 0.7 dB by beam steering technology, which greatly reduces the influence of errors. The difficulty of processing and assembly is reduced, and the reliability of the system can be improved.Finally, a test system for the insertion loss of a single-channel fiber optic rotary joints was built, and the position of the optical fiber and the gradient-index lens was adjusted with a high precision fiber alignment stage, and observed through a binocular microscope. By fitting the experimental data with the simulation data, the accuracy of the system design and simulation analysis is verified.
With the rapid development of aerospace industry, optical fiber sensing technology has been widely used. The optical fiber sensor does not contain electronic components, so it has strong anti-electromagnetic interference ability and good electrical insulation, it can be used in many kinds of environment. It uses light wave transmission in an optical fiber to obtain the outside signal and measure the relative physical quantity. Fiber optics can both transmit light and sense signals. When the optical fiber sensor detects the measured physical quantity, the parameters such as wavelength, intensity, phase and frequency of the transmitted light wave will change. The optical fiber sensor can measure hundreds of physical quantities, including temperature, pressure, strain, displacement, acceleration and so on. The use of optical fiber sensors has become more and more popular in recent years, in the military defense, aerospace, industrial control, measurement and testing, exploration and other fields have a broad market. At present, Fiber Bragg Grating (FBG) sensors and Fiber Fabry-Perot (F-P) sensors are mainly used in practical applications. The optical fiber EFPI sensor has many advantages, such as small volume, high precision and large dynamic range. At present, there are two demodulation methods for the measurement of optical fiber EFPI sensor, one is fiber laser interferometry, the other is fiber white light interferometry. The former is suitable for the relative measurement of dynamic signals, while the latter is generally used for the absolute measurement of static or slowly varying signals. White light interferometry can realize absolute measurement, which has the advantages of a large dynamic range and strong anti-interference ability. Currently, various types of white light interferometry demodulation methods have been applied, including peak method, wavelength tracking method, interference order method, principal frequency method and Fourier transform method. Because of its various advantages, this technology can be widely used in practical engineering. With the increasing demand of the latest applications, the research on high-speed fiber-optic white-light interferometry has become the main trend in the future. With the increasing demand of applications, such as the monitoring of the surface pressure of the space engine and the strain produced at the moment of explosion, these unstable static physical variables change very frequently. In order to get better measurement results, high-speed white-light interferometry is studied. But it is difficult to achieve the absolute measurement of high-speed signals and achieve the required resolution. In general, the measurement system will be limited by the scanning light source module and computer processing speed and other factors. As the number of signals to be processed increases, the measurement speed decreases. Field Programmable Gate Array (FPGA) processor can solve the speed problem. Because it contains a lot of digital logic resources and rich RAM resources, FPGA can process and analyze data at the same time. The speed of demodulation can be improved by combining FPGA with white-light interferometry. In this paper, a multichannel high-speed Extrinsic Fabry-Perot Interferometric (EFPI) sensor interrogation system based on White-Light Interferometry (WLI) and FPGA is proposed and experimentally demonstrated. The system uses a semiconductor Optical Amplifier (SOA) and a Fiber Fabry-Perot Tunable Filter (FFP-TF) to make a high-speed wavelength scanning fiber laser. The symmetrical triangular wave technology drives the tunable Fabry-Perot filter to generate a swept spectrum, the scanning frequency is 2 kHz. Use FPGA to realize high-speed signal demodulation of EFPI sensor. The system realizes high-speed demodulation of EFPI sensors with 4 channels, the demodulation speed of each channel reaches 2 kHz.
With the rapid development of the Internet of Things era, the perception and acquisition of all kinds of external information is becoming increasingly important. Traditional monitoring and warning technologies such as manual inspection, video surveillance, infrared detection, and microwave detection have been widely used in various fields. Although these technologies have succeeded, they are expensive, environmentally sensitive, and cannot be monitored over long distances. To realize real-time and environment-free long-distance monitoring, distributed optical fiber sensing technology has developed rapidly in recent years. Distributed optical fiber sensing uses the whole optical fiber as the signal's transmission medium and sensing unit. When external factors act on the optical fiber, the light wave transmitted in it is modulated accordingly, and its light intensity, frequency, phase or polarization state, and other parameters will change accordingly. Distributed optical fiber sensing has the advantages of flexible layout, high cost performance and wide measurement range. Compared with traditional methods, distributed optical fiber sensing realizes large-scale measurement at a lower cost. As an important branch of distributed optical fiber sensor, phase sensitive optical-time domain reflectometer has the advantages of high sensitivity, high resolution and simple structure. At present, φ-OTDR plays an important role in many applications, such as structural crack detection, railway monitoring, traffic flow detection, vehicle detection and intrusion detection. With the development of modern productive forces and the diversification of life scenes, simple vibration location and signal demodulation cannot meet the actual needs and user needs. Random interference in nature (such as thunderstorm, wind, hail, etc.), passing vehicles and moving personnel will interfere with the accurate recognition of intrusion signals, increasing the false alarm rate. It is hoped that the type of vibration signal can be obtained at the same time as the location of the vibration signal, so as to replace manual inspection more intelligently. Compared with traditional monitoring methods, phase sensitive optical-time domain reflectometer system has greater advantages in real-time performance and convenience. At the same time, compared with video surveillance, phase sensitive optical-time domain reflectometer system is more covert, has strong resistance to external electromagnetic and other interference signals, and has the advantages of low cost, wide range and continuity. Therefore, it is of great significance to classify and identify vibration signals, identify the types of vibration signals, and study related identification algorithms to improve the identification accuracy and response speed of vibration signals, which are of great significance to the monitoring occasions in the fields of road traffic and border security. In view of the phase-sensitive optical time domain reflectometer distributed optical fiber sensing system has difficulties in real-time performance and accuracy of signal recognition. A method based on wavelet packet decomposition and support vector machine is presented. The energy feature vector is extracted by the wavelet packet decomposition of the signal as the input samples of the support vector machine, and the energy distribution trend of different signals is analyzed. A total of 800 experimental samples of knock, shake, walk and noise signals were trained. The recognition effect was evaluated by four evaluation indexes: accuracy rate, recall rate, F1 value and accuracy. The experimental results show that the precision rate, recall rate and F1 value of the knocking signal are 94.12%, 96% and 95.05%, respectively; the precision rate, recall rate and F1 value of the shaking signal are 95.92%, 94% and 94.95%, respectively; The precision rate, recall rate and F1 value of walking signal and noise signal are all 100%. The overall recognition accuracy is above 97%. The method improves the recognition result accuracy and real-time performance in the signal recognition of phase sensitive optical-time domain reflectometer system.
Chalcogenide glass material has an ultra-broad infrared transmission window, ultrafast nonlinear response time and ultra-high third-order nonlinearity. The As2S3 material has lower cost, higher nonlinearity and a broader transmission span than other chalcogenide materials, which is a supporting factor for supercontinuum generation. In this paper, a chalcogenide As2S3 glass based Photonic Crystal Fiber (PCF) with an As2S3 glass fiber core and air-holes as the microstructure cladding was theoretically designed, and the optical performance of the As2S3 glass PCF was studied using a commercial software of COMSOL Multiphysics. The proposed As2S3 glass PCF preform was then experimentally fabricated using an improved molding method along with a chemical etching method. The As2S3 glass PCF was drawn at the temperature of 350oC under the protection of dry N2 gas. The fabricated As2S3 glass PCF has a solid hole in the center and its cladding consists of four layers of air holes arranged in regular hexagonal order. The solid core diameter of the fabricated As2S3 glass photonic crystal fiber is 10 μm, the diameter of the air-holes is 3.3 μm and the air hole pitch between the centers of proximal holes is 7.2 μm. For the fiber tapering process, a micro-tapering system using a CO2 laser along with a scanning mirror and two high precision translation stages was established, all of which are computer programming controlled. The use of a CO2 laser to heat the fiber is advantageous over standard oxyhydrogen flame-tapering systems since it allows greater control over the tapering parameters, namely the size of the irradiated zone over the sapphire capillary, the heating rate and the exposure time, and also this avoids potential further pollution of OH- and H2O into the As2S3 glass PCF. By mounting the As2S3 glass PCF on computer-controlled translation stages gives programmable dynamic control over the fiber tension, as well as the ability to control the position of the tapered section with an accuracy of ±0.5 μm. Using this tapering system, taper regions as long as 5 cm were achieved with a tapering fiber diameter reduction of 56%. The diameter of the As2S3 glass photonic crystal fiber can be tapered down to 40 μm based on the tapering method. To fabricate a pump source of supercontinuum generation, a customized mode-locked femtosecond (fs) fiber laser based on nonlinear polarization rotation effect in a home-made Ho3+/Pr3+ codoped (Ho3+∶2 mol.%, Pr3+∶0.2 mol.%) ZBLAN glass fiber (the core diameter is 12 μm, the cladding diameter is 125 μm and the NA is 0.16) was achieved and it generates 173 fs pulses at the wavelength of 2.87 μm with an estimated peak power of 25 kW and a repetition frequency of 42 MHz. Then, the tapered chalcogenide glass photonic crystal fibers were pumped using the above ZBLAN fs fiber laser to generate the mid-infrared supercontinuum spectra. After optimizing the tapered diameters of the tapered As2S3 glass photonic crystal fiber, the tapered As2S3 glass photonic crystal fiber with a waist diameter of 55 μm and a waist length of 3 cm can generate a supercontinuum spectral coverage range of 2 000 nm to 5 500 nm at the loss level of -20 dB. A theoretical model based on the well-known generalized non-linear Schr?dinger equation was also established for simulating the evolution of the proposed supercontinuum generation in the tapered As2S3 glass photonic crystal fiber over the length of 3 cm. The experimental results have a good agreement with the theoretically calculated results. This investigation constitutes a major step toward devoloping an efficient chalcogenide glass photonic crystal fiber based broadband supercontinuum light source operating in the mid-infrared region.
The ambient temperature sensing is essential in industry, agriculture, medicine, food processing and so on. Optical fiber sensors have been widely valued by scholars due to the advantages of simple fabrication, electromagnetic interference resistance, chemical corrosion resistance and easily distributed measurement. In recent years, Hollow Core Fiber (HCF) has been investigated in fiber temperature sensing due to its hollow structure. In addition, antiresonant Negative Curvature Hollow Core Fiber (NCHCF) as a special hollow photonic crystal fiber, greatly reduces transmission loss of HCF by virtue of its negative curvature structure and quite thin cladding tube wall thickness. Hence, the mechanism of Multimode Interference (MMI) and Anti-Resonant (AR) of NCHCF are significantly enhanced, so it has more potential in the field of sensing and has become the focus in optical fiber sensing. At present, the sensitivity of the NCHCF cascaded temperature sensor based on the MMI mechanism is low. And some simulations have indicated that the high temperature sensitivity based on AR mechanism can be obtained by filling temperature-sensitive liquid into the hollow core of NCHCF. In order to simplify the fabrication of devices and obtain high temperature sensing sensitivity, the temperature sensing characteristics of the unfilled cascaded device based on AR and MMI are both studied theoretically and experimentally in this paper. Firstly, Single-Mode Fiber (SMF), Graded Index Fiber (GIF) and NCHCF are fused to form the cascaded fiber sensing structure (SMF-GIF-NCHCF-GIF-SMF). And then, the temperature sensing principle of the cascaded sensor based on MMI and AR is analyzed and the formulas are deduced. As temperature increases, the tube wall thickness and the refractive index of cladding and will increase due to the thermal expansion and thermal-optical effect of fiber materials, while the refractive index of air will decrease. Therefore, the dips based on MMI and AR both show red shift with the increases of temperature. The temperature sensitivity based on AR resonant dip is calculated as 17.25 pm /℃, and the sensitivity only caused by the thermo-optic effect of the fiber cladding material is 15.50 pm/℃. It is concluded that the thermo-optic effect of the fiber cladding material should play a leading role in temperature sensing. Therefore, the confinement losses of the core fundamental mode at corresponding resonant wavelength vary with different temperatures are simulated, where only the thermo-optic effect of the fiber cladding material is considered. And the simulation result is 15.57 pm/℃, which is consistent with the theoretical calculation. Finally, the temperature sensing experimental setup is designed and built. The temperature sensor is placed in the temperature controller and the endpoints of the sensor are connected with Broadband Light Source (BBS) and Optical Spectrum Analyzer (OSA), respectively. As the transmission bandwidth between adjacent AR resonant dips of NCHCF is large, the range of monitoring wavelength of the OSA is set as 600~1 700 nm, and its resolution is 0.02 nm. Multiple experiments are performed for monitoring MMI and AR dips when the temperature increases from 20 to 80 ℃ at a step of 10 ℃. It is found that the wavelength of each dip shifts as a function of temperature, which is fitted with an error bar. And the experimental results show that the temperature sensitivity and Detection Limit (DL) based on the MMI mechanism are 7.70 pm/℃ and 2.60 ℃, respectively. As for AR based sensors, there are three resonant dips in the resonant region of the transmission spectrum, which correspond to the three AR modes supported by NCHCF. The temperature sensitivities of the three dips are 17.29 pm/℃, 17.38 pm/℃ and 17.22 pm/℃, respectively, which are consistent with the theoretical results. And the DL based on AR mechanism is 1.16 ℃. Compared with the above experimental results, the temperature sensor based on AR mechanism has higher sensitivity and more fine detection, and thus AR mechanism is more suitable for temperature sensing. The temperature sensing experiment of the cascaded device is mainly carried out in the temperature range of 20~80 ?℃. However, the relevant studies have shown that the detection temperature based on the quartz material sensor can be up to 1 100℃. Therefore, the proposed sensor can be used in a higher temperature environment theoretically. Meanwhile, it has the advantage of good stability and can be widely used in environmental temperature detection scenes. In addition, the comparative study of dual mechanisms carried out in this paper can provide the theoretical basis for multi-parameter sensing.
Considering the competition for oceanic resources among different nations, Underwater Wireless Communication (UWC) technology has a lot of potential for development. As compared to its traditional counterparts, namely underwater acoustic communication and radio frequency communication, Underwater Wireless Optical Communication (UWOC) has many advantages, such as a strong information-carrying capacity, a faster communication rate, and good confidentiality, which can better suit the practical communication requirements of high-speed and large-capacity, lower implementation costs, and lower time latency in underwater wireless communication. The effects of the UWOC channel on the received laser pulse are typically categorized into the signal power attenuation caused by absorption, scattering, and the light intensity scintillation caused by oceanic turbulence, which leads to a decline in the transmission performance (bit error rate) of the UWOC system. The most widely used turbulence channel models are only suitable for a specific turbulence state. In order to further analyze the signal characteristics and system performance of the UWOC system of the Offset Quadrature Phase Shift Keying (OQPSK) modulation under the common action of turbulence channel and attenuation channel, this paper uses the Exponential Generalized Gamma (EGG) turbulence distribution model, which is more consistent with real oceanic channel characteristics. We obtain the turbulent random noises utilizing the acceptance-rejection sampling algorithm and further establish a composite channel model taking into account the attenuation channel, turbulence channel, and the Additive White Gaussian Noise (AWGN). In addition, according to the waveform of simulating signal, varying turbulence noise parameters, system noise parameters, and attenuation channel parameters, we analyze the average Bit Error Rate (BER) characteristics of the OQPSK modulation in the UWOC system. The simulation results show that the signal waveform does not change when it passes through the attenuation channel, but the amplitude is severely attenuated; the signal envelope passing through the turbulence channel changes with time, and the speed of signal amplitude change is negatively correlated with the turbulence coherence time; the signal waveform passing through the composite channel is distorted nonlinearly. For strong oceanic turbulence of the scintillation index σI2=2, the performance of analog signals with carrier characteristics is better than the performance of the digital signal, where as compared to the Binary Phase Shift Keying (BPSK), the SNR gain of OQPSK is rough by 3 dB. For weak oceanic turbulence of the scintillation index σI2=0.2 with a water quality attenuation coefficient of c=0.151 m-1, the OQPSK system can achieve reliable communication of 50 meters at an average BER of 10-3 when the SNR is 20 dB. Under the same parameter of oceanic turbulence channel, the BER decreases linearly with the increases of the communication distance. At the same time, the quality of seawater has a great influence on the average BER of the UWOC system. In the weak oceanic turbulence channel of σI2=0.2 and turbulence coherence time τ0=10 ms, the UWOC system with the OQPSK modulation can achieve reliable communication of 40 meters at the average BER below 10-3 by increasing the SNR in the case of pure ocean water or clear ocean water, but it is noticed that the system with the OQPSK modulation can hardly achieve effective communication in the coastal ocean water.
Ultraviolet (UV) communication technology realizes signal transmission based on light scattering, which has strong anti-interference, good security performance, omnidirectional transceiver, obstacle crossing and other technical advantages. During radio silence, local secure communication or complex electromagnetic environment, UV communication can be used as a special local military secure communication means, or as a supplement to other communication means under certain conditions. It has special use value and practical significance for future war and modern national defense. In recent years, with the development of UV communication technology, Non-line-of-sight (NLOS) target location technology based on UV scattering transmission has become a hot research topic. At present, the method of network multi-point coordinate mutual calculation is often used to determine the target distance and azimuth information. This paper starts from the requirement for fast and flexible completing hardware deployment in complex combat environment. When using optical signal transmission, its polarization characteristics contain the angle information carried by unique vibration direction, and can keep this characteristic in the long distance straight line propagation. A polarization UV light NLOS target location technology is proposed,which can realize the localization of two NLOS targets. And aiming at the application requirements of target localization in specific environments, the performance of non-line-of-sight target positioning system based on single scattering of polarized ultraviolet light is studied.Firstly, the relationship between the system received light intensity and azimuth angle and positioning distance is derived by the method of matrix optics. Secondly, the effects of system outage probability and transmitter and receiver elevation angle on outage probability in different atmospheric turbulence are analyzed by establishing atmospheric turbulence model based on polarized UV light. Thus, the influence of the geometric parameters of the receiver and transmitter device on the ranging and positioning and the reference range of the system in different environments are obtained. The research results show that the increase of positioning distance, turbulence intensity and transmitter and receiver elevation angle will lead to the decline of system performance. If the outage probability of the system is required to be less than 10-2, the positioning range of the positioning system in weak turbulence and strong turbulence environment should be within 1 200 m and 600 m, respectively. In addition, the change of the transmitter and receiver elevation angle has a certain influence on the outage performance of the system. If the positioning distance is 800 m and the turbulence intensity is Cn2=1×10-16 m-2/3, in order to make the system performance acceptable, the variation range of the receiver elevation angle is 0° to 20° when the transmitter elevation angle is 30°, and the variation range of the transmitter elevation angle is 0° to 25° when the receiver elevation angle is 30°. And with the increase of the transmitter and receiver elevation angle, the outage probability increases and the system stability decline. Therefore, the smaller the transmitter and receiver elevation angle, the better the system performance. In order to achieve the purpose of NLOS positioning, the transmitter and receiver elevation angle of the system can be set to 25°.This paper mainly studies the NLOS positioning method based on polarized UV light, and the effects of geometric parameters of the system and different environments on the positioning performance of the system, and the relevant conclusions and the reference application range of the system in different environments are obtained. The research contents and results of this paper provide a theoretical basis for the engineering implementation of polarized UV NLOS target location system, which also provides relevant theoretical basis for the new direction and practical application of UV communication technology, and has certain guiding significance.
In the process of deep oil and gas exploration,low-frequency signal plays a key role. Therefore, accelerometers that can receive low-frequency signals are particularly important in low-frequency exploration. Compared with electric accelerometers, the Fiber Bragg Grating (FBG) accelerometer has the characteristics of anti-electromagnetic interference, small size, high-temperature resistance, and high resolution. Researchers have proposed various FBG vibration sensor structures, in which cantilever beam and circular diaphragm are two typical structures and have been widely studied. The early single cantilever fiber grating sensor has the characteristics of high sensitivity and weak lateral interference resistance. In contrast, the ordinary circular diaphragm structure has strong lateral interference resistance and low sensitivity. In this paper, we propose a low-frequency FBG accelerometer based on a diaphragm-type cantilever structure. It combines a plane circular diaphragm and an equal strength cantilever beam into one, which reduces the lateral interference and enhances the sensitivity coefficient. The diaphragm-type cantilever is made of a beryllium bronze sheet into a circular diaphragm. Then four symmetrically distributed equal strength cantilever beams are cut into the diaphragm. The four suspension beams jointly support a copper inertial mass block located in the diaphragm center. The edge of the diaphragm is fixed between the base and the gasket through screws. An FBG with a central wavelength of 1 539.15 nm, a reflectivity of 90% and a grating area length of 10 mm is selected. The FBG is pasted at two points and fixed on the centerline of one cantilever of the diaphragm-type cantilever with 302 glue. The tail fiber at one end passes through the gap between the diaphragm-type cantilever and the gasket and is connected with the external optical demodulation equipment. According to the theoretical analysis, the optimal size of structural parameters of the diaphragm cantilever beam is obtained by MATLAB numerical simulation. According to the optimal size, a finite element model is established by COMSOL simulation software to further analyze the vibration form of the structure. The simulation results show that when the edge of the diaphragm cantilever is set as a fixed constraint, the resonance frequency of the first mode is 49.5 Hz. The vibration form is that the central mass of four cantilever supports vibrates up and down along the z-axis, and the vibration mode conforms to the original design intention. Simultaneously, the tested results of sensing characteristics from the shaking table indicate that the system has an excellent response to low-frequency acceleration excitation signals when the natural frequency of the system is 48 Hz. The frequency response range of the system is 1~35 Hz, in which the acceleration sensitivity is 452.6 pm/g. The acceleration sensor is designed with strong lateral immunity since the sensitivity in the transverse sensitivity is only 2.16% of the sensitivity in the working direction. Therefore, the designed FBG accelerometer provides a new method for the single component FBG accelerometer in the practical application of seismic exploration.
Aiming at the difficulty of existing optical sensors to meet the requirements of high-frequency vibration of micro-seismic monitoring in oil and gas production field, a high frequency FBG accelerometer based on symmetrical flexible hinges is proposed. The accelerometer is based on a compact structure consisting of a base, double hole hinge, a fiber Bragg grating and a mass block. There are threaded holes in the base to install the geophone on the vibration test table. Two small semi-circular rings are cut out on a cylindrical stainless steel material along the transverse symmetry using a line cutting technology. The upper and lower parts of the base and the mass block are engraved with 0.5 mm grooves along the axis. FBG is placed in the the upper part of the base and the mass block of the fiber trench. Both ends of FBG are glued to the mass block and base by epoxy adhesive. The FBG certain is applied to some prestress during packaging. When there is an external vibration signal, the base of the detector vibrates with the measured object. The mass block vibration around the center of the hinge relative to the base under the action of inertial force, driving FBG to stretch and compress, leading to a wavelength drift of FBG. The principle of vibration sensing is analyzed. The sensitivity and the resonant frequency formula of the accelerometer are given theoretically and the influence of structural parameter on the sensitivity and resonant frequency of the accelerometer is discussed. The modal analysis of the geophone is carried out using simulation software. The first order characteristic frequency of the structure is 1 191 Hz, the vibration direction is x direction, and the second order characteristic frequency is 7 039.4 Hz. The vibration direction of the second order characteristic frequency is y direction. As the two characteristic frequencies are very different, the geophone has good transverse anti-interference performance. To obtain the sensing performance of the detector, the amplitude-frequency response, sensitivity and lateral anti-interference of the detector are tested. The packaged fiber grating geophone and standard acceleration sensor are fixed on the vibration table, PC control software controls the output signal of the vibration table, fiber grating demodulator and fiber grating geophone is connected, completing the signal demodulation. The demodulation signal is transmitted to the computer to complete the signal acquisition. The analysis of the experimental results shows: the resonance frequency of accelerometer based on the symmetrical hinge structure is 1 200 Hz, basically consistent with the resonance frequency results using the simulation software. The reason of the difference may be caused by the processing error of the sensor. The operating frequency band of the detector is 20~800 Hz. The sensitivity of the sensor is about 10.2 pm/g, and the linear sensitivity is 0.999 8. The cross axis sensitivity of the detector is about 5% of the main axis. The geophone has good application prospects in oil and gas exploitation field.
Optical fiber sensors have the advantages of small size, low cost, high resolution, compact structure, strong anti-electromagnetic interference ability, etc., which have been widely used in structural health monitoring. The application of optical fiber high temperature strain sensors in the high temperature and harsh environment in aerospace, petroleum exploration, industrial smelting and other fields has attracted more and more interest of researchers. At present, thermocouples and resistance strain gauges are commonly used to measure temperature and strain, respectively. However, both of them have many shortcomings. Thermocouples are very expensive, low precision and sensitive to pollution; the resistance strain gauges themselves have very high costs, short service life, complicated pasting process and low measurement accuracy. Therefore, there are so many challenges for high temperature strain sensors based on electrical type in high-temperature and harsh environments and it is urgently ask for developing other kinds of sensors to be used in these environments. Optical fiber high temperature strain sensors are one of the most important sensors due to their many advantages. For example, they can be protected by ultra-high temperature ceramics, carbon/silicon carbide and other materials with mature preparation technologies, and can be used for thermal structure health monitoring in high temperature environment.It is of great significance to explore and develop optical fiber sensors that can be used in high temperature environment. However, when optical fiber high-temperature strain sensor is used to monitor the temperature and strain of thermal structure in the high-temperature environment in real time, the sensor can respond to temperature and strain at the same time in the demodulation process, resulting in the problem of cross-sensitivity. In the process of strain measurement, temperature affects the measurement results at the same time, resulting in a large strain measurement error. How to solve this problem is particularly important. At present, there are two demodulation methods: dual-wavelength demodulation and temperature compensation demodulation.Dual-wavelength demodulation method adopts dual-parameter matrix to demodulate temperature and strain, which can cause large measurement error in high temperature environment. In the temperature compensation demodulation method, one of the sensor structures is protected by adhesive package, so that it only responds to temperature, and the other sensor structure is compensated for temperature. However, this method is only suitable for the experimental test in low temperature environment because the adhesive is not resistant to high temperature. At present, the research scheme and technical route to effectively solve the temperature-strain cross-sensitivity problem of optical fiber sensors are not clear, especially for strain monitoring at ultra-high temperature. Therefore, it is an extremely urgent problem to design the sensor structure and improve the demodulation method to realize the accurate measurement of temperature and strain in high temperature environment. The developed optical fiber high temperature strain sensors should not only have a more reliable demodulation method, but also largely solve the main technical problems left by the current optical fiber high-temperature strain sensor.In this paper, sensing mechanisms, experimental methods and packaging applications based on FBG and optical fiber interference type high-temperature strain sensors are reviewed. The response characteristics of different sensing mechanisms to temperatures and strain are summarized and the measurement parameters of various fiber optic high temperature strain sensors are compared in table, including the measure range of temperature and strain, the sensitivity of temperature and strain, and the latest development of optical fiber high temperature strain sensor is introduced emphatically. Finally, the perspective of optical fiber high temperature strain sensor is forecasted.
The ultrasonic imaging of seismic physical model is an effective seismic simulation method for on-site seismic exploration. According to a certain simulation similarity ratio, the geological model of the field geological structure is constructed in the laboratory. The experiment of seismic physical model imaging has been widely used in oil and gas exploration, such as studying the basic regularity of wave propagation and the seismic response of typical geological structures, optimizing field observation systems and exploration methods, and verifying propagation theory and mathematical calculation methods. Because the ultrasonic signal transmitted in complex models is usually weak, it is necessary to employ a high-performance ultrasonic transducer to collect the echo signals. The traditional detection method usually adopts Piezoelectric Transducers (PZTs). The mechanical resonance of PZT determines its narrow-frequency response characteristics. In addition, in the application of array sensing, PZT has difficulties in signal demodulation and is also easy to be disturbed by electromagnetic environment.In comparison, fiber-optic ultrasonic sensor can avoid most of the shortcomings of PZT. Fiber sensors have the advantages of small size, high sensitivity, wide-frequency response, anti-electromagnetic interference, etc. Therefore, the research on new fiber-optic ultrasonic sensors has very important technological significance and application value. At present, the development trend of fiber-optic ultrasonic sensors mainly focuses on high sensitivity, high spatial resolution, broadband response and other characteristics. The basic principle of fiber-optic ultrasonic sensors is the interaction between ultrasonic wave and optical fiber, causing changes in the intensity, phase, wavelength, polarization state of optical fiber transmission and reflection light. The ultrasonic information is obtained by demodulating the small changes in the above optical parameters. The demodulation methods include phase demodulation, intensity demodulation, and optical frequency demodulation. Meanwhile, preparing new optical fiber ultrasonic sensing devices in terms of materials and processes, the signal-to-noise ratio of fiber-optic ultrasonic sensing can be further improved by integrating photoelectric conversion, electrical signal amplification, signal filtering, and other technologies into the signal demodulation system.For ultrasonic echo acquisition, fiber-optic sensors have shown obvious advantages. For the excitation of ultrasonic wave source, laser ultrasound gradually emerges in ultrasonic detection. Compared with the traditional PZT, laser ultrasonic technology can excite the ultrasonic field on the surface of objects with different scales and shapes. The excited ultrasonic wave has the characteristics of wide-frequency band, multi-mode waves, high intensity and non-contact. The nanosecond pulse laser is irradiated on the photoacoustic functional material with high absorption, and the material absorbs heat to produce periodic expansion and contraction, thus generating ultrasonic waves. Based on the photoacoustic effect, a series of photoacoustic functional materials, such as noble metal nanoparticles, carbon nanotubes, graphene, and organic nanoparticles, have shown efficient photoacoustic properties. However, almost all photoacoustic functional materials are designed for biomedical applications. These photoacoustic materials need to have low toxicity, immunogenicity, high target affinity and specificity, and high biocompatibility. Coated on the surface of seismic physical models, the photoacoustic functional material can replace the conventional PZT emission source to achieve high-quality ultrasonic excitation. The material is required to have the characteristics of wide-band absorption, high thermoacoustic conversion efficiency, high laser damage threshold, low cost, easy extension in a large area, etc. Therefore, in order to meet the needs of ultrasonic imaging of seismic physical models, it is necessary to further develop efficient photoacoustic functional materials and laser excitation technology.The two technologies of high-quality laser ultrasonic excitation and high-performance fiber-optic ultrasonic sensing can be combined to realize high-intensity excitation and high-fidelity sensing of broadband ultrasonic waves. All-optical pulse-echo imaging of seismic physical models can accurately extract the internal structure information of seismic physical models. In 1990, the French Petroleum Research Institute, a world-famous comprehensive oil, natural gas and chemical research institute, took the lead in proposing the optical ultrasonic imaging technology for seismic physical models. Pulsed laser was used to generate ultrasonic waves. Laser interferometer was employed to detect the vibration and sound signals in the models. Seismic physical models are made of resin, silicone rubber, paraffin, gypsum and other materials with weak photoacoustic properties. When the pulse laser is directly irradiated on the model, it is difficult to generate high-intensity ultrasonic waves and the receiving end adopts laser interferometer. However, laser interferometer have the disadvantages of high price, low sensitivity and inconvenient use. Therefore, there have been few reports on all-optical ultrasonic imaging technology of seismic physical models in recent years. For the in-lab detection of seismic physical models, the fiber characteristics of flexibility and multifunction make all-fiber ultrasonic imaging more and more concerned.Throughout the development of fiber-optic technology in recent decades, fiber-optic acoustic sensors have made great breakthroughs in materials, structures and fabrication. Some have been successfully applied to industrial nondestructive testing, marine seismic exploration, and other fields. This paper mainly summarizes the sensing mechanism and development status of several typical fiber-optic ultrasonic sensors, such as fiber interference type and fiber Bragg grating type. The state-of-the-art of electroacoustic transducer, fiber-optic ultrasonic sensor and laser ultrasonic technology in ultrasonic imaging of seismic physical model are comparatively shown, and the existing scientific and technological problems and challenges are also deeply analyzed. By comprehensively discussing the new development of ultrasonic imaging research in seismic physical models, this paper reveals the new trends and opportunities of in-lab simulation technology, so as to improve the exploration ability and informatization level of oil and gas resources in China.
In the short-distance, multi-user quantum key distribution integrated optical fiber network scene, aiming at the problem that when the quantum channel and the classical channel are close to each other, the four-wave mixing noise generated by the classical light is easy to have a serious impact on the performance of quantum key distribution, a theoretical model is constructed to describe the deterioration of the performance of discrete variable quantum key distribution caused by the four-wave mixing effect. In order to suppress the influence of four wave mixing noise, four noise suppression schemes were built and analyzed in OptiSystem and MATLAB, which realized the simultaneous co-channel transmission of seven 10 Gbps classical signals and one quantum signal. The minimum bit error rate of classical signal is 8.13×10-11, and the quantum bit error rate is up to 1.69% at the minimum. The research results are of great significance for promoting the practical process of quantum-classical co-channel transmission system.
A temperature sensing system based on the Fabry-Pérot interference structure is designed. Air, distilled water, 5% NaCl solution, absolute ethanol, methanol and cured silicon rubber are used as the temperature sensitive materials of the sensor to improve the temperature sensitivity effectively. The experimental results show that when the medium in the cavity is air, the temperature sensitivity of the F-P interference structure is inversely proportional to the cavity length. However, when the medium in the cavity is liquid or the solid material constitutes F-P type detection probe, the length of the cavity will hardly affects the temperature sensitivity of the structure. Because at this time the main reason of the wavelength shift is the change of material’s thermo-optical coefficient, the temperature sensitivity is proportional to it. In the experiment, methanol is the liquid with the highest absolute value of the thermo-optical coefficient. And when it is filled with F-P cavity, the temperature sensitivity is -564 pm/℃. In addition, when the cured silicone rubber is used as an F-P type detection probe directly, the temperature sensitivity can be as high as 1.15 nm/℃. The temperature sensing structure has the advantages of small size, good repeatability, strong flexibility and plasticity, and has potential application values in the field of temperature sensing in the future.
A high-precision Fiber Bragg Grating (FBG) displacement sensor was proposed based on the combination of a spring and a slider. It can realize the displacement measurement and temperature compensation on a single fiber, which greatly reduces space occupation. Experimental results show that the sensor has excellent micro-displacement measurement capability, its sensitivity, accuracy and the range are 145.08 pm/mm, 1.43%, 1.55% and 10 mm, respectively. The relative error of static synthesis is 2.88%, and the overall error of linearity, repeatability and hysteresis are small. By comparing the temperature compensation effect of aluminum alloy substrate, non-substrate and quartz substrate, it is found that the sensor temperature compensation effect of quartz substrate is better which the delay time is reduced from 6.8 min to 4.3 min, and the maximum temperature compensation error is reduced from 44 pm to 40 pm. Finally, the temperature sensitivity of the sensor made of quartz glass substrate is 6.34 PM/℃ and the temperature compensation error is 0.26%. All of those implies the sensor is suitable for online monitoring of high-precision structural displacements such as mechanical equipment and civil engineering.
Distributed Brillouin optical fiber sensor has great potential for slope monitoring. The sensing fiber is easy to be broken due to extensive local strain, and it is often difficult to obtain accurate measuring results due to the limitation of the spatial resolution. In order to fit for the slope monitoring, an Optical Fiber Sensing Structure(OFSS) for the distributed Brillouin optical fiber sensing network is developed. The OFSS composes of a PVC pipe and two pulleys, constructing multiple round-trips for the sensing fiber. The length of sensing fiber for a short spatial length is extended significantly. So both the sensing accuracy and the tolerance to local deformation of the fiber are improved effectively. A comparative calibration experiment is designed in the laboratory to verify its performance with the spatial resolution of 1 m. What's more, the effect of the proposed OFSS for landslide monitoring is confirmed through an on-site monitoring test on a shallow artificial slope. In addition, since the optical cable has no contact with the outside under the protection of the modified device, the survival rate of the cable is increased significantly.
The clearance measurement environment of engine and gas turbine is harsh due to the high temperature, high pressure, and strong vibration. In such a measurement environment, the low-coherence heterodyne interferometric clearance measurement technique faces the problem of weak signal and the limited measurement range caused by the low Signal-to-Noise Ratio (SNR). To address this problem, a method using differential detection is proposed to enhance the SNR. The theoretical measurement model of the proposed method is strictly established, and the model indicates the method has the advantages of improving the SNR and expanding the measurement range. To prove the feasibility of the method, an all-fiber clearance measurement experimental verification system is built, and a comparison experiment between single-ended detection and differential detection is carried out. The experiment result shows that, under the same measurement conditions, the differential detection method improves the SNR by 4.22 times and increases the measurement range from 10 mm to 20 mm. Furthermore, the measurement uncertainty of the system is analyzed. It is theoretically and experimentally found that, due to the scanning speed instability of the optical fiber delay device, the measurement uncertainty increases with the increase of the clearance. But within 20 mm, the measurement uncertainty is less than 15 μm.
Based on previous pattern recognition research which uses Mel Cepstrum coefficient method to extract frequency characteristics of disturbance signal, a fiber intrusion pattern recognition method using 1-Dimension convolutional neural network is purposed for interferometric distributed optical fiber sensing system. Hierarchical thresholds of the restored signal are used to judge and extract the intrusion behavior, which effectively reduces the calculation time compared with the framing method. A one-dimensional convolutional neural network is constructed based on the frequency domain features of the intrusion signal after Fourier transform to extract the characteristics of the disturbance signal adaptively. A line-based Sagnac interference system is set up to acquire data. By training the network with a large number of sample data, a good classification result is obtained. The average recognition rate of the verification set reaches 96.5%. The trained convolution kernels and the convoluted intrusion signals are discussed. After zscore standardization, the one-dimensional convolutional neural network can identify some features in the frequency domain of the signals, and the recognition result of the branch tapping signal with complex frequency components is greatly improved.
The fiber grating strain sensor is developed by using the 3D-printing method. A separate model of strain coupling and transmission between the bare fiber Bragg grating sensor, 3D-printing encapsulation layer and the measured matrix structure is established. The strain transfer relationship between the fiber grating sensor and the measured matrix structure is deduced. The influencing factors of the strain transfer rate of the clamped 3D-printing fiber grating strain sensor are analyzed, including the elastic modulus of the encapsulation layer, the thickness of the encapsulation layer and the bonding length. The research results show that the average strain transfer rate is positively correlated with the elastic modulus and bonding length of the encapsulation layer, negatively correlated with the thickness of the encapsulation layer. The research results can be used as a reference for the clamped fiber Bragg grating strain measurement, error correction and sensor design, as well as the feasibility of 3D-printing technology for packaging fiber Bragg gratings.
When the fiber Bragg grating is multiplexing, the limited source bandwidth may cause spectral overlap, then affects the accuracy of demodulation. A new type of fiber Bragg grating overlapping spectrum demodulation method is proposed. This method is based on an improved manta ray foraging optimization algorithm.The Tent chaotic map is used to optimize the initial population, and the differential evolution algorithm is used to optimize the individual location update strategy, which solves the problem that the manta ray foraging optimization algorithm is easy to fall into the local optimum. The simulation and experimental results of multiple fiber Bragg gratings overlapping spectrum show that the proposed method can accurately demodulate the center wavelength of the overlapping spectrum, even in the case of spectral distortion, the maximum error does not exceed 0.01 nm; at the same time, it effectively reduces the probability of fall into local optimum,and improves the stability and reliability of the algorithm.
The Brillouin dynamic grating model is developed based on the stimulated Brillouin scattering and elastic acoustic theory. The reflection spectrum of the Brillouin dynamic grating is calculated based on the fiber Bragg grating theory, and it is demonstrated that the Bragg wavelength downshifts by Brillouin frequency shift equals the Doppler frequency shift. The reflectivity and the spectral width are calculated when the pump power ranges from 0.1 W to 30 W, the pulse width ranges from 2 ns to 10 ns and the core diameter of a single mode fiber ranges from 8 μm to 10 μm. When the power increases to 30 W and the pump pulse width reaches 10 ns, the peak reflectivity is 2.17×10-6 and 7.16×10-9, respectively. The spectral width of the reflection spectrum decreases with the increase of pulse width. When the pulse width is 10 ns, the minimum spectral width is 1.2×10-4 nm. When the fiber core diameter decreases to 8 μm, the peak reflectivity increases to 6.64×10-11. The results show that the reflectivity of the Brillouin dynamic grating is positive correlation with the power and the pulse width of the pump wave, but it is negative correlation with the core diameter of optical fiber. The spectral width of the reflection spectrum is not affected by the power of the pump wave and the diameter of the fiber core, but it is negative correlation with the pulse width.
A thin, large aperture, high gain towed line array with weak fiber Bragg gating hydrophones is proposed. A weak fiber Bragg gating array with the same reflectivities, central wavelengths and wide 3 dB bandwidths is selected according to the principle of matched interference, and the grating spacing of the weak fiber Bragg gating array is determined according to the principle of underwater acoustic sensing for 5~10 Hz very low frequency underwater acoustic signals detection. Central wavelengths of the weak fiber Bragg gating array are uniformly shifted and the grating spacing is basically unchanged when the weak fiber Bragg gating array is coated by an optical fiber coating machine. Kevlar and polyurethane protective sheath are laid outside the weak fiber Bragg gating array by an yarn binding machine and sheath machine to form hydrophone towed array. The sound pressure-phase sensitivities of the towed linear array hydrophone unit are measured as -136.97 dB (1 rad/μPa) @ 5 Hz、-139.64 dB @ 7.5 Hz、-139.36 dB @ 10 Hz. The self noise power spectrum of the hydrophone caused by flow noise is analyzed, and the spectrum value is in the range of 45~95 dB (1 μPa2/Hz) in the frequency band of 1~100Hz under 8m/s towing speed. The experimental and analytical results show that the proposed towed line array has high sensitivity and low flow noise in very low frequency band, which is expected to increase the detection function of very low frequency underwater acoustic signals for unmanned aerial vehicles.
To extract the cavity length information of optical fiber Fabry-Perot interference signal with high precision, combined with the multi-scale subdivision function of wavelet transform, the phase information of each point of the interference spectrum is accurately extracted through the wavelet ridge, and the phase compensation is carried out by using peak information, which improves the accuracy of cavity length demodulation. Simulation results show that the demodulation error of the algorithm can reach±0.06 nm in theory. The experimental results show that the cavity length demodulation resolution of the algorithm is doubled compared with that of the least square phase correction method and the resolution can reach 0.514 nm. The demodulation resolution of the acceleration sensor experiment is up to 0.9 mg. The algorithm has a certain application prospect in the high precision measurement of dynamic and static parameters of Fabry-Perot sensor.
In the wireless ultraviolet communication system, fluorescent lamp can cause power-line interference to the system. The interference can be superimposed on the useful signal received by the photomultiplier tube in the form of a sine wave, which have a bad influence on the subsequent sampling decision stage. Firstly, this article analyzes the principle of wavelet transform to remove power-line interference noise, and gives a method to calculate the signal-to-noise ratio based on the received signal waveform. Secondly, the performance of wavelet transform to eliminate power-line interference is analyzed based on this signal-to-noise ratio calculation method. Finally, the problem of selecting the optimal wavelet basis in the process of the wavelet transform to remove power-line interference is discussed. The spectrum analysis shows that wavelet transform can effectively eliminate power-line interference of 100 Hz and 200 Hz, when the wavelet basis is sym40, the best performance in removing power-line interference is achieved, and the signal-to-noise ratio is 12.22 dB after denoising.
Dense apodized fiber Bragg grating arrays were on-line fabricated on a fiber drawing tower and used for temperature detection of small-scale heat sources. The signals of the grating array sensing network were demodulated through optical wavelength time domain reflection demodulation technology and optical time domain segmented demodulation technology. The sidelobe suppression effect of Gaussian apodized grating was studied and simulated. The results indicat that the gratings with good spectral type and high sidelobe compression ratio can be prepared when the Gauss coefficient G = 4. A dense apodized grating array with the sidelobe compression ratio of 20.74 dB was fabricated with our grating array fabrication system. The temperature experiment results show that, for this apodized grating array sensing network, the time-domain segmentation can reach 1 m, the spatial resolution can reach 10 cm and the temperature sensitivity is 10.15 pm/℃. It is expected that the system can be applied to small-scale heat source temperature detection in cable corridors, subways and so on.
In order to realize dynamic and reconfigurable optical chaotic logic operations, a specific technical scheme based on Vertical Cavity Surface Emitting Laser (VCSEL) feedback by its own light and linear electro-optic modulation effect has been proposed. The normalized injection current is modulated as logic input, the transverse electric field is modulated as control signal, and the logic output is demodulated by the difference between the average value and the threshold value of the x-polarized light intensity from the output of VCSEL. By transforming the logic operation relationship between control signal and logic input, the system can switch freely among basic logic operations such as NOT, AND, NAND, OR, NOR, XOR and XNOR. When the code width is 600 ps and the noise intensity is as high as 2.75×109, the success probability of the logic operation still equals 1, indicating that the system has good anti-noise performance. And when the noise intensity equals 2.5×109, the success probability always equals 1 if the code width is at least 579 ps. The above results have great reference value for the development of fast and stable combinational logic operation devices.
A polyaniline film micro-nano optical fiber liquid hydrogen ion concentration probe is proposed, and the hydrogen ion concentration sensing characteristics of this fiber probe are investigated. The sensing part consists of a single-mode fiber, a small-core-diameter fiber and a single-mode fiber pair fusion spliced together, and polyaniline material is coated on the small-core-diameter fiber as a sensitive film. Firstly, a theoretical analysis of the sensing principle of the fiber optic probe and the structure change mechanism of the polyaniline main chain was conducted. Then, the effect of different film thicknesses on the sensing characteristics of the fiber optic probe was verified and analyzed by hydrogen ion concentration sensing experiments. Finally, experimental interference tests on the change of liquid refractive index were completed and the response-recovery time and stability performance of the probe were evaluated. The experimental results show that the main chain structure of polyaniline changes under the action of hydrogen ions. The interference spectrum of the fiber optic probe drifts in the short-wave direction with the increase of hydrogen ion concentration, and the detection range increases with the rise of polyaniline film thickness, while the sensitivity and linearity decrease significantly. The sensitivity of the fiber optic probe is -15.74 nm/mol/L for the hydrogen ion concentration range of 10-6~10-1 mol/L, and the response and recovery time are 25 s and 35 s, respectively. The experiments verify the change of polyaniline film’s optical properties after reacting with hydrogen ions and provide a new method for liquid hydrogen ion concentration detection. This fiber optic hydrogen ion concentration probe also has the advantages of high concrete film strength, simple fabrication, and low cost.
To solve the measurement problems of fiber-optic strain sensors caused by temperature sensitivity, a hybrid temperature-strain dual-parameter based on fiber Bragg grating and multimode interference of hollow core fiber was proposed. The sensor is composed of a hollow core fiber fused between two single mode fibers, and the inner diameter of the hollow core fiber is smaller than the core of the single mode fibers, besides, on the core of one single mode fiber, near the fiber end, a fiber Bragg grating is pre-written. The hollow core fiber has a length of centimeters, in which, the optical wave is propagating in a multimode form. Combining the different responsitivities of the hollow core fiber and fiber Bragg grating to the temperature and the strain, the two parameters can be simultaneously demodulated by solving of the dual-parameter coupling matrix, and the problem of temperature-strain cross sensitivity of a single fiber Bragg grating or hollow core fiber sensor can be effectively solved. A hybrid fiber Bragg grating- hollow core fiber sensor was fabricated by using of a fiber Bragg grating with a center wavelength of 1 550.172 nm and a hollow fiber of 2.5 cm long and inner diameter of 5 μm. The experiment on the strain and temperature measurement shows temperature sensitivities of 10.530 6 pm/℃ and 1.802 1 pm/℃, strain sensitivities of 0.720 7 pm/με and 1.243 2 pm/με from the hollow core fiber and fiber Bragg grating respectively.
The optical loss caused by Waveguide Sidewall Roughness (SWR) of Silicon-on-insulator (SOI) is one of the restrictions to the adoptions of silicon photonic integrated circuits. In this paper, the anisotropic SWR of an SOI waveguide is measured by Conformal Laser Scanning Microscope (CLSM) and with introduction of a Three-dimensional (3D) anisotropic SWR, the traditional theoretical model for defining the Optical Propagation Loss (OPL) coefficient, so that a more accurate theoretical model is obtained. Numerical simulations show that the waveguide structure determined Correlation Length (CL) and the SWR have the synchronous effects on the OPL. Fabry-Perot (F-P) cavity modulation resonance output is used to accurately measure the OPL, and the measured values are agreeable with the simulation result, implying the improved model has more believability. For a waveguide with a 4 μm width, when the average horizontal and vertical SWR values are 22 nm and 23 nm, respectively, the simulation results of OPL coefficient for both TE- and TM-mode are 4.5~5.0 dB/cm, while the experimental result is 4.9 dB/cm. Hence, the outcomes and conclusion obtained are very valuable to be referred for research and development of SOI waveguide devices.
pulse width modulation technology is proposed. Bilevel pulse width modulation is analyzed in terms of bandwidth requirement, junction capacitance and error performance, and compared with non return to zero on key modulation, pulse position modulation and digital pulse interval modulation.The results show that the two-level pulse width modulation and the non-return-to-zero on-off keying modulation have the same time slot error rate, but the additional power consumption is only M/2 of the non-return-to-zero on-off keying modulation, and M is the modulation order. Although the bilevel pulse width modulation requires higher signal-to-noise ratio compared with pulse position modulation and digital pulse interval modulation, the additional power consumption is only half of that of pulse position modulation and digital pulse interval modulation, and the bandwidth requirement is also less than that of pulse position modulation and digital pulse interval modulation, it has great advantages in power and bandwidth sensitive visible light communication system.
A method of fiber Bragg grating time-division multiplexing intensity demodulation based on Sagnac fiber ring is presented. The time-multiplexing sensing system, consisted of the several FBG gratings with the same central-wavelength, can demodulate the signals by observing the intensity changes of the output spectrum of the FBG gratings. In the signal processing module, the wavelet analysis and Gaussian simulation algorithm are combined to demodulate the system signal together. The experimental results show that the signal-to-noise ratio of the system signal is increased by about 36% from 8.06 dB to 11.7 dB, which effectively improves the anti-interference ability of the system and the detection accuracy for the external parameters. The proposed method effectively increases the number of fiber Bragg grating sensors in the demodulation system with less cost and can be widely applied to various sensor systems or parameter detection systems.
In order to solve the problems of high peak-to-average ratio of traditional orthogonal frequency division multiplexing technology, complex-valued signal structure cannot be directly applied to intensity modulation direct detection system, Lifting Wavelet Transform (LWT) is applied to Orthogonal Frequency Division Multiplexing (OFDM). The orthogonal wavelet basis is selected as the subcarrier. By predicting and updating the signal, the high and low frequencies of the signal are separated. Combined with the multiple reflection channel model of Visible Light Communication (VLC), the DCO-LWT-OFDM system model was established, and the iterative decomposition and renewal reconstruction formulas of LWT-OFDM signals were deduced. The performance of the DCO-LWT-OFDM system was simulated and verified by experiments. Simulation results show that in a 4 m×4 m×3 m indoor space, when the bit error rate of the system is 10-4, the performance of the DCO-LWT-OFDM system is about 5 dB higher than that of the DCO-FFT-OFDM system, and the overall efficiency is improved by 70%. When the system peak-to-average ratio is 10 dB, the value of complementary cumulative distribution function of DCO-FFT-OFDM system is close to 10-1, and the value of complementary cumulative distribution function of DCO-LWT-OFDM system is 0. In the experimental verification, 1 W LED lamp beads were selected to build the point-to-point DCO-LWT-OFDM VLC system on the optical guide, the transmission distance was 20 cm, the system modulation error rate was 11.4 dB, and the reliability could reach 10-4. This paper provides an effective way to improve the transmission rate of visible light communication, reduce the bit error rate and restrain the system PAPR.
In order to improve anti-bending property and keep the similar model field diameter of G 652, a single-mode, small-size, anti-bending fiber by designing the structure of the optical fiber preform with depressed cladding is proposed. The outer diameter of the fiber is 180 μm, the mode field diameter reaches 9.1 μm, the macrobending loss of the fiber at 1 550 nm is less than 0.4 dB and the long-term environmental performance is not larger than 0.03 dB/km. The cabling results show that the size of the cable can save 44% by using this optical fiber. The optical fiber cable could be used for crowded and narrow channels.
A method for measuring the refractive index distribution of optical fibers is presented, which uses the white light scanning interference technology and builds the same structure on the reference mirror as the optical fiber sample to overcome the limitation of the short white light coherence length, optimizes the optical path and improves the contrast between the interference fringes. For white light interference signals, Morlet wavelet which has Gaussian distribution like envelope of white light interference fringes is used as the mother wavelet of wavelet transform for fitting processing, and the relative height between the fiber and the matching fluid with known refractive index is obtained. The refractive index distribution of optical fiber can be obtained by calculation, and fitting the obtained data with the classical function of the refractive index distribution of the fiber core, the coefficients of determination of the multi-mode fiber and single-mode fiber are 0.997 2 and 0.996 4 respectively. Finally, the experimental results are compared with the official parameters, and the error is 0.01%, which shows that this method has a high precision and can be used to measure the refractive index of optical fiber.
This paper reports a microwave frequency dissemination experiment over a 56 km-long-fiber link in the laboratory, exhibiting frequency instabilities of 1.8×10-15/s and 4×10-18/104s. The phase perturbation accumulated along the fiber link is detected by comparing the round-trip signal with the reference signal. By controlling the phase of the transmitted signal in real time, the phase perturbation along the fiber link is compensated. Different modulation frequencies are used to avoid stray reflection effect. To improve the phase noise of the detection signal, we also implemented dispersion compensation.
Underwater wireless optical transmission faces the problems of power attenuation and time domain width enlargement, the long-distance transmission characteristics of blue-green light are also more difficult to test. Monte Carlo method is used to investigate the process of underwater Gaussian beam, the effects of channel parameters such as water type, attenuation distance and divergence angle on the received power and impulse response are taken into account, power distribution on the receiving surface with different seawater types and transmission distances under small divergence is also numerically compared. The simulation results show that the received power decreases gradually and the time-domain width expands greatly with the increase of seawater attenuation coefficient, attenuation distance and divergence angle, among them, the influence of divergence angle on the transmission distance of turbid harbor water above 10 m is small, while the width of time domain changes from 0.32 ns to 0.8 ns with the increase of divergence angle. Although the transmission distance is doubled, the power similarity is still up to 99% in pure sea water and clear ocean water. Then based on the difference of seawater attenuation coefficient, we propose the power similarity and area ratio to analyze underwater long-distance transmission, so as to obtain the long-distance transmission characteristics more efficient and quicker.
To improve on the poor real-time performance and low accuracy of traditional manual feature extraction and pattern recognition method in the Phase-Sensitive Optical Time-Domain Reflectometric System (φ-OTDR), a new pattern recognition method based on wavenet is proposed. This method fully analyzes the temporal causality of optical fiber vibration signals through the causal dilated convolutions, and makes the model converge faster by the residual block structure, so as to achieve higher recognition accuracy and efficiency.The experimental results show that, when three signals are recognized, namely, hand tapping, foot stepping, and stick striking, compared with the other two common methods, one-dimensional convolutional neural network structure and long short-term memory structure, the recognition accuracy as high as 99.85% is achieved with the proposed method. And it consumes the least amount of training time, as short as 96 s. Also, its signal detection process takes only 30 ms, which can meet the real-time application's requirement. The proposed pattern recognition method has high accuracy and good real-time performance. It should be of great significance for the application and popularization of φ-OTDR systems in perimeter security and similar areas.
In order to achieve high sensitivity space laser communication and improve the anti-interference ability of the transmission channel, this article combines single photon detection technology and pulse position modulation technology, the single photon detector avalanche is quenched by the combination of the gated circuit and the feedback quenching circuit. The insert frame header method was designed to modulate and demodulate the pulse position. The pulse position modulation and demodulation process was simulated by field programmable gate array, which verified the effectiveness and feasibility of the insert frame header method. On this basis, a 1 550 nm pulse position modulation laser communication experiment was built, and the performance of the single photon detector under different parameter was tested at the same time. The results show that the single photon detector has the best performance when the detection efficiency is 25%, the trigger delay is 8.00 ns, the gate width is 5.0 ns and the death time is 0.1 μs. Finally, the detection sensitivity of single photon detectors with different modulation rates was tested. The results show that the communication sensitivity is -51.8 dBm when the communication code rate is 1 Mbps; the communication sensitivity is -41.0 dBm when the communication code rate is 4 Mbps, which realizes high sensitivity space laser communication.
A fiber bundle made of 12×9 individual Ge-As-Te-Se fibers with a core diameter of 20 μm has been tested for optical properties and used to carry out the imaging experiment. A 5~11 μm continuous tunable quantum cascade laser has been used to test the bundle’s attenuation property with a cut-back method. The attenuation of bundle is nearly 1 dB/cm in the spectral region. A compact distal objective lens with length of 13.6 mm and diameter of 6 mm based on telecentric system has been designed and fabricated. The MTF of the lens showed the ablitity to transmit the image with resolution of 25 μm. A infrared image with 2 mm×2 mm and 100 μm resolution with quantum cascade lasers illumination, which has high power spectrum density and narrow linewidth, has been captured. A comparison experiment about the environmental temperature has been conducted illuminated by quantum cascade laser and incoherent black body, it proved that the quantum cascade laser illumination could help build a high SNR imaging under a high environmental temperature even above human's temperature. It would be meaningful for the next infrared endoscopy experiment in vivo way.
A thin-cladding Excessively Tilted Fiber Gratings (ExTFG) cantilever beam vibration sensor is reported in this paper. Using the ExTFG written in the standard single-mode fiber, the effects of the decrease of the fiber cladding radius on the dispersion factor of the waveguide, the effective refractive index of the cladding mode, the axial strain sensitivity factor, the axial strain sensitivity and the mode order are theoretically analyzed, and the corresponding numerical simulations are performed, which provide the theoretical foundation for the enhancement method of vibration measuring sensitivity. And then, hydrofluoric acid is used to etch the fiber cladding to fabricate ExTFGs with different diameters and the related vibration sensing experiments are conducted. The results show that in the vibration frequency range of 40~200 Hz, with the decrease of cladding diameter, the acceleration sensitivity of ExTFG vibration sensor in C band of the same order and different order TE/TM mode increases gradually, and there is a good linear relationship between them. The maximum acceleration sensitivity of the TE and TM mode with the same-order cladding mode is 100.46 mV/g and 88.68 mV/g, respectively, which is increased by 1.36 times and 1.53 times, respectively, as compared with the standard diameter one. And the maximum acceleration sensitivity of those with different-order cladding modes can reach 159.35 mV/g and 133.37 mV/g, respectively, which is increased by 2.15 times and 2.31 times.
Aiming at the problems of severe noise interference of fiber-optic vibration signals, single feature extraction and long recognition time, an improved local characteristic-scale decomposition and ant colony algorithm optimize deep belief network are proposed. Firstly, cubic B-spline function interpolation is used to fit the mean curve to improve the local characteristic-scale decomposition algorithm, and the sum of a series of intrinsic scale components is obtained by decomposing the original signal. Secondly, the fusion index is formed by kurtosis factor and energy spectrum coefficient to screen the effective component. Then, the entropy features of the effective components in the time domain, frequency domain and time-frequency domain are extracted respectively to perform feature fusion and dimensionality reduction. Finally, the integrated feature vectors are fed into ant colony algorithm optimized deep belief network for training and recognition to improve the algorithm efficiency and recognition rate. Experimental verification using measured data shows that the signal-to-noise ratio is increased by 8 dB on average, the average signal recognition rate can reach 95.83%, and the average recognition time is 0.715 s.
Fiber Bragg Grating (FBG) thermal flow sensors are only suitable for gas flow currently. In order to expand its application field, a new FBG thermal flow sensor that can be used for liquid flow measurement is designed. A heating ceramic sheet is used to provide heat at a constant power for the FBG thermal flow sensor. Different flow of liquid takes different heat when flow through the sensor. By detecting the change of FBG central wavelength, the temperature change of the sensor can be measured, and then the liquid flow can be deduced. Through temperature sensor test experiment and flow sensor test experiment, it is verified that the designed sensor can be used for liquid flow measurement. The experimental results show that the flow measurement range of the sensor is 40.575~550.664 L/h.
A new kind of polarization maintaining photonic crystal fiber with two side-holes is designed. There is only a layer of small air holes between the core and the large air hole, and two large air holes are introduced in the cladding symmetrically. The two-dimensional model was simplified by the plane strain hypothesis, the two-dimensional model of this fiber was numerically analyzed by the finite element method, and the characteristics of temperature and hydrostatic pressure sensing were studied by calculating the birefringence-induced frequency shift at different temperatures and hydrostatic pressures. As the studies show, within a large range of hydrostatic pressure and temperature, this polarization maintaining photonic crystal fiber can realize the hydrostatic pressure sensitivity of -2.135 3 GHz/MPa and it is insensitive to temperature without doping any stress material. Its temperature sensitivity is only +0.154 2 MHz/℃. In addition, the optical properties of the photonic crystal fiber were also analyzed. The proposed photonic crystal fiber satisfies the condition of single-mode transmission, has a small confinement loss and a large effective mode area. Due to its characteristics of small size, strong compatibility with other optical fibers, high hydrostatic pressure sensitivity and temperature insensitivity, it has obvious advantages in the accurate measurement of hydrostatic pressure in the environment with variable temperature and great hydrostatic pressure variation. Better optical properties make it has important reference value in monitoring application of oil well and civil engineering.
A Fabry-Perot Interferometer (FPI) was constructed by fusion splicing a Hollow Core Fiber (HCF) with a length of tens of microns at the end of a single-mode fiber and coating gelatin film at the free end of the HCF. Relative humidity measurement was achieved by detecting wavelength shift of the interference spectrum with humidity level changes. Experimental results show that high sensitivity of 192 pm/%RH has been obtained in the temperature range of 20 ℃ within the range of 20%~80%RH and the measurement accuracy and repeatability are quite good. In addition, the sensitivity of 173 pm/%RH and 194 pm/%RH were obtained by humidity increasing experiments at 15 ℃ and 25 ℃, respectively. In order to measure temperature has also been achieved by cascading a fiber Bragg grating sensor with the FPI sensor head. The optical fiber humidity sensor possesses several advantages including simple fabrication, high sensitivity, and temperature measurement. It has good potential in the field of humidity measurement.
To address the shortcomings of piezoelectric and optoelectronic pulse wave sensors that are susceptible to interference in strong electromagnetic interference environments, a flexible material encapsulated Fibre Optic Grating (FBG) pulse wave sensor is proposed for measuring human radial artery pulse signals. The optimal package thickness and the position of the optical fibre in the substrate were first investigated using comsol finite element simulation software. Based on the simulation results, the sensor thickness of 5 mm and the optical fibre encapsulation at 1 mm from the lower surface of the substrate were optimised to combine the strain transfer effect and the flexibility of the sensor. The FBG flexible pulse wave sensor was fabricated and the human flexural artery pulses were collected from ten test subjects. The denoised signals were well preserved by a modified threshold wavelet method, and the signal-to-noise ratios were above 40, and the peak, tidal and repulse waveform recognition rates were 100%, 100% and 90% respectively. The results show that the flexible substrate FBG pulse wave sensor can effectively acquire and identify pulse wave signals, providing a theoretical basis and application support for further applications.
Hydrogen diffusion is an important factor that causes additional absorption loss in optical fibers. The concave G.657 fiber with the inner cladding was chosen as the experimental fiber, and the structure and attenuation factors of the G.657 fiber were analyzed. The mechanism of deuterium gas to eliminate the hydrogen sensitivity of the fiber was explained, and the fiber hydrogen diffusion experiment was designed to carry out the deuterium gas treatment formula. By adjusting the two key parameters of deuterium concentration and processing time, the additional attenuation value of the fiber under different experimental conditions was obtained. The comparison experiment results and the tracking retest results show that 0.9% deuterium gas concentration and 80 h deuterium treatment time are suitable for reducing the hydrogen diffusion of G.657 optical fiber.
In order to improve the recognition accuracy of intrusion vibration events by distributed optical fiber acoustic sensing system based on phase-sensitive optical time domain reflectometer, a recognition method of fiber optic vibration signals using endpoint detection and signal recombination is proposed. The method first uses EMD_PCC-based denoising to denoise the signal, then performs endpoint detection on the denoised signal, reorganizes the detected vibration signal and extracts the frequency domain information of the vibration signal, uses a dual-input multi-scale convolutional neural network to extract the time domain features and frequency domain features of the vibration signal respectively, and finally uses a support vector machine for classification. The experiment proves that the recognition method can quickly complete the training of the recognition model and effectively recognize the intrusion vibration signals collected in the actual environment, and the recognition accuracy of the intrusion signals can reach 94.8%.
A reconfigurable microwave photonic mixing and phase shifting system is proposed. By adjusting the frequency of the local oscillator signal and the angle of the polarizer, it can output up/down conversion microwave signal, and its phase can be adjusted continuously within 360°. In addition, by appropriately changing the phase difference between any two mixing channels, it can be reconstructed to realize I/Q mixing, double-balanced mixing and image rejection mixing. When applied to multiple channels, it can output multi-band mixing and phase-shifted microwave signal, and the phase of each channel can be adjusted independently. Since the high-order sidebands of the local oscillator signal are involved in the mixing, the requirements for the frequency of the local oscillator signal are reduced exponentially. The simulation results show that when the frequency of the local oscillator signal and the RF signal are 10 GHz and 19 GHz respectively, the mixing range of the system is 1~79 GHz, the phase shift amplitude difference is less than 0.1 mV, the image rejection ratio exceeds 65 dB, and the signal power difference after multi-band mixing is less than 0.2 dB. In addition, the 90° electrical phase shifter and polarization controller angle adjustment deviation hardly affect the system performance, and the appropriate radio frequency signal power can help improve the system performance. Theoretical analysis and simulation have verified that the system has high flexibility, reconfiguration and practicability.
A scheme for generating 16-tupling millimeter wave signals is proposed, which uses parallel Mach-Zehnder modulators and optical attenuator without filters. In this scheme, the Mach-Zehnder modulator and optical phase shifter in parallel are used to generate the 8th-order optical sideband, and the 16-tupling millimeter wave signal is obtained through the beat frequency of photodetector. In view of ideal and non-ideal extinction ratio of modulator, when the extinction ratio is 35 dB and 100 dB respectively, the optical carrier and the fourth-order optical sideband are theoretically suppressed, and the eighth-order optical sideband signal is derived. The correctness of this deduction is verified by simulation. Furthermore, according to the simulation results, the influence of non-ideal conditions of various parameters, such as the shift of modulation index and optical attenuator attenuation value, the angle shift of electric phase shifter and optical phase shifter, on the system is analyzed. The 2.5 Gbit/s baseband signal is modulated on the 16-frequency millimeter wave signal generated by the scheme. When the transmission distance of the system is 5 km, 10 km and 15 km, the loss power is 0.35 dBm, 0.55 dBm and 2 dBm respectively, and the transmission loss is small. In addition, the relationship between laser linewidth and received power is analyzed. The received power is -22.6 dBm when the linewidth is 10 MHz, and the power losses are 0.1 dBm, 0.25 dBm and 0.6 dBm when the linewidth is 20 MHz, 30 MHz and 40 MHz, respectively.
Taking panda-type polarization-maintaining fiber as the object, the influence of drawing tension and heat treatment on its stress birefringence is systematically studied. In the multiphysics finite element modeling, we introduced the drawing tension during the fiber preparation process into the PMF photoelastic model, and we summarized the numerical law that the birefringence of the panda-type PMF decreases linearly with the increase of the drawing tension. We used white light interferometry to experimentally measure the effects of different heat treatment conditions on the birefringence of PMFs. The structural relaxation induced by boron-doped silica glass at high temperature causes the volumetric shrinkage of the Stress Applying Parts (SAP), which increases the birefringence. The deformation caused by the structural relaxation is obtained from the variation of the vacancy defect concentration of the glass network, and then the birefringence after the annealing of the fiber is calculated. The numerical simulation results are consistent with the experimental measurements. This work provides a theoretical basis for the design, preparation and birefringence control of PMF.
In order to obtain more precise icing thickness distribution within a span, a special icing monitoring technique for tight tube Optical Fiber Composite Overhead Ground Wire (OPGW) is proposed and preliminary verified by experiments. This method utilizes the distributed strain sensing capability of Brillouin optical time domain reflectometer. The strain distribution of OPGW was obtained by measurement, and then the weight distribution was derived based on mechanical analysis method. the specific icing thickness distribution was obtained via the conversion between weight and icing thickness. In the simulation with a suspended optical cable, the errors of the experimental results are 2.27% and 15.77% for two uniform loadings of 5 kg and 2.5 kg, which preliminarily verifies the effectiveness of the proposed method. The error analysis shows that the effect of the proposed method is better for longer span range in practice, because it allows longer spatial resolution which can improve the strain measurement accuracy.
Based on the synthetic method of rainfall attenuation time series, the dynamic rainfall attenuation time series at 28 GHz、30 GHz and 38 GHz in Beijing are simulated, and the power spectraare estimated by using fast Fourier transform and Kaiser Window function. The probability distributions of rainfall attenuation are obtained by simulating rainfall attenuation events for many times and statistical analyzing of the simulation series. Compared with the model recommended by ITU-R, it can be used for the prediction of 5G millimeter wave rainfall attenuation.
In free-space optical communication system, the scintillation of optical intensity from atmospheric turbulence can seriously affect communication performance. In order to reduce the scintillation effect caused by turbulence, based on the time-domain characteristics of light intensity scintillation caused by turbulence, a simulation method which can generate dynamic scintillation series is presented. And its statistical characteristics can be used in the study of free-space optical communication against turbulence effects. In this paper, the time series model are established which can reflect the dynamic characteristics of the received optical intensity. The time series model combine double generalized Gamma distribution and the time signal covariance function of atmospheric turbulence by using generation algorithm of the non-Gaussian colored spectrum signal. When the pointing error is considered in the optical communication system, the time series simulation of the light intensity scintillation under different turbulence conditions is analyzed, and the results are in agreement with the theoretical results. And the formula of average BER under joint probability density is given. The communication system average bit error rate and outage probability can be analyzed by the generated time series involved.
A linearized self-interference cancellation scheme based on a dual-polarization dual-drive Mach-Zehnder modulator is proposed to cancel the self-interference and reduce the nonlinearity of the signal of interest for in-band full-duplex systems. The received signals consist of the signal of interest and self-interference. They are injected into the radio frequency ports of the upper arms of the two sub-dual-drive Mach-Zehnder modulators of the dual-polarization dual-drive Mach-Zehnder modulator, while the constructed reference signals are injected into the radio frequency ports of the lower arms. The two sub-dual-drive Mach-Zehnder modulators are respectively biased at the maximum transmission point and the quadrature transmission point. When the two optical signals from the two dual-drive Mach-Zehnder modulators are detected in two photodetectors, two electrical signals are generated. If the reference signals and the self-interference are the same, the two electrical signals from the photodetectors are self-interference free. The two self-interference-free electrical signals are sampled and further processed in the digital domain to obtain the signal of interest without self-interference and nonlinear components. In addition, an optimization algorithm to reduce the distortion caused by the power mismatch of the two electrical signals is proposed. The feasibility of this scheme is verified by simulations and experiments. The experimental results show that when the signal of interest is a 4-dBm two-tone signal with frequencies of 10 MHz and 12 MHz and the self-interference is a 4-Mbaud quadrature phase-shift keying signal with a center frequency of 11 MHz, a cancellation depth of around 25.6 dB for the self-interference and a cancellation depth of around 17.3 dB for the third-order intermodulation products can be achieved.
In the satellite-to-earth coherent optical communication system, in order to compensate for the wavefront distortion of signal light caused by the random disturbance of atmospheric turbulence, a wavefront coherent compensation technology based on a dual detector array and a stochastic parallel gradient descent algorithm is proposed. Taking the regional light intensity ratio as the beam evaluation standard, an optical communication simulation system integrating signal light wavefront compensation and demodulation is built, and the 65-order Zernike polynomial is used to simulate atmospheric turbulence wavefront distortion. The simulation system adopts quadrature phase shift keying modulation, and the data transmission rate is 10 Gbps. The results show that when the bit error rate is 10-9, the number of photons per bit required by the communication system is reduced to 65.6%, and when the number of photons per bit is 14, the bit error rate is reduced to 2.6%. The wavefront compensation technology based on the dual detector array can compensate the signal light wavefront distortion, reduce the optical communication system error, and provide a guarantee for the signal transmission of the satellite-to-ground optical communication link.
From the perspective of micro-nano optical fiber coupling, an optical fiber sensor is designed by using single mode optical fiber 2×2 coupling. By changing the tapering parameters of optical fiber coupler, the over-coupling state is reached. At this time, the over-coupling structure of optical fiber itself is affected by the environment, so that the wavelength of light has a higher response to the change of external temperature and strain, realizing the measurement of temperature and strain. In the experiment, optical fiber over-couplers with overcoupling length of 22 000 μm and 22 500 μm are tapered, and their transmission spectra were compared. The temperature and strain sensing tests results show that the lowest wavelength response to temperature is about 104 pm/°C, and the lowest wavelength response to strain is about -21 pm/με.
For the space applications of fiber amplifier, the radiation performance of fiber amplifier in radiation environment was investigated experimentally. The evolution laws of output power and spectral character are investigated experimentally by irradiating the gain fiber of an erbium-ytterbium co-doped fiber amplifier with gamma rays. The investigation of the noise characteristics of erbium-ytterbium co-doped fiber amplifier has been carried out by means of frequency spectrum measurement. In the on-line irradiation experiments, with a total radiation dose of 50 krad, it is found that the laser output power, the power of peak wavelength and optical noise decrease with the increment of radiation dose. The noise characteristics of the fiber amplifier are analyzed by the optical noise model after irradiation. Compared with the results before irradiation, the coefficients of relative intensity noise of relaxation oscillation noise and the intermediate frequency part are increased by 1.625×10-4 nW·mW-2·Hz-1 and 3.122×10-4 pW·mW-2·Hz-1, respectively, while the coefficients of shot noise are reduced by 0.900 pW·mW-1·Hz-1 and 0.035 pW·mW-1·Hz-1, respectively. Therefore, it is necessary to pay attention to the suppression of the relative intensity noise for the practical application of fiber amplifier in space.
A optical fiber sensor based on modular interference is proposed to measure temperature, refractive index and axial strain. Waist-enlarged taper is formed at the splice point of a single-mode fiber and a double-cladding fiber, which is then cascaded with two long-period fiber gratings with different periods. Due to the mismatch of the mode fields, the higher-order cladding modes are excited,three resonance peaks are formed, which have different sensitivity responses to different parameters. Therefore, by demodulating the wavelength shift of the three resonance peaks and using the coefficient sensitivity matrix, temperature, refractive index and axial strain can be measured.The experimental results show that the sensitivity is 60.07 pm/℃, 6.47 pm/℃ and 103.83 pm/℃ in the temperature range of 25℃~75℃, the refractive index is in the range of 1.335 5~1.359 5, and the sensitivity is -56.64 nm/RIU, 34.02 nm/RIU and -214.84nm/RIU, the axial strain is in the range of 200~1 400 με, the sensitivity is -2.14 pm/με, -3.61 pm/με and -2.59 pm/με.The resolution is 1.29℃, 0.000 42 RIU and 21.42 με respectively. The sensor has high sensitivity, good linearity and other advantages, can be widely used in the field of multi-parameter measurement.
A kind of Fabry-Perot optical fiber humidity sensor is fabricated by normal single-mode optical fiber and Graphene Quantum Dots.By the established experimental system, the humidity response experiments were carried out in the relative humidity range of 11% RH ~85% RH. The sensitivity was 0.560 6 nm/%RH with 0.999 47 linearity at humidity rising, and 0.565 5 nm/%RH sensitivity with 0.999 36 linearity at humidity falling. The experimental results showed that the humidity sensor has high humidity response sensitivity, good linear response characteristics and measurement repeatability. Furthermore, the temperature responding characteristic is also experimentally investigated. The good linear temperature responding results are got with 0.035 nm/℃ sensitivity, 0.012 41 residual sum of squares, 2.305×10-4 sensitivity stand error. The humidity response sensitivity is about 17 times of temperature response. Typical tests are given for dynamic response characteristics. The dynamic response data of interference spectrum wavelength drift under 43% RH showed fast dynamic response characteristic. The response time and recovery time were 6.5 s and 9.0 s respectively. The research results provide a beneficial exploration for developing low cost, easily fabricated and high sensitive optical fiber humidity sensor.
A combined algorithm of amplitude normalization and minimum mean square error for the cavity length interrogation of fiber-optic Fabry-Perot sensors is proposed. An amplitude normalization method is introduced to realize the equiamplitude of both the reflected and the simulated spectral signals, which helps improving the interrogating accuracy. Significant increasing of interrogating accuracy compared with conventional algorithms is verified through numerical simulations. And, a 0.72 nm resolution of cavity length interrogation is achieved in real experiment.
To improve coupling efficiency from blackbody source into fiber, a coupling model of light intensity from the blackbody cavity into a spherical optical fiber was proposed. Influence of parameters of spherical fiber and blackbody cavity on coupling efficiency was analyzed. Results show that coupling efficiency firstly increases and then decreases with the increase of radius of spherical fiber. When radius is around 85 μm, coupling efficiency reaches the maximum. Spherical fibers with radius from 65 μm to 105 μm were fabricated by fusion splicer. Experiments were carried out with spherical fibers and a fiber with flat end face. Results show that compared with the fiber with flat end face, coupling efficiency of spherical fiber is greatly improved, when radius is around 85 μm, coupling efficiency is improved by approximately 60%. The experimental results are consistent with the theoretical analysis.
There is an error between the measured strain of the fiber Bragg grating sensor bonded on the surface of bending host material and the real strain of the host material. As a result, the issues of the deformation mechanism and the relationship between measured strain and real strain of the fiber Bragg grating sensor received considerable critical attention. First of all, the interaction mechanism between the fiber Bragg grating sensor and the host material was studied. Then, finite element solution, experimental value and theoretical solution were used for comparison and verification. Also, the causes of the errors were analyzed. Finally, the influence of the parameters (e.g., Young's modulus, thickness, bonding length) on the measurement effect of fiber Bragg grating sensor was studied. The results reveal that finite element solution, experimental value and theoretical solution exhibit the same variation trend. The error between finite element solution and theoretical solution is controlled within 2%, while the error between experimental value and theoretical solution is controlled within 7%. The average strain transfer rate increase with an increase of both the Young's modulus of the host material and the bonding length. Opposite conclusion held for the decreasing elastic modulus of the adhesive and increasing thickness of the adhesive. This theory has a certain guiding significance for the design of fiber Bragg grating sensors used for the strain measurement of the bending host material.
In this paper, a dual optimized down taper structure based on fiber Mach-Zehnder interferometer is designed. This structure is manufactured by using insufficient arc discharge splicing between single-mode fiber and polarization-maintaining fiber method, achieving simultaneous measurement of strain and temperature. The spherical core which located at the middle of the down taper can further regulate the light power distribution in the core and cladding area. The optimized structure can obtain a larger interference fringe extinction ratio of 16 dB, which is greater than the dual-down taper structure under the same parameters. Sensing experiments show that the proposed structure has high resolutions of ±1.616 με and ±0.79℃ in the range of 0~244.35 με and 25~50℃, respectively. The measurement error of both parameters is less than 1×10-3% due to cross-sensitivity. This structure provides a promising method for simultaneous measurement of strain and temperature, which can be applied to precision instrument measurement.
The limitation of peak power caused by two kind of nonlinear effect, self-phase modulation and stimulated Brillouin scattering, in G.652 single-mode fiber is studied. It shows that dispersion and self-phase modulation lead to distortion of optical pulse waveform and its effect is determined by optical pulse peak power.Stimulated Brillouin scattering results in fast decay of pulse power in fiber and further shorter sensing distance, and its effect is determined by optical pulse energy. Experiment data shows that self-phase modulation puts more stringent limit on optical pulse lauch power. Based on the analysis of experimental data, empirical formulas for power upper limit are given, which can be used to estimate the maximum optical pulse incident power in distributed fiber sensing systems. For a typical distributed fiber sensing system with range of 25 km, the incident power upper limit of optical pulse peak is about 1 W.
An experimentally study on microwave frequency transfer over a 50 km long fiber was reported, which frequency instabilities reach 4.38×10-15@1 s and 2.80×10-18@65.5×103 s. By using the well-known Doppler noise cancellation method, the fiber noise was measured by comparing the phase of the signal double-passed fiber and the reference one at local site, and compensate the fiber noise with a home-made fiber-based variable delay line, which includes a fiber stretcher driven by a piezo-transducer for fast action with 1 kHz bandwidth and a temperature controlled fiber spool with a delay adjustment range of 5 ns. Compared with electronic phase compensation technique, this approach is insensitive to microwave leakages which is difficult to avoid in such a case, consequently, enables a better instability for long terms. In addition, we synthesized the back and forward transferred signals into two different frequencies to avoid effect introduced by stray optical reflections, and employ dispersion-compensation fiber to reduce the penalty due to the chromatic dispersion.
In order to achieve a larger amplification bandwidth, while increasing the output gain and maintaining a small gain flatness, a second-order Raman fiber amplifier is designed, which used a second-order pump and four first-order pumps to perform distributed Raman amplification on 100 channels of signal light. First, numerically solve the second-order Raman coupled wave equation. Simultaneously, in order to further improve the output performance, the performance parameters of the second-order Raman fiber amplifier are optimized by using particle swarm optimization algorithm. Then, under the same pump parameter configuration, compare the first-order RFA with the second-order RFA. Finally, the factors affecting the output gain of the second-order Raman amplifier are analyzed. Through experimental simulation, within the 1 510~1 610 nm gain bandwidth range, the designed second-order Raman fiber amplifier has an average output gain of 23.768 0 dB, a maximum output gain of 24.124 4 dB and the gain flatness is 0.911 2 dB.
The forward stimulated Brillouin scattering based fast light in small-core photonic crystal fibers is theoretically investigated. Three-wave coupled wave equations of forward stimulated Brillouin scattering in frequency domain were derived to calculate the group refractive index and gain coefficient in small-core photonic crystal fibers by Fourier transformation, then optical and acoustic field distribution, advancement and broadening factor of signal pulses induced by forward stimulated Brillouin scattering were simulated by the finite element method. Tight confinement of the optical fundamental mode and acoustic modes strengthens nonlinear interaction in the small-core photonic crystal fibers and results in strong SBS and large advancement of time. The time advancement grows nonlinearly with the transmission distance of signal light increasing, and the signal pulses are compressed. The pulse broadening factor gradually levels off with the growth of the initial pulse width. The time advancement of 21.76 ns and pulse broadening factor of 0.77 are evaluated at the transmission distance of 70 m, the initial pulse width of 200 ns and the pumping pulse power of 600 mW.
By annealing at high temperature (850~950℃), the fiber Bragg grating can be erased at high temperature and grow again to form a regenerated fiber Bragg grating, which can work stably in a high temperature environment of more than 1 000℃. However, the mechanical strength of the regenerated fiber Bragg grating after annealing at high temperature is significantly lower than that of common fiber Bragg gratings. In this paper, the axial stress of fiber Bragg grating and the change of quartz molecular components in fiber Bragg grating are studied and analyzed by single mode quartz fiber experiment. The results show that compared with the unannealed fiber Bragg grating, the compressive stress at the fiber core decreases by 80 MPa, and the tensile stress at the cladding far away from the fiber core gradually decreases by 22 MPa. At the same time, with the increase of oxygen content in the atmosphere of hot annealing, the regenerated fiber grating SiO2 generated after annealing gradually increased, and the proportion increased from 52.99% to 69.92%. Although SiO2 has a high density and its mechanical strength is greater than Si2O3, the brittleness of the regenerated fiber grating after hot annealing still increases. Therefore, it is inferred that the change of components has no significant effect on the increase of brittleness of regenerated fiber Bragg grating, and the main reason for the increase of brittleness is the stress relaxation caused by high temperature. This paper provides a reliable theoretical and experimental basis for improving the mechanical properties and solving the brittleness problem of thermal regenerated fiber Braggg ratings.
The influences of the geometric deformation of the fiber such as ellipse, misalignment and diametrical nonuniformity on the performance of Orbital Angular Momentum (OAM) modes propagated in the Hollow Ring-core Polymer Optical Fiber (HRC-POF) are studied by full vector finite element method. In addition, the maximum deformation that the fiber can withstand under the condition that maintains the stable transmission of the OAM mode is also studied. The results show that the ellipse and misalignment will cause the mode walk-off upon propagation, leading to the decrease of the purity of synthesized OAM modes and the increase of the crosstalk. Numerical results show that the purity of synthesized OAM modes is more than 99.02% and the crosstalk is less than -20.08 dB when the ellipticity or misalignment is within 1.0%. The diametrical nonuniformity of the fiber will only affect the number of OAM modes supported in the HRC-POF. The larger the core radius is, the more OAM modes can be transmitted in the fiber. Besides, the original 26 OAM modes can be supported in HRC-POF when the diametrical nonuniformity is -3% to 10%.
A photonic microwave phase-shifting system with continuously tunable phase shift and the frequency multiplication factor is proposed. Without optical filters, the scheme is mainly consists of two integrated dual-polarization dual-parallel Mach-Zehnder modulators. By adjusting voltages of the radio frequency driving signal and direct current bias signal on the dual-parallel Mach-Zehnder modulators and phase modulator, frequency-doubling or frequency-tripling, ..., frequency-sextupling microwave signal can be generated, with 0° to 360℃ontinuously tunable phase shift. The simulation results show that, when the radio frequency signal frequency is 10 GHz, the output microwave signals with the frequency 20, 30, 40, 50, 60 GHz can be obtained respectively. The ratio of direct current bias voltage and half-wave voltage of the phase modulator is set to vary from 0 to 1, corresponding to phase shift of microwave signal vary from -180° to 180°. In addition, the effects of the extinction ratio of the modulators on the optical sideband suppression ratio and electrical spurious suppression ratio of the output microwave signal, as well as the effects of the phase balance of the 90° hybrid coupler on the phase drift and amplitude variation of the microwave signal are analyzed.
It is difficult to make receiver locating in the center of the light spot for high-speed laser communication, which results in difficultly establishing a stable underwater optical communication links. Monte Carlo simulation statistics method is used to analyze the distribution of the received light intensity of laser photons transmitted in seawater, and then the light spot image at the receiving end is sampled and analyzed through experiments, and the relationship between the receiver position and the received light intensity is obtained by curve fitting. The simulation and experimental results show that the received light intensity distribution is still approximately Gaussian distribution after 25 m underwater transmission. Based on the nonlinear estimation algorithm (extended Kalman filter) under the basic state control feedback theory, the distance between current position and position of the maximum light intensity spot is estimated according to received light intensity, and active tracking alignment between the receiver and the spot center is achieved by feedback algorithm. The simulation results show that the alignment error of the receiving end is less than 2 mm and the receiving efficiency is more than 98% after alignment.
A scheme is proposed for millimeter-wave signal generation without filters. No optical or electrical filters are used in the scheme, and the scheme can obtain 16-tupling millimeter-wave signals. The scheme adopts the structure of cascading a three-parallel Mach-Zehnder modulator structure and a single Mach-Zehnder modulator. All redundant sidebands can be suppressed very well by adjusting the parameters of the system and only 8th order optical sidebands are left. The scheme can obtain 16-tupling millimeter-wave signals without any optical or electrical filters. The theoretical analysis for this proposed scheme are provided, and the effects of modulation depth, extinction ratio, phase shifter offset and modulator bias on the system are verified by simulation. The 10 GHz radio frequency signal is mixed with the 2 Gbit/s non-return-to-zero pattern data as the driving signal of the Mach-Zehnder modulator, it is verified that the power cost of the system link through 50 km optical fiber transmission is only 1.0 dB. The system can meet the needs of communication systems with good transmission performance. The scheme has certain reference value for the generation of high frequency millimeter wave signal without filters.
An hybride optical fiber sensor is presented for the measurement of temperature and pressure in high temperature environment. The sensor is based on the configuration of an extrinsic Fabry-Perot interferometer(EFPI), which is formed by a Hollow Core Fiber(HCF) sandwiched between a section of Single Mode Fiber(SMF) and a section of of Photonic Crystal Fiber(PCF), and an intrinsic Fabry-Perot Interferometer(IFPI), which is formed by a section of PCF. Temperature measurement is achieved by thermal expansion effect and thermooptic effect, while pressure measurement is realized by the change of refractive index of gas. The demodulation of the sensor was realized by a self-made white light interferometry demodulator. In the environment of different temperature and pressure, the temperature and pressure optical fiber sensors whose cavity length is 306 μm and 1 535 μm were measured continuously. The experimental results show that the pressure sensitivity decreased with the increase of temperature. 1 460.5 nm/MPa is achieved at the temperature of 28 ℃ and the temperature response of the EFPI cavity is 17.4 nm/℃. The sensor is able to operate stably at temperature of 28~800 ℃ and pressure of 0~10 MPa.
In this paper, a novel photonic analog-to-digital conversion scheme with differential encoding based on vector superposition is proposed and demonstrated. Two pulse sources with different center wavelengths are employed to modulate the input signal by a phase modulator, the modulated signal is sent to a delay-line interferometer to achieve two differential modulated signals with a specific phase difference. By adjusting and combining the intensity of two differential modulated signals with a vector superposition module, the desired phase shifts among different transfer functions can be obtained. Compared with most existing photonic analog-to-digital conversion schemes, the proposed scheme can differentially encode the input signal with improved bit resolution; furthermore, this scheme features its relative simpler configuration, because only one phase modulator, one delay-line interferometer and a vector superposition module are required; in addition, since the desired phase shifts of transfer functions are realized by attenuating the signal intensities, the proposed scheme can effectively alleviate the problem of phase bias-drift induced by modulators. Proof-of-concept experiment of a 4-bit photonic analog-to-digital conversion system based on the proposed scheme is successfully carried out, which demonstrates the feasibility of the approach.
In order to solve the problem that the accuracy of scale factor measurement of Resonance Fiber Optic Gyroscope (RFOG) is limited by the performance of turntable, a new method based on sawtooth equivalent input was proposed. By adding sawtooth wave to the RFOG's phase modulator, which works as equivalent input angular velocity, the RFOG closed-loop transfer functions under turntable input and equivalent input were analyzed, relationship between the parameter of sawtooth and equivalent input angular velocity was given, the scale factor and non-linearity were obtained by the scale factor test system. The scale factor is basically identical to the test result on the turntable, and the non-linearity is also improved from 0.42% to 0.26%. The test results demostrate that the scale factor test system based on sawtooth equivalent input can accurately measure the scale factor of RFOG, and effectively eliminate the measurement errors due to the vibration and imperfect precision of the turntable.
A double square wave and B-spline wavelet for demodulation of a Weak Fiber Bragg Grating (WFBG) was proposed and demonstrated. A period of single square wave is set as the round trip time of laser transmission in the fiber between two adjacent WFBGs. The burst operation is conducted to the single square wave to form a double square wave, for which the rear square wave reflected by former WFBG and the front square wave reflected by latter WFBG overlap and interfere. B-spline wavelet transform is used to reduce the noise of interference signal. Hilbert transform is applied to produce π/2 phase-shift of the interference signal. Arc tangent operation is conducted to the ratio of the interference signal with the phase-shifted signal to obtain the phase signal of the interference signal. A 5-WFBG array with 50 meters equispaced length is put on a wooden platform, and received sinusoidal sounds with different amplitudes and frequencies respectively. The experimental results can reflect the information of the sounds well. The advantage of the proposed demodulation method is that the optical structure and data processing is simple.
Aiming at the problem of the stress and the displacement of the fiber embedded in the flexible optoelectronic substrate can change, which affects the coupling efficiency of the optical path and the effective refractive index of the fiber which can result in the transmission performance to change under lamination process, he finite element method software was adopted to conduct coupling analysis of stress modules, heat transfer and electromagnetic field of fiber embedded flexible substrate. Simulation results show that the maximum stress of the fiber embedded in the trapezoidal groove flexible optoelectronic substrate was 68.336 7 MPa. The fiber displacement embedded in trapezoidal groove is 1.430 4 μm largest among the three types grooves. The maximum stress of the fiber increases from 52.667 MPa to 71.907 MPa with the increasing of groove width. The maximum stress of the fiber increased from 51.589 MPa to 53.567 MPa as groove spacing increases. The maximum fiber stress decreases from 52.667 MPa to 47.793 8 MPa firstly and then increases to 67.349 6 MPa with the increase of groove depth. With the increase of temperature and pressure, the effective refractive index of single-mode fiber in the X direction increased from 1.446 249 977 to 1.446 259 084 and increased from 1.446 326 398 to 1.446 393 041 in the Y direction. The difference of effective refractive index increases with the increas of temperature and decreases as the pressure increases. With the effective refractive index increases, the fiber core's ability to limit light energy increases which can better reduce the radiation of light energy and the bending loss of the fiber. The research conclusion has certain reference value and guiding significance for designing the embedded structure of flexible optoelectronic printed circuit boards.
Based on the formation mechanism and distribution characteristics of the pulse wave characteristics, this paper proposes to use PDMS packaged fiber grating flexible sensor to detect the human wrist pulse wave signal, aiming at the four most common types of pulse waves: obvious, hidden and partly obvious. The pulse wave signal feature extraction method based on the time-domain differential period ratio uses the relative position and proportional relationship of each feature point in the pulse wave time-domain differential signal as feature parameters, and realizes a comprehensive algorithm from pulse wave detection to feature point extraction. The results show that for the 4 050 pieces of experimental data collected, the algorithm can accurately identify the characteristic points of the starting point and the crest. In the resting state, the identification accuracy of the tidal wave d and e points is 98.28% and 97.25%. The recognition accuracy rates of points f and g are 98.14% and 99.19%; in the state of exercise, the recognition accuracy rates of tidal waves d and e are 94.23% and 90.77%, respectively, and the recognition accuracy rates of dicrotic waves f and g are 91.93% and 95.38% respectively.
In order to measure the low-frequency vibration parameters of the structure, a thin-diameter Excessively Tilted Fiber Grating (ExTFG) cantilever vibration acceleration sensor is proposed, and the characteristics and optimization methods of the sensor are studied. Firstly, the characteristics of axial strain and bending strain of the thin-diameter ExTFG and the sensor model of vibration of cantilever beam are analyzed theoretically. Then, the influence of axial tension and bending strain on the vibration of the cantilever beam is analyzed by static experiments. Finally, the dynamic vibration test of cantilever beam is completed and compared with the standard-diameter ExTFG sensor. Experimental results indicate that under the effect of axial tension and bending stress, the axial strain sensitivity of TE and TM mode of the thin ExTFG is -4.68 pm/με and -3.55 pm/με, respectively, the wavelength based sensitivity of bending strain is -8.82 nm/m-1 and -7.71 nm/m-1, respectively, the intensity based sensitivity of bending strain is 6.71 dB/m-1 and 0.95 dB/m-1, respectively, and the thin ExTFG has a higher strain sensitivity than that of the standard ExTFG. In the cantilever vibration detection experiment, the maximum acceleration sensitivity at the 3 dB point is more than 500 mV/g, which is about 5 times of the standard-diameter ExTFG sensors under the same conditions. Compared with the spectral diagram after FFT transformation, the harmonic component of the thin-diameter ExTFG sensor is less than those of the standard one. Therefore, in the vibration sensing, the thin-diameter ExTFG has stronger anti-noise interference ability and higher detection accuracy.
According to the characteristics of the system that the wavelength of the optical signal is calculated by the phase of the modified frequency shift interference system, a digital mixing method based on the modified frequency-shift interference system was proposed, and the FBG wavelength demodulation was experimented. In the demodulation process, the algorithm combines the advantages of the frequency shift interferometric technology to accurately locate the grating, improves the frequency consistency between the local oscillator signal and the original interference signal in the signal mixing stage, and realizes the high-precision phase demodulation of the interference signal. A FBG sensor system was built based on the modified frequency-shift interference structure. In the simulation and experimental test, the wavelength information contained in the initial phase of the interference signal is accurately obtained by digital mixing method. The demodulation error of the 1.5 nm wavelength change signal is less than 0.26%. At the same time, the temperature sensing experiment of quasi dynamic signal is carried out, which has a good effect of wavelength demodulation, and has a certain application prospect in sensor monitoring.
In order to explore the influence of the number of cascade stages of the same structure cascade sensor on the sensitivity of the sensor, a comparative study on the same structure based two-stage and three-stage cascaded Mach-Zehnder(M-Z) optical fiber sensor was carried. In the experiment, we fabricated the two-stage cascaded and three-stage cascaded M-Z sensor based on the waist-enlarged structure and misaligned structure. In order to study the difference between the two-stage cascaded structure and three-stage cascaded structure, the strain performance was also measured. The experimental results show that the strain sensitivity of the two-stage cascaded M-Z sensor with waist-enlarged structure is -0.76 pm/με, and that of the three-stage cascaded M-Z sensor is -0.75 pm/με, the sensitivity change of strain is 0.01 pm/με. The strain sensitivity of the two-stage cascaded M-Z sensor with misaligned structure is -0.73 pm/με, and that of the three-stage cascaded M-Z sensor is -0.77 pm/με, the sensitivity change of strain is 0.04 pm/με. Therefore, the sensitivity of the three-stage cascaded structure is not improved obviously compared with the two-stage cascaded structure. The study shows that the multiple cascaded M-Z structures can not improve the sensitivity.
To discuss the strain error caused by the relative slip between the flexible Fiber Bragg Grating sensor and the substrate material, ANSYS finite element simulation software was used to analyze the relationship between the relative error of axial strain at each point of the core and the forward pressure. The parameters affecting strain transfer of the sensor, including core material, coating material, substrate material and size were analyzed, and the effect of relative slip on the relative error of FBG axial strain were explored. The results show that the relative error of axial strain at each point decreases with the increase of forward pressure, when the pressure is in the range of [0.1 N, 10 N], which presents the trend of small in core and big in two ends. In practical application, it should be combine the positive pressure to select the core material with less elastic modulus and the substrate material with higher friction coefficient, the relative error of axial strain could be less than 10%; select coating elastic modulus 2.4×1010 Pa, 0.062 5 mm thickness, the relative error of axial strain could be reduced to 8.57%; when half length of the FGB is more than 40 mm, the axial strain relative error of the core could be less than 20%, at this point, it could be considered that the coating and silica gel are completely bonded.
Frequency division multiplexing based on long-period fiber grating Mach-Zehnder Interferometer(MZI) is an important way to implement fiber multiparameter sensing. The Fourier method for the composite fringes in freqency division multiplexing was investigated, with emphasis on the relationship between the value of different frequency and sum frequency in the Fourier frequency spectra and the structral parameters of the single grating in fiber MZI and then, an effect method for depressing the different and sum frequencies was proposed. The results show that the singal to noice ratio can be effectively improved by properly selecting the grating structual parameters to reduce the fringe contrast at central wavelength to a certain extend, which improves the effectiveness of filtering in freqency domain.After adjustment and optimization, the singal noice ratio of the frequency spectrum is doubled comparing to the original value. A comparison between the cosine curves of recovered phases and the orignal fringes confirms that the tuned frequency spectra remain sufficient phase information that characterize the original fringes. The spectrum optimization method developed in this paper can provide theoretical and technical guidance for multiparameter sensing based on fiber grating MZI and freqency division multiplexing.
The Ultraviolet (UV) communication network which is based on the space division multiplexing technique will be faced with the problem of inter-link multi-user interference. For a set of typical communication link models, based on the theory of multiple-scattering, and Monte Carlo method was used to analyze the relationships between the probability of error and the distance from transmitter to receiver, the angle between two links, transmitters′ elevation angle and receivers′ elevation angle, respectively. The results show that, the probability of error increases with the increase of the distance from transmitter to receiver, the communication distance should be less than 120 meters when emission power is 100 mW; with the increase of the inter-link angle, the probability of error decreases first, then remains the same gradually, and then increases, the inter-link angle should be within 60°~120°; the influence of the elevation angle of transmitter and receiver on the probability of error is almost the same, that is the probability of error decreases first and then increases rapidly with the increase of the elevation angle, the minimum probability of error at 15°.
The output characteristic of a new-designed bidirectional feedback Brillouin-Raman fiber with Brillouin pump variation was studied.The Brillouin-Raman fiber laser consists of a segment of 7 km long dispersion compensating fiber, 1455 nm Raman pump, tunable laser and double feedback loops.The BP wavelength must be closer to Raman peak gain to obtain more Stokes lines when RP is fixed at 250 mW.The power difference between the neighbouring Brillouin components and Rayleigh components was reduced by enhancing the BP power, the average intensity of Stokes lines was improved to the saturation value, simultaneously.Affected by the cross-gain in DCF, firstly the number of output lasing lines increase and then decrease when the BP power was adjusted from 1.8 dBm to 6.9 dBm.Maximum 37 output channels was obtained when BP power fixed at 4.4 dBm, the channel spacing is 0.078 nm.
A novel method for the design and production of the optical receiving antenna was proposed. The hologram called Holographic Mirror, which has the dual function of gathering beam and filtering ambient wave, is made from holographic material by using the principle of interference and diffraction. The angular selectivity, spectral selectivity and diffraction efficiency at any point of the novel optical receiving antenna have been analyzed by coupled wave theory and k vector closed legitimate, the results of simulation show that the field of view and spectral bandwidth at different points of the Holographic Mirror are 0.8°-13.4° and 4.4 nm-7.4 nm respectively, and the overall diffraction efficiency of more than 95.3%. Compared with the combination of a converging lens and a filter into the conventional optical receiving antenna, small size, light weight, and low cost of the holographic mirror can be used as optical receiving antenna for indoor visible light communication.
The band-gap maps of As2S3, Ge20Se65Sb15 and As2Se3 chalcogenide glass fibers at different air-fillings were analyzed by using the plane wave expansion method,. The results show that when increased to 0.75, the photonic band gap and air lines have appeared intersection mode, the band gap is wide, and an appropriate laser transmission mode in the core of fiber is formed. Under different fiber core diameter, using the finite element method, the fundamental mode confinement loss and effective mode area of homemade Ge20Se65Sb15chalcogenide glass hollow-core photonic crystal fiber were systematically studied. The results show that the confinement loss is the lowest and the effective mode-field area is very small when the core diameter is 9.2 μm. By optimizing the structural parameters of fiber, a hollow-core photonic crystal fiber with low confinement loss (0.00472 dB/m) and effective mode-field area (58.046 μm2) at 4.3 μm was obtained.
A shear displacement sensoring device for monitoring rock sliding based on the principle of optical fiber Bragg grating.The displacement sensor was designed by attaching the fiber grating to equal strength cantilevers.Then the sensor was embedded inside a Φ50 polyvinyl chloride (PVC) tube.Three test models were made using Φ75 PVC tube with the Φ50 PVC tube mentioned before in it.Cement mortar was grouted into the gap between the Φ50 and Φ75 PVC tubes as ratio at 1: 1.Indoor shearing tests on the models were carried out.The curve of the relationship between the sliding distance and the grating wavelength shift was also measured.Results show that the sensitivity of this sensor is 0.5 mm and the maximal sliding distance could reach 30 mm while the grating wavelength could shift up to 1200 pm, of which indicates that the sensor could be well applied into sliding displacement measurement which needs a very high sensitivity.It is proved that the features of the fiber Bragg grating (FBG) sensor we designed are more sensitive and reliable, better anti-interference ability and larger measurement range.The structure of the sensor is simple, and it also can realize remote, real-time monitoring.Thus, the sensor can be used for landslide early warning monitoring.
Based on the the general expression of the beam wander variance modeled by Andrews and Philips, the expression of wander variance for partially coherent Gaussian-Schell model beam considering the outer scale was derived using beam width formula of partially coherent Gaussian-Schell model beam propagating in slanted atmospheric turbulence, combining the turbulence structure constant model varying with height, numerical calculations were conducted and the spreading and wander variance of partially coherent beam and fully coherent beam were comparatively analyzed. The results show that, compared to the fully coherent beam, the partially coherent beam spreads faster and is less affected by turbulence under the same propagation conditions, and the beam spreading effect becomes weaker as the initial beam radius becomes larger or the height of the receiver becomes higher. As the propagation distance increases, the beam wander variance decreases with the increase of initial beam radius, the beam wander variance caused by different coherence have little difference. The wander of fully coherent beam is less affected by the wavelength, while the longer the wavelength of the partially coherent beam, the more obviously the beam wanders.
The detection and alignment of polarization axis is a key technology in PMF’s application.In order to improve the accuracy of the end face detection of polarization axis,a sub-pixel edge extraction method based on surface fitting was proposed to improve the accuracy and the normal distribution of the detection result was used to improve the precision.Based on the normal distribution of the detection result,a aligning method based on normal distribution was put forward.An automatic aligning system was established.The experiment shows the sub-pixel edge extraction method can decrease the error by 45% and the ellipse fitting can reduce the error by 15%.The alignment experiment figures out that the bias of aligning method based on probability is less than 0.1°.
A kind of single-core PCF splitter based on tellurite glass was proposed, defected holes were arranged in the outer cladding, they can bring about strong coupling effect between defected mode and the fundamental mode. FEM was used to analysis its characteristics. The simulation results show that this kind of splitter can split light in the bands of 1.3 and 1.55μm, propagate along the orthogonal polarized directions. For a propagation distance of 15 mm, the -20 dB bandwidth of crosstalk in 1.3 and 1.55 μm bands, reach 44.2 and 67.1 nm. The confinement loss at the wavelength are 0.063 dB and 0.0 48dB. The proposed splitter has low crosstalk and low confinement loss.
A novel method for measuring electrodeposition stress was presented.Its mechanism is that the center wavelength of metal coated Fiber Bragg Grating sensor shifts under the action of the electrodeposition stress when the FBG is used as cathode in electrodeposition;based on the shift,the stress can be calculated.In the measurement,the center wavelength shifts of the FBG during deposition are recorded by FBG interrogator and these records are divided into many time segments with equal interval.Based on average depositing velocity and the relationship of FBG′s stress sensitivities with deposition thickness, the stresses producing in every time segment are calculated individually.Stress evolution is obtained by adding up these segmental stresses.The process is stress evolution during electroplating nickel was tested by using electroless Ni-P coated FBG sensor.The result shown that the FBG′s sensitivity is higher than 7pm/MPa under the condition that the nickle thickness is less than 50μm,and the degree of accuracy is more than 0.14 MPa.After deposition for 6 000 s,the accumulative pressure stress is 173.049 9 MPa.
Pseudo millimeter wave ultra wide band signals have the characteristics of narrow pulse width, low signal strength , which make it is not easy to detect. To deal with such problem, two optics assisted envelope detection schemes were proposed. Firstly, a fiber Bragg grating was used for filtering one of the first sidebands at the output of the modulator, then the envelope of PMM-UWB signals for decision were obtained after photo-detection and low pass filter. Based on the method of mathematical derivation and numerical simulation, the effects of phase modulator or intensity modulator on the output envelope signal amplitude were analysed respectively. The results show that intensity modulator requires bias control to maintain a stable optical operating point, when the bias point of intensity modulator was switched to minimum transmission point, the maximum amplitude of envelope signals were acquired;phase modulator do not include the bias control, the amplitude of envelope in phase modulator is equal to the maximum amplitude in intensity modulator approximately. Compared with intensity modulation scheme, the phase modulation scheme has more advantages, such as simple structure, low insertion loss.
With finite element method,a hexagon High Birefringence Photonic Crystal Fiber (HB-PCF) with two zero-dispersion dots and CdSe/ZnS quantum dots film was designed.The dispersion and loss characteristics of the designed HB-PCFs with the different thickness of CdSe/ZnS quantum dots films were analyzed.The results show that HB-PCFs with CdSe/ZnS quantum dots films exist the fundamental modes along the x- and y-axes of fibers.As pump wavelength increases,the birefringence of HB-PCFs with the same thickness of CdSe/ZnS quantum dots films increases gradually.And their dispersions increase first and then decrease along the x- and y-axes of fiber.The losses of HB-PCFs are close to zero in the visible region while they increase gradually in the infrared region.For the different thickness of CdSe/ZnS quantum dots films,the birefringence of HB-PCFs decreases gradually with the increase of quantum dot film thickness for the same pump wavelength.Their dispersions decrease gradually along the x- and y-axes of fiber,and their two zero-dispersion dots are close to each other,and their losses increase gradually.These research results indicate that the dispersions and losses of the HB-PCFs can be controlled effectively in experiment by depositing the different thickness of CdSe/ZnS quantum dots films and choosing a suitable pump wavelength.
The demodulation speed has become the key issue limiting the application of fiber optic Fabry-Perot nonscanning correlation demodulator.A fast fiber-optic Fabry-Perot (F-P) nonscanning correlation demodulator based on Super Luminescent Diode(SLD) was proposed,its principle and signal characteristics were analyzed.To improve the signal stability,envelope detection based on wavelet was presented.Theoretical calculations show that the error of the result is only 0.33% with this algorithm,while the error is 4.33% without it.A experimental prototype was made and tested,when the demodulation speed is 1.5 KHz,the demodulation erorr is only 5 nm with this algorithm.The experimental results indicate that envelope detection based on wavelet can improve the demodulation stability and accuracy of the fast fiber optic Fabry-Perot nonscanning correlation demodulation system,providing a stable and efficient algorithm for the fast fiber optic Fabry-Perot nonscanning correlation demodulation system.
In monitoring intrusion incidents based on phase-sensitive Optical Time Domain Reflectrometer(φ-OTDR), the methods of combining elements, which are the past records of a single point, the data of neighboring points, and the ratios of peak values, were introduced. The advantages and disadvantages of those methods by accuracy of identification, time consumption, complexity and stability were measured. The experimental results demonstrate the method combining the past records of a single point, and the data of neighboring points achieve the best performance, the identification accuracy rate can be as high as 100%, this method has a good adaptation in different frequencies. The research will contribute to the signal processing part of applying φ-OTDR to security system.
The pilot-aided method was adopted for estimation and compensation of sampling clock frequency offset in CO-OFDM systems to improve the system′s performance .And the pilot arrangements were simulated and discussed by comparing with five different location of pilots inserted,the optimal location of pilot inserted was obtained.The simulated results show that the method can work quite effectively even with large sampling clock frequency offsets,and the loss of optical signal-to-noise ratio is less than 1 dB.Therefore,it can reduce the cost of system;the location of pilots will impact the performance of the algorithm,the average insertion of pilots is optimal.With increasing sampling frequency offset,if the method can not work well,the pilots should be inserted at the low frequency sub-carriers.
Through the analysis of the intensity ratio of anti-stokes and stokes light of the backscattering light,a multimode optical fiber Raman temperature sensing system had been developed based on Raman scattering and optical time domain reflection principle.Using a new type of dynamic temperature calibration program for fitting low and high temperature zone respectively,which improved the measuring-temperature accuracy up to ±1 ℃.This paper respectively conducted the system temperature resolution,temperature measurement precision,spatial resolution and repeatability experimental verification.The experimental results show that the temperature resolution of 1 ℃,the spatial resolution of 1m,the system stability is good and can adapt to the complex environmental change.
A novel octagonal duel-core photonic crystal fiber based on tellurite glass was proposed.Using the Full-vector finite Element Method (FEM) and coupled-mode theory,impacts of structural parameters on characteristics of the coupling was analyzed.The results show that the coupling length decreases significantly as the value of hole-pitch decreases,but the relative coupling length changes slightly as the value of hole-pitch decreases;the coupling length increases slightly as the value of air hole diameter increases,the relative coupling length increases significantly as the value of air hole diameter increases;the coupling length increases slightly as the value of ellipticity increases,the relative coupling length increases significantly as the value of ellipticity increases.The performance of the design polarization splitters was desired when the relative coupling length is 1.Then a kind of polarization splitters based on the proposed dual-core PCF was obtained.The two polarized lights are separated entirely with 139 μm fiber,simultaneously the polarized light extinction ratio is -53.46 dB at the wavelength of 1.55 μm.Besides,the bandwidth is over 120 nm when the extinction ratio is less than -20 dB,which exhibited high performance of splitting one light into two orthogonal polarization states comparing to the other duel-core polarization splitter with highly extinction ratio and short length.
For protecting the transmission performance of the Polarization Multiplexed-Quadrature Phase Shift Keying (PM-QPSK) optical transmission system, an improved Optical-Signal-Noise-Ratio(OSNR) monitoring method based on high order statistical moment was proposed. By using different calibrations, this method is transparent for modulation formats. Using numerical simulation the proposed method is verified in 100 Gb/s PM-QPSK coherent receiving system. In the range of 5~25 dB OSNR, the OSNR measurement error is within 0.5 dB for the modulation formats with different duty cycle, and the system has 2 400 ps/nm of chromatic dispersion tolerance and 62 ps of first-order polarization mode dispersion tolerance for 14 dB OSNR with measurement error in 0.5 dB. The improved monitoring method has such advantages of small monitoring error and high tolerance to chromatic dispersion and first-order polarization mode dispersion.
In order to realize the quick and accurate calibration for strain and temperature response coefficients of distributed sensing optical fibers, a novel method for simultaneous calibration of strain and temperature was proposed. The temperature of sensing fiber and metal tube was controlled by the thermostat. The strain of optical fiber twined around the metal tube was controlled by the linear thermal expansion of the metal tube. The temperature change of optical fiber around the metal tube was compensated by the loose fiber in the same thermostat. So that the simultaneous, quick and accurate calibration of strain and temperature can be realized. The strain and temperature response coefficients of Brillouin frequency shift of bare single-mode fiber were calibrated using a thermostatic waterbath and a stainless steel tube. The calibration span of strain was 620 με, the calibration range of temperature was 35~75℃. The strain and temperature response coefficients of Brillouin frequency shift of bare single-mode fiber were 0.048 MHz/με and 1.06 MHz/℃ respectively. Experiment results indicate that the method can be used to calibrate the strain and temperature of minor diameter distributed sensing fiber simultanously.
In order to study the active spontaneous emission of erbium-doped fiber pumped by arbitrary waveform pulse with breaking the limitation of pump waveform before. By dividing the time into tiny periods, the expressions for level population and the average power of active spontaneous emission were obtained with rate equations. Simulations and experiments show that pump glitches have less effect on the active spontaneous emission just like having a “high frequency filter”;the waveforms of the pump light and active spontaneous emission light are alike when the pump power is large relatively, this research can be used to all-optical modulation for the optical fiber laser. The results are consistent with the theoretical analysis, which show the analysis is reliable.
The reflection spectra of polarization-mode conversed light in Magnetooptic Fiber Bragg Gratings (MFBGs) dependent on the magnetooptical coupling intensity is investigated.According to the magnetooptic coupled-mode theory combined with the propagation property of fiber Bragg grating,the magnetic control characteristics of the MFBGs are analyzed numerically.The 3 dB bandwidth tunable filter is obtained by adjusting the magnetooptical coupling intensity.The clock extraction of 40 Gbps RZ data signal by utilizing the comb filtering of MFBGs under bias magnetic fields is simulated and the jitter performance of the MFBGs is also analyzed.
To suppress multiple access interference and multiple noises existing in optical code division multiple access system,a system with hard-limiter and parallel interference canceller based on maximal value decision is presented.The effect of suppress multiple access interference is examined while APD noise and thermal noise exist.Optical orthogonal code is applied as address code,APD is used as opto-electro detector,and bit error rate of the system is derived and analyzed.Simulations based on the expression are done and comparison with other multi-user interference suppressing methods is made.Compared with other effective methods such as single bit with single hard-limiter,single bit with double hard-limiters,multiple-bit transmitted method,multiple-bit transmitted with hard-limiter method,parallel interference canceller(without hard-limiter) based maximal value decision method and so on,simulation results show that the effect of suppress multiple access interference of the system is superior to parallel interference canceller without hard-limier and multiple-bit transmitted method with or without hard-limiter.If the received power is low,the system is also superior to double hard-limiters method.So,it is effective for suppressing multiple access interference and multiple noises.
A novel optical comb filter is designed with two cascaded phase-shifted linearly chirped fiber gratings.According to the transmission matrix method and Fabry-Perot resonant cavity theory,the characteristics of reflection spectrum of the optical comb filter are numerically analyzed for the different combinations of two chirp gratings,the value of chirp,the index modulation and the phase-shift.The results show that the distribute density of reflection peaks is different for the different combinations of two chirp gratings,that is,the spacing between reflection peaks is equal if the signs of two chirps same,it can be used to design a uniform filter; on the other hand,the distribute of reflection peaks shows the characteristic density gradient if the signs of two chirps opposite,which can be used to design a non-uniform filter.And the greater the chirp of gratings,the wider the bandwidth of reflection spectrum of the filter,and the lower the reflectivity; however, the reflectivity will be improved with the increase of index modulation; furthermore each of the reflection peaks moves with the change of phase-shift,which is useful to select a filter window by changing the phase-shift.
In order to analyze influence of the external factors to the tracking precision of space-ground optical communication,the platform vibration,atmospheric turbulence,and background light are studied,and can be attributed to the influence of tracking precision.Tracking simulation system is founded on the basis of measured platform vibration data.The compositive influence of various factors to tracking precision is analyzed under different conditions.Based on the assumptions,coarse tracking can well suppress the vibration of mobile platform on ground and ensure that laser beam can enter fine tracking field;the middle atmospheric turbulence increases the standard deviation of fine tracking error to about four times;if low altitude area is selected as examination spot,atmospheric turbulence will increase standard deviation of fine tracking error to about two times.
A novel optical packet switching scheme,in which an optical orthogonal frequency division multiplexing (OOFDM) signal is generated as a label,is proposed and experimentally demonstrated.The 10 Gb/s OOK optical payload and the 2.5 Gb/s OFDM optical label are produced by intensity modulation of two continuous waves with different wavelength,respectively.In the experiment,the eye diagrams of the payload,constellation diagrams of the label and the BER curves of the label and payload are obtained,when the optical packets were transmitted on optical fiber or not.It is experimentally demonstrated that the power penalties for the payload and the label after 40 km fiber transmission are 1 dB and 0.5 dB,respectively.The scheme is easy to implement,so it can be used in an optical label switched network.
To the point of security threats against lightpath establishment process in ASON, an efficient secure lightpath establishment protocol is presented. This protocol uses integrated strategy of wavelength reservation, and makes use of digital signature and message feedback security mechanisms to protect the integrity of important object in GMPLS RSVP-TE message and prevent malicious or selfish actions from inner node. In addition, in view of the close coupling character of routing and signaling module in ASON, this protocol adopts PKLSA message of OSPF-TE to distribute nodes public key certificate which the lightpath establishment protocol demanded. Through simulation experiment and analysis, it is proved that this protocol can ensure the security of lightpath establishment,and has better performance than the old RSVP-TE protocol in terms of connection block probability, lightpath connection setup time and message overhead.
The performances of two-stage double-clad Er3+/Yb3+ co-doped fiber amplifier(EYDFA)are analyzed based on the rate equation and light propagation equation.The optimum length and the influence of the ratio of forward and backward pump power on gain and noise characteristics of two-stage double-clad EYDFA are obtained with numerical simulation.The gain improvement of 4 dB and the noise figure reduction of 3 dB is obtained by choosing the optimum position of the isolator and the the ratio of forward and backward pump power.
The linearly chirped long period grating with special refractive index for EDFA gain flattening is presented. Runge-Kutta algorithm is used to numerically solve the coupled-mode equations of this type of grating. The grating structure parameters including grating length, period, linear chirp coefficient and the core refractive index profile are optimized by Nelder-Mead arithmetic. A linearly chirped long period grating with appropriate parameters is designed to flatten the gain spectrum of EDFA. The flattened gain spectrum has a bandwidth of 35 nm in C wave band and a fluctuation in ±0.5 dB.
Pr3+,Ce3+:YAG crystal fibers were grown by laser heated pedestal growth method as the white LED materials.The Luminescence properties of the materials was investigated,and the results show that in Ce3+ ions and Pr3+ ions codoped fluorescence emission process the Pr3+ ions 610 nm fluorescence intensity can be increased by Ce3+ ions sensitization. High efficient white LED was gotten by combined Blue LED and grown Pr3+,Ce3+:YAG crystal fiber. The white LED was improved with the CIE color coordinates (x=0.322, y=0.335) and 84.3 of the CRI. It can be used for high effient power white LED in future.
Based on theoretical analysis of Sagnace interferometer, a noncontact optical fiber measuring technology is proposed. The related demodulation technique is developed in accordance with the characteristics of trap wave. The results show that, the noncontact optical fiber measuring technology based on Sagnac interferometer can realize the identification of transient signal,and realize the location of sound signal source according with the relative location precise of 1.5%.
Quasi-periodic superstructure fiber Bragg gratings (SFBG) of 0~9 Fibonacci order with the central wavelength 1 541.86 nm were written into single mode photo-sensitive fiber by phase-mask scanning technique. The fabrication parameters were optimized by simulating the UV induced refractive index change in the fiber core. Both theoretical and experimental results show that the transmission spectra were multi-fractal, self-similar and the transmission coefficients have six-cycle property. The fabricated SFBG can be served as C-band multi-frequency optical device in optical integrated circuits and optical communication system.
The rapid numerical difference recurrence algorithm is used to analyse the nonlinear Schrdinger equation (NLSE) in which the chromatic dispersion and the nonlinearity act together with polarization mode dispersion.And it is used to study the optical pulse transmission with polarization mode dispersion.The calculated results by using the rapid numerical difference recurrence algorithm are contrasted with the analytical results and the results of the split-step Fourier method (SSFM) and indicate that the rapid numerical difference recurrence algorithm is very accurate and more reasonable.It is concluded that the new method is scientific and reasonable for the study of light pulse propagation with polarization mode dispersion.Moreover,it is used to study the pulse distortion and the pulse broadening in the optical pulse transmission with polarization mode dispersion .As a result ,some valuable curves are obtained to help design the optical transmission system.
Theoretical study on channel crosstalk due to gain saturation in Semiconductor Optical Amplifiers (SOAs) was presented.A significantly higher penalty was paid at more channels because of the gain saturation characteristic of SOAs.Crosstalk mitigation techniques in SOAs were studied.Reduction of crosstalk in SOA by amplifying dispersed WDM signals was confirmed experimentally.Based on the results of the experiment,a novel dispersion management scheme was proposed to prevent the interchannel crosstalk caused by cross-gain modulation.
Based on the generalized nonlinear Schrdinger equation desiring pulse propagation in fiber,the supercontinuum generation in high nonlinear fiber pumped at the normal dispersion region was investigated.The results show that the pulse width,peak power and initial chirp of the pump pulse are very important to flat wideband supercontinuum generation. The supercontinuum generated in high nonlinear fiber with the normal dispersion,the effect of the third,fourth order dispersion even the higher-order dispersion on supercontinuum spectrum can be ignored,the self-steepening was effect on supercontinuum spectrum generation than other higher-order nonlinear effects. A broadband and flat supercontinuum is 400 nm at-20 dB,the spectral intensity fluctuation is 10 dB and without residual pump generated from high nonlinear fiber pumped at the normal dispersion region.
Variable retardation plates (VRP) were analyzed by quaternion theory and equivalent rotatable quarter-wave plates and rotatable half-wave plates were constructed from sequences of adjustable linear retarders with fixed retardation axes.To avoid reset,a polarization controller (PC) based on adjustable linear retarders using polarization controlling algorithm for rotatable plates was proposed and demonstrated.Simulation results show that the 4-plate polarization controller can transform any varying general input state of polarization (SOP) into an arbitrary linear output state of polarization,and the fluctuation of output light intensity is less than 1% while the rang of retardation is confined into 2π.This transformation is continuous,and reset-free and the phase shifts are smooth.
A phase-shifted chirped fiber Bragg grating with two π phase shifts was studied theoretically and experimentally.The grating was calculated by F matrix,based on which the properties of the spectra were analyzed.The phase-shifted chirped fiber Bragg grating has a transmission spectrum with dual-wavelength peaks.The wavelength position of the peaks is directly dependent on the position of the π phase shifts.The linewidth of the peaks increases only with the chirp of the grating,and is not related with the wavelength difference of the two peaks.Due to the local character of chirped fiber gratings,the design of the phase-shifted chirped fiber Bragg gratings is simple.A phase-shifted chirped fiber Bragg grating with two π phase shifts was fabricated by ultraviolet scanning with a phase mask.The wavelength difference,the extinction ratio,and the 3dB linewidth of the two peaks in the transmission spectrum are 8nm,20dB and 0.08nm,respectively,which agree with the theoretical design.
The design and fabrication for a loop optical circulator was proposed,which can take two forms with different optical arrangements.Its working priciple and optical performance are analyzed by theoretical simulation,and the effectiveness was validated by experimental results.Results indicate that the scheme can make device more compact only using one stage of Faraday Rotator (FR).Compared with traditional circulators,the optical performance is similar to a circulator with two stage of FRs.This compact design can also reduce the cost for the device and ease the assembling process.
Taking into account the evanescent wave of nanofiber,the distribution of light intensity near the two bending nanofiber was numerically calculated using the FDTD method.The coupling efficiency between the bending nanofiber versus diameter,bending radium and the distance between the two fiber was given.It is shown that there is evidence of coupling phenomenon between nanofiber.
The performance of 2-level pulse amplitude modulation and analogue pulse width modulation (PWAM) system based on self-error-correction code was analyzed. The system performance was evaluated by considering the signal to noise ratio (SNR) of 50 dB for analogue signal transmission and bit error rate (BER) of 10-9 for digital signal transmission as a lower bound. Computer simulations show that the receiver sensitivity of PWAM system with self-error-correction code can get 1.1 dB improvement without complexity.
Experiments on the correlation analysis of backward diffractive patterns in different azimuths to determine the polarization axes of PANDA fiber were made in order to solve the problem to built a standard library of diffractive patterns.The experiments show that there are three characteristics of the patterns:the diffraction patterns in the fast axis direction is different from that in the slow axis direction; the diffraction patterns near the fast or slow axis are similar,and the diffraction patterns in the fast axis or the slow axis direction are symmetric.Based on the three mentioned characteristics,a new method without standard library of diffractive pattern was put forward to determine the special azimuths of polarization axes of PANDA fiber with image cross correlation.It is not necessary to build a standard library for the diffractive images to determine the special azimuths of polarization axes of PANDA fiber.
The nonlinear crosstalk due to cross-phase modulation was studied by a new analysis model in single-sideband radio over fiber (ROF) systems, and the results were compared with that of the pump-probe method,which could be used to analyze different links, in particular, electrooptical upconversion links. Detailed analyses of the crosstalk due to cross-phase modulation for links with optical single-sideband modulation were presented. The nonlinear crosstalk due to cross-phase modulation have relation to the modulation frequency, the power of the pump and probe and the channel space. The simulation results agree well with the theoretical analysis.
A scheme of Orthogonal Modulation was studied.FSK was used to produce 80 Gbit/s payloads and IM was used to produce 2.5 Gbit/s labels.Using EAM to erase old labels,new labels were inserted in the center of the web node.The optical packets could transmit long distance in the web.This scheme was proposed and demonstrated by simulation and theoretically prove.
Long period fiber gratings were mechanically fabricated through periodic rectangular pressure grooves method. The rectangular periodic pressure grooves were made by mechanical line processing technology with the groove period of 600 μm and 60 grating periods. The pressure and temperature characteristics of the fabricated LPFG were studied with different pressure and temperature, respectively. The resonant wavelength could be adjusted by tilting the angle between the fiber and the grooved plate. The fabricated LPFG′s maximum resonant peak is up to 15 dB and the range of tunable resonance wavelength exceeds 12nm by adjusting the angle between fiber and grooved plate.
All-optical 2R regeneration based on self-phase modulation in the fiber and subsequent offset filtering was investigated numerically and experimentally to restore optical data signals degraded due to amplified spontaneous emission (ASE) noise.Numerical results show that Q factor and amplitude jitter can be improved for 40 Gb/s return-to-zero (RZ) signals with Q factor between 5 and 12 when the fiber input average power is chosen suitably.The regeneration experiment on 10 Gb/s RZ ASE-degraded data signal was demonstrated using a highly-nonlinear dispersion-shifted fiber,and a power penalty of only 0.1 dB was measured at 10-9 bit-error-ratio.
Based on analyzing Shared Risk Link Group (SRLG) constraints and overlapped protection scheme,and adjusting on each link weight when computing a working path or protection path dynamically,a novel protection scheme called SRLG-disjoint-based Overlapped segment Shared Protection Algorithm (SOSPA) was proposed.SOSPA provides several overlapped protection segments for a working path with the consideration of SRLG disjoint constraints.A new approach to select these appropriate protection segments was given.The results of analysis and simulation on SOSPA demonstrate that,compared with other traditional protection schemes,SOSPA can improve the reliability of network connection and enhance the efficiency of resource utilization by sharing backup resources among protection segments which belong to SRLG-disjoint working paths.
A bi-directional ROF system based on double-sideband with optical carrier suppression (DSBOCS) scheme to generate optical millimeter-wave signal via electronic-optical phase modulation (EO-PM) was demonstrated.The communicated bandwidth could be doubled to realize Ultra Width Band & Ultra High Frequency (UHF) communication via phase modulation.The BERs of downlink and uplink via adjusting the phase deviations of the phase modulators could be successfully controlled.The simulation result shows that the bi-directional 2.5 Gbit/s data modulated with a 24 GHz UHF microwave in a light beam with a transmitted power of 3 dBm is successfully transmitted over a 40 km SMF-28 fiber with an attenuation of 0.25 dB/km and a dispersion of 20 ps/(nm·km).
The fluorescence fiber temperature measurement system was proposed.Using the double light paths and double channels measurement,the micro fluorescence probe was made by the chemical conversion coating technique and heat treatment process combination.The wavelet transform was used to eliminate the noise in the fluorescence signal.The results show that this system has higher precision and resolution by the temperature experiment,the verification experimental and the repetitive experiment.
A systematical approach was proposed to evaluate the impact of linear and nonlinear phase mismatch on tunable paiametric wavelength converts based on degenerate four-wave mixing in high-nonlinearity fibers.The result shows that,for any media fiber with a given dispersion wavelength,there exits a corresponding optimal signal wavelength,at which the tuning range can be maximized.Then the analytical expressions of the optimal signal wavelength and the maximum tuning range were deduced.Based on these expressions,a systematic optimization method was proposed to enhance the tuning range and minimized the gain ripple.
The relationship between acoustic wave direction,laser incident angle,and laser diffraction angle of TeO2 acousto-optical deflector with acoustic frequency was analyzed based on the TeO2 crystal acousto-optical characteristic and Dixon equations under abnormal Bragg diffraction.A simple linear model for the fixed diffraction angle was set up by numerical calculation and parameter fitting.Using the linear model,acoustic wave frequency for fixed laser diffraction angle could be obtained easily.
A method of dispersion of light prism-based half kind simulation system was proposed for laser fuze.The lens optic coupling system were adopted to couple the light beam of sending window into the receiving window,Then real-time power attenuation simulation was realized by means of a Light switch and a Electric-controlled intensity attenuator.After being attenuated,the optical signals returned to windows of receiver.In the system,the optical coupling system is used to propagate laser pulses,so it can accurately simulate the reflective laser signal which laser fuze has transmitted such as passing across the target.At the same time,it can testify the signal processing algorithm used in laser fuze in the laboratory through controlling attenuation of laser signal in real-time.By theoretical analysis,system composition and detailed design of every part were proposed,and then the design was verified by conclusion.
The design and implementation of ROADM based on the FBG optical switch (OSW) was investigated. A new fiber Bragg grating (FBG) collimator was designed, and the construction of which is introduced. The FBG optical switch (OSW), composed of FBG collimators, was the integrated package of FBG and OSW. A new ROADM, which is compacter and smaller, was designed based on FBG OSW. Having tested the ROADM and simulated the system according to the performance standards of ideal OADM, a conclusion can be drawn that the newly designed ROADM is simple in structure and easy to be operated. In addition, this new ROADM, applied with mature technology, can minimize the facture difficulties and reduce the cost.
Based on improved TOAD and introducing DC assistant light which solves two groups signals to compete in SOA and realizes transformations from three condition to two condition,a novel scheme to implement header recognition for NRZ was proposed.Simulation results shows that this program can get 13.29 dB extinction ratio(ER) of the output at 40Gb/s with common SOA.The analysis of impact of nonlinear phase shift difference departure from π and 2π in actual system shows that when a fluctuant rang of nonlinear phase shift difference is confined within 0.1 rad,and 13 dB extinction ratio of recognition signal is still obtained with assistant light injection.Therefor the demander of debugging is degraded greatly.Its validity is confirmed through experiments at 2.5 Gb/s.The experimental result is agree with the theoretic analysis.Further more,the influencing factors to the ER of recognition signal was researched in experimentatlly.
Based on the characteristic of multi-granularity networks,two shared protection schemes for multi-granularity optical networks were proposed,which provide protection for the working paths based on waveband granularity and wavelength granularity respectively.The simulation results show that the proposed schmemes have better performance than dedicate protection schemes.Based on the analysis of these simulation results,the effect of various network parameters on the two schemes were investigated,which can be used as references in choosing protection schemes.
The polarization-temperature characteristic of single-mode (SM) fiber coupler was investigated theoretically and experimentally.An analytic expression shows that the factors affecting the extinction ratio (ER) of the output light of SM fiber coupler include the extinction ratio of the input light and the mode-coupling ratio of the fibers is derived.Then a series of experiments on measuring the extinction ratio of the output light by changing temperature rapidly and using several light sources with different degree of polarization (DOG) were performed,and the above experimental results demonstrate that the light sources with low DOG are useful to reduce the mode-coupling effect in SM fiber coupler caused by temperature change.Moreover,the output errors of fiber optic gyroscope (FOG) owing to the mode-coupling effect were calculated and simulated with changing the extinction ratios of light sources,which is helpful on how to choose suitable light sources for different FOG systems.
Using Maxwell′s equations, surface modes in a fiber with left-handed material core and right-handed material cladding under weak guidance were discussed. Some dispersion equations for TE (TM) mode, EH modes and HE modes were obtained. According to these equations, some curves for different surface modes and oscillating guided modes were plotted. These dispersion equations and curves are compared with each other. Many novel characteristics of surface modes for LHM fiber were obtained.
The propagation properties in a symmetrical three slabs air waveguide with left-handed materials as substrate and cover layers were studied.The fast and slow waves can both propagate in the waveguide system.There is no base mode for fast wave.The slow wave mode can exist in zero or one order TE or TM mode only.TE and TM mode were degenarated when the permittivity equal to permeability of the left-handed material.And the degenarate was disappeared.Two propagation modes with the same mode number can coexiste in the waveguide.
A novel three-port band-pass tunable filter based on DWDM angle-tuned thin-film filter was proposed.The tunable filter consists of three single-fiber collimators and one particular designed angle-tuned narrowband multiple cavities thin film filter.By the controlling of a stepping motor,the tunable filter can choose any particular wavelength to the drop port,and reflected wavelengths can go out from another port without any interrupt.With the theoretical calculation and the experiment research,the characteristics of the three-port tunable filter was analyzed,especially the crosstalk was calculated in detail.In the experiment,the channel isolation was at about 30dB and its tunable range was over 26 nm,which was coinciding with the result of theoretical design.
The compensation of Composite Second Order Intermodulation (CSO) caused by Self Phase Modulation (SPM) and chromatic dispersion in 1550nm long distance fiber CATV systems were discussed.The CSO formula in the long fiber link with casecaded EDFA′s was obtained,and then modified in the case of using chirped fiber grating for dispersion compensation.The analysis and experiment demonstrate that the effect of dispersion compensation is sensitive to the compensator position.The optimal position can be deduced through a theoretical model and verified by simulation and test.
The influences of first-order polarization-mode dispersion (PMD) on systems using differential phase-shift keying (DPSK) and on-off keying (OOK) with 40 Gb/s non-return-to-zero (NRZ) or return-to-zero (RZ) formats were studied.The eye open value of modulation formats on the different PMD effects were compared and analyzed.The simulation results show that when either the NRZ or RZ format is used,DPSK exhibits higher eye open performance than OOK in the presence of first-order PMD; NRZ-DPSK,as compared with RZ-DPSK,incurs smaller eye open due to PMD effects; CSRZ-DPSK format has better tolerance than RZ-DPSK format to PMD for a given bit rate.
A 3-dimensional strain sensor with fiber Bragg grating (FBG) was designed and realized.The sensor was based on cylinder structure,with three FBGs fixed on surface of cylinder by 120° angle intervals.This 3D sensor was theoretically demonstrated with material mechanics,and experiment was taken with stress of serials on a random direction.Experiment results show that the sensor can measure strain in any direction in 3-dimension space.The measure results have high linearity and error below 1.58% in measure range of 0-5N,and its′ measure precision can reach 9.2×10-3N.
Four-wave mixing and wavelength conversion in the photonic crystal fiber was studied.25 meters of photonic crystal fiber was adopted due to it′s high nonlinearity and flat dispersion characteristics.When the pump power is 19.8 dBm,100 nm of the tunable wavelength conversion bandwidth and -20 dB of the conversion efficiency were attained.The experimental results show that the four-wave mixing with two signals input simultaneously does not affect each other when the signal power is below the threshold of four-wave mixing.
For the applications of the infrared laser communication system in the quasi-horizontal propagation path near the ground,diurnal variations of the atmospheric visibility and the refractive index structure constant were continuously measured in Jinan,Shangdong Province,for seven days.The key atmospheric parameters impacting on the performance of laser communication system were obtained.The influences of the extinction and turbulence channels on BER (Bit Error Rate) and ES (Error Second) were also analyzed for one kilometer communication chains.Furthermore,the feasibility and reliability of communication system were discussed under the complex atmospheric conditions.The methods for attenuating the effects on the performance of laser communication system were summarized and discussed.
The fabricating technique of blazed fiber Bragg grating was presented.BFBG fabrication technique was discussed.By phase mask method,several BFBG in different tilt angle were successfully fabricated on H2-doped photo sensitive fiber.Their transmission spectrums were also presented.A fiber Bragg grating sensing interrogation system using BFBG as the core wavelength division component was proposed.A photo detector was put on the focal plane of the lens to detect the light.BFBG was used to tap light out of the fiber core to fiber cladding.The scheme and result was discussed in detail.
The principle of reflection volume holographic storage technology was introduced.A new reflection holographic sonrage beam path that was different from conventional transmission beam path was designed and established.Based on this principle,a new angle-multiplexing scheme was suggested,which was two-lens confocal method.Two thousand holographic images were stored continuously in a signal-point using this system.This method was performed experimentally that been controlled easier and finishing images memory continuously.And it achieved high diffraction efficiency and improved the quality of images.The tow-lens confocal scheme decreased the complexity of system and conldbe multiplexed with high-accuracy.It expands multiplexing dimension from one to two and increases memory density.
The possibility of frequency overlapping was discussed when the Fourier transformation of deformed fringe for discontinuous object that contain mutation was executed by the method of dual-frequency grating.The condition of separation between f1 of low frequency grating and f2 of high frequency grating was deduced,and frequency overlapping of the same grating because of the nonlinearity of detector was analyzed.For the overlapping between f1 and f2 is the chief condition,computer simulation and experiment on the condition prove that f1 and f2 are overlapped when f22f1.
A 3 × 3 coupler demodulation algorithm was used to detect destruction along the fiber.Based on confirming the occurring of the destruction,by switching the broadband to narrowband source,Mach-Zehnder interference system works.The cross-correlation method was used to realize precise positioning,thus achieving the goal that defenders are led to stop acts of sabotage to prevent the occurrence of malice.The optical path was presented,and the detecting and locating principle was analyzed in detail.
The preparation method for graded index polymer optical fiber was analyzed using co-extrusion methods.According to the calculation theory about two refractive index and the function of refractive index distribution of the GI-POF,the dopants′ mass was calculated.Based on the unstable diffusion theory and finite difference method,the forming of refractive index distribution was described and the mathematical model was obtained.The general approximate way to describe the forming of refractive index distribution of the GI-POF was attained.
The MCVD and solution-doping technology for the Er-Yb co-doped double-clad fiber was reported.Using the MCVD manufacture technics,the fiber isolation layers(SiO2-P2O5-F) and the porous frits(SiO2-GeO2-P2O5) were deposited.The porous frits were soaked in the solutions of YbCl3 and ErCl3.Two fiber samples with 1:13 and 1:8 of Molar ratio Er/Yb were fabricated respectively.The maximium effective absorption coefficient of the sample 2 at the pump wavelengh 976 nm is 2 dB/m.The attenuation spectral and fluorescence characteristic were discussed.
Modulation scheme of atmosphere laser communication was discussed.Typical modulation schemes OOK,PPM,DPPM,DPIM and DH-PIM were introduced and its performances were analyzed.A new type of modulation called H-PPM was proposed,which was compared with the modulation scheme mentioned above.This type of modulation inherits the excellent bit error rate performance of standard PPM modulation scheme,it also need not the symbol synchronization when the information was demodulated at receiver.In this way,the design of receiver was simplified; and the difficulty of system was much reduced.Theoretical analysis and simulation results show that H-PPM is the best type of modulation for short instance,high reliability and little content laser communication between autos.
Using multiple importance sampling technique, the power penalty and outage probability of 10Gbit/s lightwave transmission system impaired by polarization-mode dispersion (PMD) and polarization-dependent loss (PDL) were investigated. The simulation results show that optical filter has significant influence on the power penalty, and the system with Fabry-Perot filter always performs better than that with four-order Gaussian filter. It is also found that the power penalty generally increases with the increasing of chirp factor, but the system performs best when chirp factor is about 0.5. It also demonstrated that DPSK is still more robust to PMD and PDL than OOK even though PMD and PDL coexist.
In order to improve the utilization rate of optical network,an approach of selecting the top quality web services composition based on Petri nets was presented.A Petri net model,which represented the data dependent relationship between the service composition according to consumers' demands and the current available services,was obtained.By calculating the T-variables relationships of the Petri net,plans of the service composition were obtained.And,the generalized stochastic Petri net (GSPN) was employed to analyze the performance of the plan.Compared with other approaches available,the results show that the presented method takes full advantages of Petri nets,such as easy description,analysis and evaluation of the systems,and can obtain the best plan of Web service compositions.
Based on Michelson-Gires-Tournois interferometer (MGTI) structure of interleaver that both of reflecting mirrors were replaced by two mirror Gires-Tournois etalons (GTEs).The appropriate value of structural parameters such as GTE front surface mirror reflectance,the cavity length of GTE and interferometer arm length difference were chosen.Four groups of spectrums that the ratio of wide bandwidth port to narrow bandwidth port at 3 dB passband (bandwidth ratio) was 1.69,1.83,2.13 and 2.14 respectively,were obtained.They have high channel isolation,wide flat-top passband and different periods.The effects of these structural parameters upon output spectrum characteristics were systematically investigated.The numerical simulation results indicate that asymmetrical interleavers of different bandwidth ratios could be obtained using this MGTI.GTE front surface mirror reflectance influences the passband bandwidth and channel isolation.And the two kinds of influences are different.Choosing appropriate GTE cavity length and interferometer arm length difference,different frequency periods can be obtained.
An all-optical non return-to-zero (NRZ) to return-to-zero (RZ) modulation format converter using single semiconductor optical amplifier (SOA) and an optical band-pass filter (OBPF) was proposed. By adopting the ultra-fast SOA model associated with optical system software, the 10 Gbit/s NRZ-to-RZ format conversion was successfully demonstrated with simulation. The proof-of-the-principle experiment at 10 Gbps was also demonstrated using the test SOA and OBF converter. The result shows that the BER is 1.0×10-9 when the power of RZ is -15 dBm,which are well coincidence with simulated results.
Based on the recording mechanism of photopolymer,the effect of different recording modes on the diffraction efficiency of gratings was investigated.The existing first-harmonic diffusion model of grating formation in photopolymer was simplified,then the analytic expressions of refractive index modulation for holographic exposure,dark enhancement and uniform post-exposure were educed.The kinetics of refractive index modulation for these three recording modes under different exposure intensities were numerically simulated by using the analytic expressions.Experiments were done in a new blue-sensitized photopolymer material.The gratings were recorded by different recording modes with exposure intensities of 4 mW/cm2 and 2 mW/cm2 respectively.The results showed that if the intensity was high,the dark enhancement mode yielded to a high saturated refractive index modulation,while uniform post-exposure made the course of saturation fast.
A theoretical model of a dual-channel polarization optical transmission system is analyzed,using two semiconductor optical amplifiers (SOAs) to achieve all-optical polarization modulations with two input data streams.In this model,the signal polarization modulation is completed by utilizing the cross polarization modulation (XPolM) effect in the SOA.A rate equation of the tensile-strained bulk SOA is adopted in the model.With this model,the dynamic characteristics of the system in respects of the amplification and the phase difference of the probe light in SOAs,signal polarization multiplexing and demultiplexing are investigated numerically.The simulated and experimental results are well consistent.
The influence of stimulated Raman scattering (SRS) on high-power (peak power ~1 000 W) pulse with a pulse-width of ~1 ns propagating in a single-mode fiber is investigated numerically by solving coupled nonlinear Schrdinger equations including Raman gain,pump depletion,self-phase modulation,cross-phase modulation and group velocity dispersion.The evolution of input pulse and Stokes pulse is analyzed,and suppression of SRS using large mode area fiber is shown.The results show that SRS can be suppressed effectively when using photonic crystal fiber (PCF) as transmission medium for inertial confined fusion (ICF).Compared to conventional single-mode fiber,the signal loss is smaller,and the central-dip in output pulse is lower.
A rotatable biprism is introduced in phase mask technique to change the inscribed Bragg of grating.In this system,the gratings are inscribed by the UV interference fringes of 248 nm derived from a rotatable biprism,where the phase mask is used as a beam splitter,and the biprism is rotated to change the intersection angle of two beams.For initializing the reference quantity of Bragg wavelength,the vertex angles of the biprism are determined by the ±1 diffraction angle of phase mask and the refract index of the biprism.Because the change of grating period caused by the asymmetrical rotation of two beams is limited to 5×10-4 nm at a tunability of the Bragg wavelength of ~100 nm,the tilt of grating caused by the rotation of the biprism can be ignorable.As the shift of Bragg wavelength is 1 nm,the maximum rotation angle of the prism is ~1 degree,and the minimum rotation angle is ~3 min.By contrasting with the rotation angle ~23 s/nm of the mirror in Talbot interferometer,the rotation precision of the prisms is decreased by two or three orders of magnitude in this phase mask interferometer.
Propagation constant and field evolution in the tapered region are discussed using numerical method based on waveguide theory.GNSE is resolved by the split-step Fourier method to study the transmission characteristics of ultra-short pulse in the tapered region.The calculated results show that at the beginning of the tapered fiber,propagation constant decreases gradually while in the end it drops dramatically.After the “transition point”,energy redistributes in the fiber,and it becomes very large in the end.It is found that pulse width is broadened along the tapered region when the pulse width is smaller than 80fs.
A controlled teleportation scheme of a three-particle arbitrary state is proposed,in which a seven-particle entangled state is used as quantum channel.After projective measurement,the sender announces measurement results.Under the control of the controller,the receiver performs unitary operations on particles to reconstruct the original state.Using the controlled teleportation scheme,some controlled quantum communication protocols can be realized.
The three-channel transmission of 42.8 Gbit/s nonreturn-to-zero differential phase-shift keying (NRZ-DPSK) signal is demonstrated.The 410 km transmission link consists of four spans using standard single-mode fiber (SSMF).The dispersion compensation fiber (DCF) and erbium-doped fiber amplifier/distributed raman amplifier (EDAF/DRA) hybrid amplification are used.Both optical power spectrum and eye diagrams of the DPSK signal are given in back-to-back configuration and after transmission (center channel).The curves of bit error rate (BER) of the center channel also is given,both in back-to-back configuration and after transmission,which is compared with the curve in single-channel transmission configuration.The BER of the DPSK signal of the center channel is 1.0E-3 after transmission using single-ended detection.张帆|fzhang@pku.edu.cn
Square-array microstructured polymer optical fibers with polystyrene core and polymethylmethacrylate cladding are fabricated.The image and grating function of the fiber are investigated with home-made specialized system.This new type of fiber provides strong potential for applications in imaging fiber and grating measuring.
Based on the coupled-mode theory,the complex characteristic equation of LPFG coated with weak absorption double-layer films is set up.By use of the perturbation method,the complex root is obtained.The result is coincident with the result of D.V.Ignacio.The film thickness and refractive index have a great influence on the resonant wavelength,but the extinction coefficient has a little influence on the resonant wavelength.And,the two methods of optimizing the film parameters are presented in order to obtain the larger shifts of the resonant wavelength.The results show that when the film parameter combinations are h3=122.76 nm,h4=400 nm and n3=1.572 2,h4=400 nm,the wavelength shifts are 10.35 nm and 11.74 nm,respectively,which is much larger than the value of 2.78 nm obtained from the LPFG coated single film.It proves that the selection of the thicker film increase the sensitivity.顾铮天|zhengtiangu@163.com
A novel multi-ary wireless optical communication technique is proposed, which utilizes a set of mutually distinguishable spatial patterns instead of convergent facula to transmit information.The communication capacity of the proposed communication scheme is analyzed.The results show that the spatial freedom of the wireless optical communication system can be utilized to increase the communication capacity highly by reasonably selecting parameters of the system geometry and designing signal patterns.A typical electro-optical system scheme of the proposed communication is presented.The signal processing scheme is described in detail, which includes the following three processes as 2D correlation, comparison and M/2 conversion.The error performance of the proposed communication scheme is analyzed and the average probability of error decision is obtained.For reducing error decision, the fundamental rules of designing spatial pattern signal are formulated.Finally, the proposed communication is experimentally demonstrated with a lenslet array processor.潘卫清|welcome.pan@163.com
All optical wavelength conversion based on four-wave mixing in a SOA for OFDM optical signal is theoretically and experimentally investigated.2 Gbit/s OFDM is used to modulate directly on the signal lightwave by an external intensity modulator.The modulated signal lightwave and the parallel pumps are coupled and then injected into the SOA for wavelength conversion based on four-wave mixing (FWM).Experimental result shows that the newly converted wavelength sideband carries OFDM signals and its conversion efficiency relates with the wavelength spacing between the pumps,the wavelength spacing and polarization angle between the pumps and signal lightwave.The BER curves and receive constellation are also measured.卢嘉|liliuchen12@vip.163.com
Based on a light source with adjustable pulsewidth,a dual functional fiber Raman distributed temperature sensor was proposed.Short pulses were used to achieve high spatial resolution for peak temperature detection.Wide pulses were adopted to obtain high temperature resolution for average temperature monitoring.The proposed method can achieve either high spatial resolution or high temperature resolution for various applications.The experimental results indicate that the measurement time can be saved greatly for the same temperature resolution comparing to the conventional method with constant pulsewidth.
A coupling model of imaging fiber and photo-detector array devices was established, and its coupling characteristic was studied via simulating method, which introduces different variable factors to represent the irregular arrange of imaging fiber. Based on 9×9 μm photosensitive cell, the influence to resolution was analyzed as fiber′s structure factors changing. A reasonable economic matching structure was obtained when fiber diameter was about one third size of the cell, that showed the optimal designing problem must be considered in the system design. Results of the computer emulation also explained the existence of the stripe and the formed reason of partial background structure in imaging fiber digital system, and the relevant method of numerical analysis was given.
For the problem of spectral distortion in all fiber Fourier transform spectrometer caused by the phenomenon of polarization fading on the output interferometric signal,the effect of optical fiber birefringence which were induced by stress,bending and twisting to light polarization state were analysed by Polariztion Optics theory,and find the reason of spectral distortion in FFTS.Finally,in order to stabilize the fringe visibility,the method to establish feedback control system of polarization state was proposed.Experimental results demonstrate that the impact of polarization state change has been eliminated by the method.
The possible application of multi-phase modulation technology in optical fiber communication based on Carrier-Suppressed Return-to-Zero (CSRZ) format was studied.With analyzing the characteristics of CSRZ format, Differential Quadrature Phase Shift Keying (DQPSK) and Eight Differential Phase Shift Keying (8DPSK), formula of CSRZ-DQPSK and CSRZ-8DPSK modem methods was got and the methods in their specific achieving processes were presented.Spectra based on CSRZ, CSRZ-DQPSK and CSRZ-8DPSK modulation methods and their eye diagrams following demodulator using Matlab simulation were obtained.The results show that the CSRZ-DQPSK and CSRZ-8DPSK modulation methods, as the possible new methods, had narrower spectra with higher spectrum efficiency.Their performance of eye diagrams was also much satisfactory after demodulation, suggesting some possible applications of the methods in the next generation of optical fiber communication.
A novel assemble algorithm was proposed for Optical burst switching networks.Burst assembling was a very important technique in OBS.Burst assemble algorithms was introduced and then a new assemble algorithm based on the traits of the current algorithms was proposed.The algorithm was discussed and the feasibility by formulation was proved.The algorithm dynamically changes the assemble threshold based on the arrival rate of data and the performance of the network.From the result of the formulation.It can be found that the algorithm can obviously improve the performance of loss rate and the delay for the OBS networks and hold real-time data service.
The relative motion of communication satellites which are not on a plane was analyzed.The acquisition probability and acquisition time was improved by compensated the point angles for terminal gimbal.And the charge coupled device target plane coordinates which decided by the receiving laser beam were modified to reduce the constraint for fast steer mirror.Numerical simulation resolving the relative motion model compensates the excursion to antenna rotation angles and charge coupled dirice coordinates.The curves show that the relative motion can be eliminated depending on compensation model analysis.The system structure was simplified that not needs to consider actuator cooperation with other device.That decreases the complexity of the whole system and saves energy.And numerical compensation method is convenient to realize for satellite computer program.
Based on the particle swarm optimization (PSO) algorithm,a novel FWM power estimation method was proposed.And a technique to design the channel frequency allocation in order to minimize the crosstalk due to FWM in optical dense-wavelength-division multiplexing (DWDM) systems was presented.Which is able to support unfixed fiber dispersion parameters along the links and arbitrary span configuration and more than 16 channels frequency allocation.An effective 16 channels allocation method was obtained,and was compared with other similar methods.The result shows that it can restrain FWM effect much more significantly.
A fiber optic sensing demodulation system based on a swept fiber laser was reported.A HCN gas cell was adopted as an embedded wavelength calibrator for the swept laser.The relation between the scanning voltage applied to the fiber Fabry-Perot tunable filter (FF-PTF) and the lasing wavelength was given out by a three order nonlinear fitting.The scanning voltage and multi-channel spectroscopy signal from sensors were sampled simultaneously and calibrated in real time.The interrogation wavelength resolution of 1.4pm and the long term repeatability of 3.2 pm were achieved.Demodulation test results for FBG and F-P sensors show that the system is capable of measuring the reflected peak wavelength of FBG and cavity length of fiber EFPI sensor with high accuracy.
作者简介:马春生,Tel:0431-885168240-8307 Email:mcsheng@163.com
Erbium-doped fiber amplifiers (EDFAs) with low-cost long-period fiber gratings (LPFGs) fabricated by high-frequency CO2 laser pulses are investigated theoretically and experimentally. Results shows that Performances of EDFAs for both single-wavelength and multi-wavelength applications can be improved via inserting a LPFG into the erbium-doped fiber (EDF) as an ASE noise filter or a gain equalizer. Optimization of an in-line EDFA with a LPFG is also provided. The NF of such an in-line EDFA is improved by ~0.5 dB and its small-signal gain is enhanced by 7 dB, compared with that without filtering. A multi-wavelength EDFA with a gain flatness of ~1.5 dB is simply achieved via inserting a LPFG into the middle of an EDF as a gain equalizer, accompanying with a gain improvement of ~1 dB and a NF reduction of ~0.1 dB compared with the EDFA having a LPFG located after the EDF.
In a typical triangular arrangement of photonic crystal fiber (PCF), each air-hole is replaced by a pair of twin air-holes. Two neighboring air holes with the same spacing and fixed axes direction come into being twin air holes pairs and it is the basic cell in the cladding of PCF.In the central of section, not missing one air hole but missing a pair of twin air holes with higher refractive index material can localize optical field and become the core of PCF. All twin air holes are regularly arranged according to triangular arrangement in the cladding. In this novel PCF, an asymmetry character is aroused because of all twin air holes with identical axis direction, and birefringence relevant to axes direction of twin air holes is formed. Finite difference (FD) method is used to analyze modal character of PCF. The results of numerical calculation indicate that, the effective refractive index difference between two orthogonal directions Δneff can reach the magnitude of 10-4. Especially, the structure parameter of twin air holes can change birefringence in slight, such as increasing air hole size or reducing spacing of two air holes within every twin air holes all can increase birefringence effect.
An ultra-broadband amplified spontaneous emission (ASE) source was proposed and demonstrated by using a combination of a novel 247 cm Bismuth-based Erbium-doped fiber and a conventional 10 m Erbium-doped fiber in a configuration of bi-directional pumps. The physical mechanism was explained and the performance was compared with those for some other similar configurations. Without any external spectral filters and with a total pump power of less than 240 mW,an ASE source with a wavelength range of 96 nm (1 522 nm~1 618 nm, over -20 dBm/2 nm)and a total output power of over 11 dBm was achieved by optimizing the EDFs' lengths and the pump powers. The -10 dB bandwidth and the peak power density of the output spectrum are over 87 nm and -2.5 dBm/2 nm, respectively.作者简介:CHEN Da-ru,Tel:0571-88206515-227 Email:daru@coer.zju.edu.cn
作者简介:张新亮,Tel:027-87792242-803 Email:xlzhang@mail.hust.edu.cn
The residual third-order dispersion of the optical phase conjugation process is detrimental to conjugated polarization solitons propagation in single-mode dispersion, resulting in more timing jitters. The dispersion-shifted fiber with positive TOD was proposed to use for compensating for the residual positive TOD at the fiber output. The numerical results prove it practicable and the physical mechanisms were also analyzed.
Transmission Control Protocol (TCP) performance over optical burst switching (OBS) is experimentally investigated on OBS testbed. The effect of burst losses on TCP performance over the OBS testbed is studied. The result shows that burst losses will lead to a network wide drop in TCP throughput and higher burst loss probabilities will result in lower available TCP bandwidth. Then the effects of delay penalty and correlation benefit are investigated in detail. Taking into consideration these effects, experimental results show that there exists an optimal assembly period to maximize the TCP bandwidth, and the optimal value is about 250 μs in the testbed, which is independent of the burst loss probabilities. To make the OBS testbed work steadily and improve the performance of TCP over the testbed to the greatest extent, the chosen burst assembly period should be equal to or slightly greater than the optimal value.
Based on the analysis solution of a W-type double-cladding chiral optical fiber, the optical power characteristics of several lower-order guided modes have been investigated. The optical power in the core as a function of chirality parameters in the core, the inner and outer claddings is calculated for different inner cladding thickness. The effect of chirality parameters on the optical power in the core for guided modes with different index signs is discussed. Thicker inner cladding results in larger optical power in the core. With increasing the chirality parameter in the core, the optical power of HE-11 mode and HE-21 mode (HE01 mode) in the core increase slowly, but the power of HE11 mode and HE21 mode (HE02 mode) in the core decrease gradually. The effect of the chirality parameter in the cladding is reversed with that in the core.
The possibility of using a hollow-core Bragg fiber with cobweb- structured cladding to obtain low-loss transmission in wavelength range 0.65 μm~ 1.55 μm and 200 μm~ 500 μm is proposed and analyzed. The results show that the transmission losses of the hollow-core Bragg fibers with cobweb cladding are far less than the absorption losses of constituent plastics. Thus, using inexpensive plastics, it allows the fibers to meet the needs of transmission longer-distance and higher-bandwidth and to realize the wavelength division multiplexing(WDM).
Based on extended nonlinear Schr(o)dinger equation, the effects of third-order and fourth order dispersion on cross-phase modulation instability (XPM) are investigated, considering the fiber loss and high-order dispersion. The results show that the third-order dispersion does not affect modulation instability. But because of fourth-order dispersion, XPM occurs at two spectrum regions in both the normal and the anomalous dispersion regimes under certain conditions. Gain spectra of the two regions of anomalous dispersion regime are larger than those of normal dispersion regime, and gain spectrum of the second region of anomalous dispersion regime is near zero than that of normal dispersion regime. The research also shows that fiber loss reduces the frequency range of the gain spectrum, and the frequency range becomes smaller with the increasing of the propagation distance.