Submarine cable communication, as an important approach of contemporary international communication, transfers more than 95% of international communication services including internet, voice, and multinational private line services. It is the main carrier of international information, and the monitoring of its operation status is of great significance. Next-generation submarine cable systems shall realize loss monitoring as well as provide early warnings of damage events. Therefore, an coherent optical time domain reflectometry submarine cable (C-OTDR) online monitoring system integrated with phase-sensitive OTDR (Φ-OTDR) is designed to achieve the synchronous monitoring of vibration and loss over a 127-km span. In order to find a solution to the conflict between Φ-OTDR and C-OTDR on the polarization state problem, an optimal selection algorithm of polarization diversity amplitude is proposed. The proposed algorithm can effectively suppress the influence of fading noise on the sensing system and ensure the reconstruction accuracy of external disturbance events. Simulation tests on water flow impact and anchor damage dragging are conducted to verify the effectiveness of the enhanced C-OTDR system.

High-temperature sensors have important research and application value in aerospace, nuclear power, metallurgical industry, and other fields. To apply fiber Bragg gratings (FBG) to high-temperature sensing, this paper studies the preparation technology, annealing process, and temperature-wavelength fitting method of FBG arrays. First, a wavelength division multiplexed FBG array is prepared by the femtosecond laser point-by-point method, and FBG arrays with 9 different wavelengths in the range of 1510-1580 nm are prepared with optimized process parameters (femtosecond laser pulse energy, fiber moving speed, and FBG length). Then, the effect of annealing temperature and time on the center wavelength of FBG is examined, and the wavelength stability of FBG is improved by high-temperature and long-term annealing (700 ℃, 195 h), with the wavelength drift rate of FBG at 700 ℃ less than -2 pm/h. Finally, the high-temperature response of FBG with different center wavelengths are studied, and the general temperature-wavelength fitting functions of these FBG are obtained. The experimental results show that the temperature measurement accuracy of the FBG array sensor is better than ±1.8 ℃ at 700 ℃. The sensor is expected to be used for high-temperature measurement in extreme environments such as aeroengines, high-speed aircraft, and nuclear reactor cores.

Optical frequency domain reflectometry (OFDR), featuring high spatial resolution and precision, has been regarded as an important technique in the monitoring and diagnosis of fiber-optic links and networks and distributed fiber-optic sensing. The sweep nonlinearity and phase noise of laser are the key issues to be dealt with for the broadband optical linear frequency sweep in OFDR systems. In this work, we report on laser sweep nonlinearity and phase noise control based on delay self-heterodyne optical phase locking, which allows highly linear laser frequency sweep with an 8 GHz sweep range and a 160 GHz/s sweep rate with low phase noise. Experiments on both fiber-optic link monitoring and distributed fiber strain sensing are carried out based on such laser source. In the former experiment, we achieve long-distance OFDR with a dynamic range of 27 dB and a high spatial resolution of 4.3 cm over a 240 km fiber-optic link. In the latter, distributed strain sensing with 5 cm spatial resolution is realized. The experiments verify the effectiveness and superiority of the proposed method in controlling laser frequency sweep errors by phase locking.

The key techniques for the high-sensitivity detection of CO2 in the 2 μm band are studied based on photo-thermal spectroscopy in a hollow-core optical fiber, and a low-sharpness Fabry-Perot interferometer based on anti-resonance hollow-core optical fiber is adopted to demodulate the photo-thermal phase. For the R(18) absorption line of CO2 at 2004.02 nm, the noise equivalent volume fraction is about 4.7×10 -8 with a pump power of 180 mW and integration time of 1 s. To improve the stability of the detection system, we suppress the intra-cavity interference of pump light by applying the anti-reflection coating to the fiber end face and lock the probe wavelength to the quadrature point of the interferometer. Within 1 h, the signal fluctuation corresponding to CO2 standard gas with volume fraction of 10 -5 is about 4.7%. Noise analysis shows that the noise mainly comes from the phase noise and intensity noise of the probe laser in the current detection system.

Surface-mode resonance coupling effect and high-temperature sensing characteristics of hollow-core photonic bandgap fibers are proposed and studied herein. The surface-mode coupling effect in the photonic bandgap fibers was observed experimentally, and the generation principle of the effect was theoretically explained. Multiple resonance peaks were formed in the fiber transmission spectrum, were subjected to temperature- and strain-sensing experiments, and unique temperature and strain sensing characteristics were observed. The resonance peak was insensitive to low temperatures between 20 ℃ and 150 ℃, whereas it was sensitive to high temperature between 150 ℃ and 260 ℃. This temperature sensitivity observed between 150 ℃ and 260 ℃ reached -0.26 dB/℃. Simultaneously, the wavelength of each part was insensitive to temperature, and the intensity and wavelength of the resonance peak were insensitive to strain. The hollow-core photonic bandgap fiber sensor addressed the temperature-strain cross-sensitivity problem, effectively can realize real-time intensity detection in a high-temperature environment, and has many advantages, including a simple structure and ease of use.

Since the concept of random fiber laser was proposed, it has received extensive attentions in the fields of fiber lasers, fiber sensing, etc. Combining random fiber laser with fiber grating, high performance point-sensing system can be realized based on spectral change information of fiber grating. Therefore, the spectral characteristics of this system determine key parameters such as sensing distance, accuracy and multiplexing capability. Aiming at the key issue of high-order random laser multi-point sensing system, i.e., spectral characteristics simulation, this paper improves the traditional high-order random laser power balance model, and the spectral simulation results of the proposed model are in good agreement with the experiment. Based on the simulation model, a high-order random fiber laser multi-point sensing system with a length of 150 km is designed and realized. At the same time, the fitting linearity of the sensing signal wavelength and strain obtained from the experiment is as high as 0.999, which fully verifies the system’s sensing performance.

The measurement accuracy of ultralong fiber interferometers is mainly limited by the 1/f noise in the ultralow frequency band from mHz to kHz. The sources and distribution principle of 1/f noise need to be thoroughly studied to suppress measurement noise and improve measurement resolution. For this reason, a balanced polarization-maintaining fiber Mach-Zehnder (M-Z) interferometer was built for differential detection, in which the arm length is over 100 m. By testing optical fibers with different lengths as well as different types of narrow line-width laser utilizing this interferometer, we obtained the distribution characteristics of ultralow-frequency phase noises respectively induced by laser frequency drift and fiber thermal noise on the spectra. According to the experimental results above, the distribution principle of 1/f noise in the ultralong fiber interferometer was also explained. The results show that the amplitude of phase noise increases rapidly from the order of 1 μrad/Hz 1/2 @1 kHz to the order of 10 mrad/Hz 1/2 @1 mHz within a 260 m arm-length M-Z interferometer, which is due to the laser frequency drift and fiber thermal noise. It can be concluded that for the frequency band from mHz to kHz, the phase noise of the ultralong fiber interferometer satisfies 1/f1+β spectral distribution.

A method is proposed to extend the dynamic strain range of fiber-optic distributed sensing based on linear frequency modulation (LFM) pulse sideband modulation. Based on inverse ratio between the frequency-modulated bandwidth and the coherent time-domain signal envelope shift, multiple modulated sidebands with different frequency-modulated bandwidths are generated in a single pulse for sensing. Coherent time-domain signals of each sideband are digitally band-pass filtered and decomposed to achieve simultaneous measurements of events with different dynamic strain ranges. LFM signals with frequency modulated bandwidths of 40 MHz and 200 MHz are used to modulate sidebands in experiments, respectively. The results show that the system can simultaneously measure the maximum amplitude of 7 nε and 350 nε sinusoidal dynamic strain events, which provides a dynamic range expansion scheme for fiber-optic distributed acoustic sensor system.

In this paper, a denoising algorithm based on local mean decomposition is proposed to improve the signal-to-noise ratio (SNR) in Brillouin optical time-domain analysis (BOTDA) sensing systems. First, the signal collected by a BOTDA sensing system is adaptively decomposed into product function (PF) components with real physical meaning. Then, the PF components containing signal energy are reconstructed to get the denoised signal after the distribution of the signal energy on each spatial scale is calculated. To further improve the denoising performance of the algorithm, we introduce a Chebyshev digital band-pass filter to filter and reconstruct the PF components in the frequency domain. The experimental results show that compared with that of the original signal, the SNR of the signal denoised by the algorithm is improved by at least 10 dB, and the algorithm provides a simple and effective denosing scheme for the sensing systems.

A resonant optical fiber gyroscope (RFOG) is an optical sensor that measures the angular rate of rotation proportional to the difference between the resonant frequency of the clockwise and counterclockwise optical paths generated by the Sagnac effect. A narrow linewidth laser with a continuously adjustable center frequency is a key element for developing RFOG, and the frequency noise characteristics of the laser directly affect the performance of this gyroscope. In this study, the frequency noise power spectral density of different types of lasers and the lasers before and after frequency locking was measured using an unbalanced Mach-Zehnder interferometer system based on a 3×3 coupler. The 1/f and white noise characterization coefficients of different types of lasers and the lasers before and after frequency locking were obtained. The study results can provide an important reference for selecting RFOGs with different accuracies and optimizing resonant frequency servo loops.

For a fiber-grating time-division multiplexing (TDM) sensing system adopting the phase-generated carrier (PGC) modulation and demodulation schemes, the influence of the structure of the sensor array and the modulation and demodulation parameters on the system phase noise is investigated. The theoretical analysis results show that the array will constrain system structure design of modem design and selection of parameters, and by using polarization modulation in the fiber Bragg grating sensing system means to suppress the induced polarization signal fading. When the number of time-division multiplexing is larger, the system frequency modulation demodulation of the lower limit. The simulation analysis of the high-frequency aliasing effect in the PGC modulation and demodulation algorithms show that the smaller the modulation frequency, the greater the background phase noise of the system. In the experiment, four sets of interferometric fiber Bragg grating time-division multiplexing sensor arrays with different design structures are constructed. The PGC modulation frequencies are 6, 10, 16, and 50 kHz. The experimental results show that the phase noises of the first time-division multiplexing channel for the four systems at the frequency of 1 kHz, are -93, -96, -98, and -99 dB/Hz 1/2, respectively, which are consistent with the theoretical analysis results.

The distributed optical fiber sensor based on Rayleigh spectral demodulation has the advantages of high linearity and capability for both dynamic and static signal sensing, but the traditional demodulation method based on cross-correlation requires a large amount of calculation. Here we propose a new demodulation method based on an artificial neural network (ANN) instead of cross-correlation operation to boost the speed in the demodulation of Rayleigh backscattering spectra, in which an ANN model is first constructed and trained to map the Rayleigh scattering pattern to the frequency of the corresponding probe laser. Then, the strain or temperature information of the fiber is figured out from the input Rayleigh scattering curve to be demodulated. In the verification experiment, the Rayleigh backscattering traces at the detection position under different probe laser frequencies are obtained using time-gated digital optical frequency domain reflectometry in which chirped pulses and matched filters are employed. It is verified that the strain applied to the fiber can be correctly demodulated by the data-treatment method based on the ANN algorithm. Compared with the demodulation method based on cross-correlation, the proposed method possesses a lower signal-noise ratio and the calculation speed increased by two orders, which ensures the realization of the fast detection of dynamic signals.

As a kind of clean energy, hydrogen has been widely used in industrial field. However, due to its light molecular weight, it is prone to leakage. When volume ratio of hydrogen in the air reaches 4%, there is a danger of explosion. Therefore, detection of hydrogen concentration is very important in practical applications. In view of the above situation, this paper proposes a Fabry-Perot interferometer (FPI) hydrogen sensor based on polydimethylsiloxane (PDMS) filling. By introducing an arrayed waveguide grating (AWG), it can detect hydrogen concentration at multi-point simultaneously. The proposed sensor is formed by fusion splicing a single mode fiber and a hollow core fiber (HCF), filling the HCF with PDMS and covering the outer surface of PDMS with Pt/WO3. When Pt/WO3 reacts with hydrogen chemically, released heat increases local temperature of the hydrogen sensor, and the thermal expansion of PDMS results in shortening the length of air cavity inside FPI, which will cause wavelength shift of interference spectrum. By connecting the sensor with the AWG, wavelength shift can be demodulated into light intensity change, so as to achieve simultaneous multi-point hydrogen concentration measurement.

A distributed vibration sensing system with a high signal-to-noise ratio (SNR) based on an ultra-weak fiber Bragg grating is proposed in this paper. The system uses an online inscribed chirped grating array with a 3 dB bandwidth greater than 3.4 nm and a reflectivity of 10 -5 as the sensing fiber, and phase demodulation is realized through typical dual-pulse modulation and a 3×3 coupler. To eliminate interference noises from optical components including laser, modulator, and optical amplifiers, we employ the least mean square algorithm and a symmetric detection structure for adaptive filtering. The experimental results show that compared with the wavelet denoising method, the method can greatly decrease the system noise. It can reduce the high-frequency noise floor of the 1 kHz demodulated signal by 40.1 dB at the maximum, and the SNR is as high as 96.5 dB. For the 2 km long sensing fiber, the strain sensitivity of the system reaches 13.66 pε/√Hz at 1-5 kHz.

The thin-walled grapefruit microstructured fiber and single-mode fiber are welded by fiber fusion technology to make a probe-type pressure sensor. The open air hole at one end of the microstructure fiber can communicate with the external pressure to realize the pressure response. The pressure sensitivity of 0.70 nm/MPa is obtained and the pressure response has good linearity. Through the comparison between the finite element simulation and experimental result, it is found that the microstructured fiber sensor has the interference of LP01 and LP11 modes, and it is proved that the sensor has good stability and repeatability.

With the development of offshore wind power generation technology, the powers of wind turbines are increasing, leading to higher demands for the bearing capacity of their pile foundations. Thus, load performance test on pile foundations is needed to ensure the safety and reliability of engineering design. In this study, we first established a Brillouin optical time-domain reflectometry (BOTDR) system with a spatial resolution of 1 m. Then, this system was used to conduct horizontal thrust-loading tests on the pile foundation of an offshore wind turbine in the Jiaxing region of Hangzhou Bay. The deformation and displacement distributions of the pile foundation were determined by measuring and analyzing the strain distributions and positions of the maximum strain of the pile foundation under different horizontal loads. Results show that the horizontal bearing capacity of the tested pile foundation can withstand the expected maximum load of 700 kN, and the BOTDR distributed optical fiber sensing technology in pile foundation tests for offshore wind turbines is validated.

Mode splitting caused by the coupling between whispering gallery mode microcavities will lead to the increase of Q factor of the device, and hence the performance of the device can be enhanced. As a new type of microcavity coupling form, an in-fiber coupled microsphere resonator improves the integration and stability of fiber devices. In this paper, an optical fiber sensor based on the fiber coupled double microsphere resonators is fabricated and investigated, which consists of the single mode fiber, silica capillary, and two barium titanate microsphere resonators. The whispering gallery modes of two microsphere resonators are coupled to each other, where the induced mode splitting increases the Q factor from 8×10 3 to 2.4×10 4. As mode splitting is beneficial to improve the sensing ability of small changes, the device is demonstrated to own a good temperature response stability in temperature sensing experiments, with the temperature sensing sensitivity of 11.7 pm /℃, and the detection limit is as low as 0.03 ℃.

In this study, the mode equation of an optical fiber-assisted circular thin-film waveguide was deduced theoretically based on a planar waveguide approximate model, from which the discrete thin-film waveguide modes are obtained. Combined with the full vector mode theory, new insights into optical fiber mode transition are obtained. It is found that the fiber cladding mode is transited into the phase-matched thin-film cladding mode supported by the circular thin-film waveguide with a certain thickness, in the mode order from high- to low-order rather than the well-known low- to high-order. The further increase of thickness of the circular thin-film waveguide eliminates the phase-matching condition and the generated thin-film cladding mode transforms back to the adjacent low-order optical fiber cladding mode, resulting in a periodic mode conversion process. These theoretical results were validated by analyzing the spectrum evolution of an indium tin oxide film-coated tilted fiber Bragg grating. An asynchronous excitation process with the same period of the orthogonally polarized thin-film waveguide modes (P- and S-polarizationand) and its periodic tuning property for optical fiber polarization are realized. These results are consistent with those of previous experiments and are expected to provide a new method for fiber-optic vector parameter sensing (e.g., vibration, twisting, pressure, acoustic field, etc.), fiber-optic biosensing (discriminative sensing of volume refractive index and surface refractive index), and polarization-dependent optical communication filters.

In the application of fiber optic perimeter security systems, realizing intelligent vibration sensing requires both accurately identifying specific types of sensing events and providing targeted processing solutions for such events. In this paper, we propose a signal feature-extraction algorithm that includes multidimensional time information features, combining these features in a convolutional long short-term deep neural network (CLDNN) that identifies and classifies specific vibration-sensing events. First, we stack and intercept the collected optical fiber sensing event information to obtain a broad picture containing multidimensional time characteristics of the sensing event. Next, we input these collected data into the CLDNN structure. We define five distinct types of events or signals for our identification and classification experiments: knocking, crashing, waggling, kicking, and no intrusion whatsoever. Experimental results show that the proposed algorithm can effectively recognize and classify these five types of signals with an average recognition rate of over 96% and a recognition response time that can be limited to 0.006 s.

Optical frequency domain reflectometry (OFDR) is an optical detection technique based on frequency-domain analysis. It can achieve high-spatial-resolution distributed fiber temperature/strain sensing through a cross-correlation algorithm and has great application prospects in the structural health monitoring of spacecraft, flight vehicles, etc. Common cross-correlation algorithms have problems of small range and large amounts of computation in OFDR fiber temperature and strain sensing demodulation, which limits the large-range application of OFDR and the improvement of the demodulation rate. For this reason, a cross-correlation algorithm based on an adapted range was proposed. The algorithm not only improves the range of temperature and strain measurement but also enhances the demodulation efficiency of the system. Experimental results show the feasibility and efficiency of the proposed algorithm. The demodulation efficiency was increased by 2.25 times. The large-range temperature curve from 20 ℃ to 500 ℃ was measured, and the residual error of cubic curve fitting was less than 0.1 ℃. The test error was no greater than 5 με in the strain range of ±3000 με.

This paper proposes an intelligent optical fiber dynamic strain sensing system based on a semiconductor optical amplifier (SOA) fiber ring laser, and uses adaptive photorefractive two-wave mixing technology to demodulate it, and there is no need to align the static strain during the demodulation process. And temperature for any active compensation. When the reflection spectrum of the FBG (Fiber Bragg Grating) sensor changes due to dynamic strain, the wavelength of the laser output will move accordingly, then converted to a corresponding phase shift and demodulated by the InP∶Fe photorefractive crystal two-wave mixing interferometer. Experimental results show that the sensor system can measure dynamic strains between 50 and 464 kHz and the sensitivity is higher than 0.5με Hz -1(ε is strain). In the InP∶Fe photorefractive crystal, the dynamic strain of three FBG sensors is demodulated at the same time. The experiment proves that the multiplexing of the two-wave mixing interferometer is feasible.

To satisfy the requirements of flow drag measurement in a wind tunnel experiment and airspeed monitoring of aircrafts in a complex electromagnetic environment, a differential-pressure fiber-optic airflow sensing system based on white-light interferometric technology was developed to perform relevant experimental investigations. The proposed system comprises a differential-pressure fiber-optic airflow sensing probe and a miniaturized white-light interferometric interrogator, which realizes synchronous high-speed, high-precision differential-pressure measurement. The sensing probe realizes single-channel differential-pressure sensing and can detect flow drag by obtaining the difference between the surface pressure of the target measurement object and the static pressure in the flow field. In addition, the sensing probe can be coupled with a pitot tube to measure flow velocity. The white-light interferometric interrogator comprises a wavelength scanning laser, a field-programmable gate array module for control and acquisition, and a photoelectric detector. The proposed system was used to measure and analyze the classical model of flow around a circular cylinder in wind tunnel testing. The results are similar to those obtained using a standard multichannel electronic pressure transducer. The proposed fiber-optic sensing system uses only optical fiber as the medium to sense and transmit signals, which can effectively resist electromagnetic interference. Thus, the proposed system enables using the optical method to accurately analyze the flow drag in an airflow field in a strong electromagnetic interference environment and has a practical potential application in future aerodynamics research and in-flight monitoring.

This paper proposes a new method of phase unwrapping based on auxiliary light, which is used to break the limitation of traditional phase unwrapping algorithm and expand the measurement range of phase signal. First, based on the wrapped statistical phase in coherent detection based phase optical time domain reflectometer, the wrapped differential phase the reference position and the subsequent position can be solved. Then, based on the joint pulse sequence, the wrapped differential phase is unwrapped according to the traditional phase unwrapping algorithm, and the phase change curves retrieved from the unwrapped differential phases are superimposed together. The position of fiber that correctly implements the traditional phase unwrapping algorithm can be obtained from the linear feature of the superimposed phase change curve. Therefore, the phase signal corresponding to the disturbance source can be correctly obtained based on this linear feature of phase change. In the experiment, a phase signal with a maximum absolute phase difference of 3.7154 between adjacent sampling points can be perfectly reconstructed.