ObjectiveSNR of Raman distributed fiber optic sensing system gradually decreases with the increase of sensing distance, the power of a single pulse will be limited by the nonlinear scattering threshold, which can't be infinitely improved. Although the incident optical power can be increased by increasing the pulse width, the spatial resolution of the system will be weakened, which makes it difficult to realize the high spatial resolution temperature sensing at longer distances to satisfy the engineering requirements. In this paper, a strong autocorrelation pulse coding technique is proposed. The Golay complementary sequence with autocorrelation characteristics is introduced into the Raman distributed fiber sensing technology, and anti-Stokes scattering signal is analyzed and reconstructed by Fourier transform to restore the autocorrelation of the Golay complementary sequence destroyed by transient effect of EDFA. This technology can significantly increase the optical power on the premise of no stimulated Raman scattering, which effectively improves SNR of the system and realizes the long-distance Raman distributed fiber optic sensing technology.MethodsIn this paper, Raman distributed fiber optic sensing system based on the strong autocorrelation pulse coding technique (SAC-codes) is established (Fig.1), and the correctness of the technique is verified by numerical simulation experiments, which achieves the accurate identification of temperature demodulation and localization of FUTs (Fig.4). The dominances of the technique are verified by analyzing the SNR after decoding. The scheme of temperature demodulation is utilized to obtain the temperature information in FUT, which verifies the performance enhancement of the technique in terms of temperature resolution (Fig.5), and obtains the final spatial resolution of the system (Fig.6).Results and DiscussionsThe correctness of the SAC-codes scheme is verified by numerical simulation experiments, and the accurate identification of temperature demodulation and localization of the FUT is achieved. The performance of the single-pulse scheme, the Golay-codes scheme affected by transient effects, and the SAC-codes scheme are quantitatively analyzed, numerical simulation experiments show that the SNR at the fiber tail is improved from 9.14 dB to 10.29 dB (Fig.3), which verifies the superiority of the SAC-codes scheme. Meanwhile, the performance enhancement of the SAC-codes scheme is experimentally explored, which dramatically improves the temperature resolution of the system, which improves from 22.69 ℃ to 4.12 ℃ (Fig.5), eventually, a spatial resolution of 1 m was achieved based on a 100 km sensing distance (Fig.6).ConclusionsIn this paper, a Raman distributed fiber optic sensing technique based on strong autocorrelation pulse coding is proposed to realize the need of long-distance temperature monitoring. The feasibility of this technique is verified by numerical simulation, and finally, it is proved experimentally that this technique obtains a large improvement in performance indexes such as SNR and temperature resolution compared with the traditional single-pulse scheme, which provides a new solution for the research of long-distance Raman distributed fiber sensing system.

ObjectiveVibration monitoring based on distributed fiber sensing technology has great application prospects in the current linear engineering field. Asymmetric dual Mach-Zehnder interferometer (ADMZI) based distributed fiber sensing effectively extends the vibration sensing distance. However, the inherent asymmetry of the system leads to the failure of positioning by directly using the cross-correlation algorithm. By extracting the phase variation characteristics of the interference signals, the asymmetry of the sensing system can be eliminated, which in turn enables vibration positioning along the fiber. To further enhance the positioning accuracy, a new positioning method is proposed in this paper.MethodsIn this study, with the ADMZI distributed vibration sensing system (Fig.1), a high-accuracy positioning method based on the wavelet-based synchrosqueezing transform (WSST) is proposed (Fig.2). First, endpoint detection is performed on the two interference signals, followed by the extraction of large bandwidth sections of the interference signals, whose 20 dB bandwidths are obtained through the power spectrum analysis. Then, the time-frequency analysis of the two interference signals is carried out using the WSST, and the effective phase variation characteristics are extracted adaptively in combination with the 20 dB bandwidths. At the same time, the asymmetry of the sensing system is eliminated. Finally, the time delay between the interference signals is obtained through the cross-correlation algorithm, and the vibration position is demodulated based on the ADMZI sensing principle.Results and DiscussionsTo verify the feasibility of the proposed positioning scheme, a series of field tests are conducted on the sensing system. A total of 100 knocking tests are performed at the end of the armored fiber optic cable over 128 km long sensing fiber, and the positioning results obtained using the CWT and proposed WSST positioning method are compared. The experimental results show that the ADMZI vibration sensing system achieves higher positioning accuracy when utilizing the WSST positioning method. The scheme can precisely locate vibrations over 128 km long sensing fiber with a positioning accuracy of approximately 35.43 m. ConclusionsIn this paper, a high-accuracy positioning method based on the WSST is proposed to enhance the positioning performance of the long-distance ADMZI vibration sensing system. First, the time-frequency analysis of the two interference signals is performed using the WSST in combination with power spectrum analysis to extract the effective phase variation characteristics. The operation solves the inherent asymmetry of the ADMZI sensing system. WSST concentrates the energy of the time-frequency spectrum while eliminating the effects of substrate noise. Compared with CWT, WSST has higher time-frequency resolution and stronger noise immunity, which contributes to improving positioning accuracy. Adaptive characteristic extraction based on the bandwidth of interference signals simultaneously suppresses the disturbance of low-frequency noise. Then, a cross-correlation algorithm is performed to acquire the time delay between the interference signals using the extracted phase variation characteristics, and the vibration position is demodulated based on the ADMZI sensing principle. To verify the effectiveness of the proposed positioning scheme, a series of field experiments are conducted on the ADMZI sensing system. The experimental results show that the proposed scheme can precisely locate vibrations over 128 km long sensing fiber with a positioning accuracy of approximately 35.43 m. This scheme provides a new research direction for improving the positioning accuracy of long-distance distributed fiber vibration sensing technology. Furthermore, polarization effects and environmental changes introduce phase variations in the interference signals, known as phase noise. By investigating real-time phase noise compensation methods, the positioning accuracy of the ADMZI sensing system is expected to be further enhanced.

ObjectiveAmong distributed optical fiber sensing technologies, optical frequency domain reflectometry (OFDR) has garnered widespread attention in aerospace, medical interventional devices, civil engineering, and other fields owing to its high spatial resolution and dynamic range. However, affected by the non-ideal phase tuning of the practical tunable laser light source, the interference signal between the local oscillating light and the scattering light at a specific position in OFDR will expand from a single frequency to a spectral band, thereby resulting in degradation of spatial resolution and demodulation accuracy. While the existing research on non-ideal phase tuning has focused on how much the broadening process can be compensated through software and hardware optimization, there is a lack of analysis on how the non-ideal tuning leads to degradation. The above motivates this paper to analyze the influence of different non-ideal tuning forms on the OFDR demodulation results, to point out the deficiencies of the traditional auxiliary interferometer compensation method, and to preliminarily verify the potential of deep learning in non-ideal tuning compensation.MethodsTaking the possible time-varying characteristics of the tuning rate and the random jitter of the initial phase into account, this work investigates the non-ideal phase forms of the tunable laser source from 30 sets of measured instantaneous frequency-time curves of the tunable laser source. The non-ideality of phase tuning caused by the jitters of the tuning rate/initial phase are analyzed by leveraging polynomial least squares fitting and zero-mean Gaussian distribution, respectively. From this, parameters can be introduced to simulate and semi-quantitatively analyze the impact of the non-ideal tuning on the strain-sensing performance of OFDR. In addition, taking advantage of the convenience of numerical simulation in generating and processing pseudo-random numbers, the convolutional neural network structure based on the Unet encoding and decoding mechanism is further trained to compensate for the non-ideal phase tuning.Results and DiscussionsThe results indicate that the phase tuning non-ideality resulting from the time-varying tuning rate can be approximated by a polynomial fitting curve. It affects localization more significantly in the case of smaller deviations and exhibits a dual effect on both localization and demodulation in the case of larger deviations. On the other hand, the tuning non-ideality caused by random phase jitter mainly impacts the strain demodulation accuracy and has a negligible influence on localization.ConclusionsThis paper semi-quantitatively analyzes the impact on a typical OFDR strain sensing system, considering the time-varying nature of the tuning rate during the phase tuning process of a tunable laser and the random jitter of the initial phase. The results also reveal that the conventional compensation method utilizing auxiliary interferometer interpolation resampling can effectively mitigate the spreading due to the polynomial non-ideal form of tuning. However, it cannot completely eliminate its effect on the localization outcomes, nor can it eliminate the accuracy degradation caused by the random phase jitter of the light source. Based on this, the secondary compensation of the interpolated results through a convolutional neural network model brings the distributed strain measurement results closer to the given ideal values, demonstrating the feasibility of deep learning for the secondary compensation of residual phase non-ideal tuning forms.

ObjectiveCompared of conventional acoustic sensors, vector acoustic sensing technology offers more comprehensive sound field information, enhanced sound source localization capabilities, and superior monitoring accuracy. It plays a crucial role in various applications, including oil and gas exploration, perimeter security monitoring, and geophysical and oceanographic research. While vector acoustic sensing technology is currently experiencing rapid development, it faces challenges such as complex structural designs, the need for individual calibration, and high maintenance requirements, which limit its suitability for large-scale, distributed acoustic wave detection. In this paper, we present a novel approach utilizing scattering-enhanced fibers and elliptically spiral winding fiber optic cables to address the limitations of conventional fiber distributed optic acoustic sensing (DAS) technology, which is incapable of performing vector detection, and realize distributed fiber optic vector acoustic sensing.MethodsThe vector sensing mechanisms of HWC and EWC are analyzed and compared, and their directional sensitivity is also simulated and calculated (Fig.1-Fig.2). The results show that due to the non-central symmetry of EWC, it has omnidirectional sensitivity to the three directions of vector signals. Therefore, EWC is chosen as the vector sensing unit in this article, and the corresponding vector signal decoupling scheme is studied (Fig.3). Firstly, an appropriate spatial coordinate system is constructed using the different sensitivity functions of the EWC vector sensing unit to obtain the EWC sensitivity curve. The time-domain signals detected by the vector sensing unit are separated and normalized to obtain the vector spatial coordinates, which are then used to decouple the vector acoustic angle information using the minimum distance matching algorithm in the appropriate spatial coordinate system.Results and DiscussionsAn experimental setup was built to verify the sensing performance of the proposed elliptically wound cable (Fig.4). Firstly, the response linearity was tested by changing the sound pressure from 0.5 Pa to 1.5 Pa. And the R squares were all above 0.9978 (Fig.5 (a)-(c)). Next, the directional responses were measured with different φ (rotating the tube in 10° intervals). Then the incidence angles were demodulated through the algorithm mentioned in the previous section. Results show that the angular positioning can be realized with a 1° accuracy and ~3.8° RMS error. Furthermore, the acoustic information in the x and y direction were restored with the angular positioning results above. And the errors of the sound pressure are all less than 0.15 Pa (Fig.7). In conclusion, it’s proved that the elliptically wound cable has great feasibility in three-component acoustic sensing.ConclusionsA distributed vector acoustic detection method based on spiral wound optical cables was proposed, which meet the demand for distributed vector acoustic wave detection. By changing the fiber winding angle, multiple sets of acoustic data were obtained. Based on the directional response of different winding angle fiber, directional decoupling was used to achieve vector acoustic detection. The spiral winding structure with triple winding angle fiber was tested and verified, and the results showed that the scheme achieved vector acoustic wave detection in a two-dimensional plane, with a root mean square error of 3.8° in detection direction and a signal amplitude error of less than 0.15 Pa.

ObjectiveIn view of the limitations of the current fiber coupler quality detection technology, that is, the traditional destructive dissection method leads to sample loss, and the lumped optical power test is difficult to locate the spatial distribution of internal defects. An innovative detection method based on the collaborative analysis of thermal excitation and Optical Frequency Domain Reflection (OFDR) strain measurement is proposed in this study to achieve millimeter-level spatial analysis of the deformation characteristics of the inner fuse cone region, the geometric symmetry of the rubber layer and the interfacial stress field of the fiber coupler. The quantitative defect evaluation system based on thermal strain gradient can directly provide data support for process optimization, thereby improving device reliability and reducing failure risk. This method breaks through the technical bottleneck of traditional trial and error process optimization and lays a technical foundation for the long-term stable operation of optical communication and optical sensing systems.MethodsIn this study, OFDR distributed sensing technology combined with a wide temperature domain stepped temperature loading (-35 ℃ to 65 ℃, at intervals of 10 ℃) was used to invert the mechanical behavior of the internal structure of the device. In the experiment, each temperature point was kept warm for 30 minutes to ensure the uniform distribution of internal temperature, and the multi-point temperature sensor was used to monitor the temperature difference between the shell and the rubber layer in real time. Based on the OFDR strain measurement system, the frequency shift of Rayleigh scattering spectrum along the fiber is accurately captured, and the high-resolution strain distribution curve is demodulated by the linear relationship between the frequency shift and strain. Further, the characteristics of the internal melt cone change, the symmetry of the rubber layer and the interface stress concentration are retrieved by the curve characteristics and the trend of change, so as to achieve the internal structure characterization of the device.Results and DiscussionsThis study systematically summarized the key characteristic parameters of the three groups of samples: The length of the cone zone, the maximum peak-valley difference and the asymmetric index, as shown in Tab.1. The experimental results show that the results of the first device reveal the influence mechanism of the thermal strain of the device when the coefficient of thermal expansion of the rubber is mismatched, as shown in Fig.4. Further analysis shows that the asymmetric characteristics of the thermal strain distribution curve (Fig.6) can effectively characterize the process defects of the coating uniformity of the package adhesive layer. The asymmetric index of the third device is as high as 45.7%, revealing the typical defects of the uneven thickness of the adhesive layer. In addition, the study also found that the low temperature environment has a particularly significant impact on the thermal strain of the device. For example, when the temperature is lower than -35 ℃, the strain of the rubber layer at both ends of the melt cone of the third device has an additional double peak rise phenomenon, and the change rate of the peak and valley value also changes sharply, and the strain change rate of the left peak and valley value surges to 4.2 times of that before the change. This phenomenon reveals that too low temperature will not only lead to a sharp decline in the mechanical properties of the adhesive, but also affect the stability of the cone region structure, and then affect the optical properties of the coupler.ConclusionsThrough the analysis of thermal strain distribution in a wide temperature range, the correlation model between the internal structure of the fiber coupler and the thermo-mechanical response is established successfully. Based on the analysis of the characteristics of the thermal strain distribution curve and the change law, the feature identification and defect location of the internal structure of the coupler are realized. The strain results of the three devices at low temperature are analyzed emphatically and the meaning of the peak and valley values in the curve are explained. The internal structural parameters of the coupler, such as the thermal expansion difference of multilayer rubber, the thickness uniformity of the rubber layer and other potential defects, are determined and the corresponding optimization suggestions are given. This method can also be used to characterize the internal structure of passive fiber devices such as polarization-maintaining fiber couplers and wavelength division multiplexers.

ObjectiveHydroacoustic detection of multi-type targets is of great significance in marine safety, maritime rescue, marine ecological protection and fishery development. In recent years, the phase-sensitive optical time domain reflectometer (Φ-OTDR) has received extensive attention in hydroacoustic detection due to its large-scale networking capability, flexible array reconstruction, and uniform wet-end structure. The working principle of Φ-OTDR is based on the interference effect of intra-pulse Rayleigh backscattering (RBS), which can quantitatively demodulate the acoustic information at each fiber position and thus provide a good solution for obtaining the target voiceprint information and orientation. The current research has realized the information detection of a certain type of ships to varying degrees. However, in practical applications, hydroacoustic targets are usually multi-type, such as ships, divers, and organisms. It is still an important challenge to obtain and master the hydroacoustic characteristics of multi-type targets. To this end, this paper detects and analyzes the hydroacoustic signals of multi- type targets in a non-anechoic pool through frequency-diversity Φ-OTDR and suspended sensitized cables. In addition, the Φ-OTDR system uses a phase demodulation method based on orthogonal I/Q or Hilbert transform, and the obtained phase is wrapped in the range of -π to π, so it is necessary to recover the actual phase signal by phase unwrapping. However, due to the value of the maximum detectable phase of Φ-OTDR is π, when there is a phase spike or strong background environment interference, the lack of dynamic range will cause the additional phase jumps and results in further distortion. For this, this paper constructs a time-space two-dimensional Kalman algorithm model to unwrap the phase to achieve high-fidelity phase detection.MethodsBased on the two-dimensional continuity of the time domain and spatial domain of detection data of Φ-OTDR system, the time-space two-dimensional Kalman algorithm for unwrapping phase is constructed. In the non-anechoic pool, the Φ-OTDR system based on frequency diversity is combined with the suspended sensitized cable to detect and analyze the hydroacoustic signals of multi-type targets (Fig.5). The spectral characteristics of multi-type targets such as remote operated vehicle (ROV), bionic fish and divers simulation are analyzed by the proposed algorithm (Fig.6).Results and DiscussionsThe time-space two-dimensional Kalman algorithm is proposed to break through the limitation of phase wrapping characteristics on dynamic range and repetition frequency of Φ-OTDR, and ensure the high-fidelity detection of multi-type targets information. To verify the effectiveness of the algorithm, the experiment of multi-type hydroacoustic targets detection in non-anechoic pool is carried out. The phase unwrapping results of ROV show that the proposed algorithm could improve the dynamic range of Φ-OTDR system and restore the phase information accurately. Furthermore, the spectral characteristics of ROV, bionic fish and divers simulation are analyzed. The results show that the ROV is a continuous signal, the frequency range is 200-1700 Hz, and there is a line spectrum with equal frequency interval (~24 Hz) in the spectrum (Fig.9). The signal interval of the bionic fish is 0.5 s, corresponding to the tail swing period, and the frequency range is 1500-1700 Hz (Fig.10). The signal interval of divers simulation is 0.25 s, corresponding to the bubble ejection period of the oxygen cylinder, and the frequency range is 150-400 Hz (Fig.11).ConclusionsThis work make full use of the two-dimensional continuity of time and space domains of the detection data of Φ-OTDR, and innovatively introduce the time-space two-dimensional Kalman algorithm. The proposed algorithm can effectively break through the limitation of phase wrapping characteristics on dynamic range and repetition frequency. In order to verify the feasibility of this method in the field of actual hydroacoustic sensing, the Φ-OTDR system with frequency diversity is combined with the suspended sensitized cable to detect and analyze the hydroacoustic signals of multi-type targets. The results show that the time-space two-dimensional Kalman algorithm can restore the spectral characteristics of ROV, bionic fish and divers simulation with high fidelity. This work is based on the hydroacoustic characteristics of multi-type targets, which can further promote the development of marine target classification and recognition.

Significance As one of the most successful optical transmission media, optical fibers owe their widespread applications to their exceptional low transmission loss characteristics, which enable long-distance transmission. However, in practical applications, fibers are often exposed to environments with fluctuating temperatures, leading to variations in transmission phase and delay. This thermally-induced instability accumulates with increasing fiber length, potentially causing signal distortion, timing mismatches, and measurement errors in physical quantities. To mitigate the adverse effects of temperature sensitivity on system performance, it is common to either control the environmental temperature fluctuations surrounding the fiber or employ complex transmission schemes and data post-processing techniques to compensate for the optical signal, which often increase system complexity and cost. As a novel type of optical fiber, hollow core fibers have demonstrated unique light-guiding mechanisms and low-loss transmission characteristics over years of research, including reduced thermal sensitivity. With the rapid development and maturation of hollow core fibers over the past two decades, the feature of low thermal sensitivity has been further explored, holding promise for applications aimed at reducing environmental disturbances and enhancing temperature stability in optical transmission systems.Progress First, the physical mechanisms underlying thermal sensitivity in optical fibers are introduced, which are summarized as the thermal expansion effect and the thermo-optic effect. The causes of the thermal expansion effect are essentially the same for both solid core fibers and hollow core fibers. On the other hand, for the thermo-optic effect, it primarily arises from the thermal properties of the silica material in solid core fibers, whereas in hollow core fibers, it originates from the combined contributions of the core gas and the microstructured cladding. Quantitative analysis reveals that the thermal sensitivity of solid core fibers is predominantly governed by the thermo-optic effect, while that of hollow core fibers is mainly determined by the thermal expansion effect. Depending on specific designs, the thermal coefficient of delay and thermal coefficient of phase in current hollow core fibers can be 3 to 20 times smaller than those of solid core fibers.Next, we present recent advancements in reducing the thermal sensitivity of hollow core fibers. Broadly, the methods can be categorized into two types: direct approaches and compensation techniques. Direct approaches aim to reduce thermal sensitivity by directly minimizing the thermal expansion coefficient of the fiber. Specific methods include operating the fiber at extremely low temperatures, winding the hollow core fiber onto spools with near-zero thermal expansion coefficients, and reducing the thickness of the fiber coating. Compensation techniques, on the other hand, utilize a negative-thermal-sensitive thermo-optic effect to counterbalance the positive thermal sensitivity caused by thermal expansion effect. The thermo-optic effects used for compensation include the shift of transmission windows induced by heated microstructures (applicable only to photonic bandgap type of hollow core fibers) and refractive index changes caused by gas flow in fiber core.Finally, this review highlights recent advancements in exploring practical applications of hollow core fibers with low thermal sensitivity. Newly identified application scenarios include highly stable optical interferometers, fiber optic gyroscopes, data center clock synchronization, microwave photonic devices, and time-frequency transfer systems. In these applications, the reduced thermal sensitivity of the fiber directly translates to decreased thermal sensitivity in key performance metrics, offering the potential to enhance the precision of device and reduce temperature control costs.Conclusions and Prospects With the gradual maturation and transition of hollow core fiber technology toward practical applications, significant progress has been made in understanding and addressing its thermal sensitivity. The physical mechanisms underlying thermal sensitivity have been comprehensively elucidated. Research efforts are ongoing to further reduce thermal sensitivity, aiming to achieve complete thermal insensitivity, though current challenges lie in enhancing practicality. Applications leveraging the low thermal sensitivity of hollow core fibers are being actively explored, and their advantages are becoming increasingly evident. With breakthroughs in key metrics such as propagation loss in anti-resonant type of hollow core fibers over the past three years, it is anticipated that the low thermal sensitivity advantages of hollow core fibers will deliver substantial value in future applications.

Significance Surface Plasmon Resonance Imaging (SPRI) is an irreplaceable technology in the field of biosensing, which is based on the principle of Surface Plasmon Resonance (SPR), where a surface plasmon wave is excited at the interface between a metal and a dielectric medium by the strong interaction between photons and free electrons on the metal surface when irradiated by a light of a specific frequency and angle. This surface plasma wave is exceptionally sensitive to small changes in the refractive index near the interface, and it is this property that enables SPRI to monitor biomolecular interactions in real time with a high degree of accuracy. Through SPRI technology, researchers can observe the dynamic changes of biomolecules on the cell surface and reveal the interaction mechanism between cells and biomolecules, so as to deeply understand the microscopic mechanism of life phenomena. Secondly, SPRI technology has the advantages of non-labelling, high sensitivity and high specificity, which can achieve ultra-sensitive detection of trace biomarkers. This is crucial for early diagnosis and treatment of diseases, as early detection of disease markers can significantly improve treatment effects and patient survival rates. In addition, SPRI technology also has high-throughput detection capability, which enables the detection of a large number of samples in a short period of time, which is of great significance in the fields of drug screening and disease monitoring. Therefore, the development of SPRI technology not only promotes the progress of biosensing technology, but also brings revolutionary changes in the fields of life science, medical diagnosis, and environmental monitoring.Progress In recent years, SPRI technology has made remarkable progress in various aspects. In terms of in-depth understanding of the basic principles and application expansion, the research on SPR principle and SPRI principle has become more thorough, so that its application scope in biomolecular detection has been continuously expanded. In terms of technology classification, prism-coupled SPRI has been continuously optimised, its stability and reproducibility have been further improved, and its accuracy in quantitative analysis of biomolecular interactions has been continuously enhanced; non-prism-coupled SPRI has been developing rapidly, and grating-coupled SPRI has made new breakthroughs in the integration of microfluidic chips for detection and the construction of biosensor arrays because of its compactness and flexibility of design, while waveguide-coupled SPRI has made new breakthroughs in the integration of microfluidic chips for detection and the construction of biosensor arrays because of its high integration and flexibility. The high integration and good spatial resolution of SPRI show more advantages in micro-bioanalysis systems and biomolecular imaging and localisation applications. The improvement of imaging technology is remarkable, through the optimisation of optical system, such as the use of high-quality lenses, improving the performance of detectors and reducing light scattering and other measures, the clarity of imaging has been effectively improved; the design and application of nanostructures, such as nanoparticle arrays and nanopore arrays, have successfully broken through the diffraction limit of traditional optical imaging, and greatly improved the spatial resolution. Multimodal imaging fusion has become a new development trend, combining SPRI with fluorescence imaging, Raman imaging, infrared imaging, terahertz imaging, etc., which realizes the simultaneous acquisition and comprehensive analysis of multi-dimensional information of biomolecules and enriches the connotation of detection information. Real-time dynamic imaging has also made great progress, high-speed detectors, fast data acquisition systems, efficient data processing algorithms and microfluidic technology synergies, making it possible to accurately capture biomolecular interactions and cellular activity and other dynamic processes of the instantaneous changes, for in-depth study of the dynamics of biological processes provide a powerful means. In the field of biosensing, the application has been deepened, in cell research, real-time monitoring of cell surface receptor-ligand interactions, cell adhesion and migration processes, as well as intracellular biomolecule changes, which provides comprehensive technical support for cell biology research; in the field of biomarker detection, the detection of biomarkers for various types of diseases is constantly improving the sensitivity and specificity, and the scope of the detection is constantly expanded; in the field of bacterial and In the field of virus detection, it can not only rapidly and accurately detect the presence and number of pathogens, but also deeply study the interaction between pathogens and host cells, opening up a new pathway for the research and development of antibacterial and antiviral drugs.Conclusions and Prospects Surface plasmon resonance imaging, as a revolutionary tool in the field of biosensing, has profoundly impacted many areas of life sciences, medical diagnostics and environmental monitoring with its ability to monitor biomolecular interactions in real time, and its advantages of non-labelling, high sensitivity and high throughput detection. Looking ahead, with the development of technology optimisation, multimodal imaging fusion, application expansion, miniaturisation and integration, as well as intelligence and automation, SPRI technology will further enhance its detection performance, broaden its application scope, provide more accurate and efficient solutions for scientific research and social development, and continue to promote technological progress and innovation in related fields.

Significance Optical fiber is the cornerstone of information technology and the foundation for the development of a future intelligent society. As the core material of optical transmission and photonic devices, optical fiber has been widely used in optical communication, laser, optical sensing and other fields. With the continuous expansion of the application field of optical fiber, higher requirements are put forward for the performance and function of optical fiber, and traditional optical fiber materials cannot meet the increasing needs of people. Under this background, multimaterial fibers came into being. Multimaterial fiber is a new type of fiber material developed in recent years, which aims to integrate different functional materials into a single fiber to achieve the multifunction and high-performance of a single fiber, providing new opportunities for expanding the application of conventional optical fibers in many fields. In particular, multimaterial fibers, which have the functions of photoelectric conversion, transmission and collection, have attracted more and more attention due to their wide applications in advanced optoelectronic devices, optogenetics, smart fabrics, multifunctional sensing and flexible wearable electronics.Progress First, the preparation method, structure, properties and application of multimaterial optoelectronic functional fibers are reviewed, and the common preparation method, design strategy and working principle of multimaterial fibers are introduced. Then, the latest progress of multimaterial fiber in the fields of photoelectric detection, biochemical sensing, intelligent fabric and photogenetic neural regulation is summarized. Finally, the development prospect and existing challenges of multimaterial fibers are prospected and analyzed.Conclusions and Prospects In this paper, the recent progress of multimaterial fibers in multifunctional sensing applications is reviewed, including the design strategies and the fabrication methods, as well as the research progress. With the development and progress of material processing technology, a variety of functional materials such as metals, semiconductors, crystals, polymers, glass, micro devices, and gels can be integrated into a single fiber or fiber array to achieve multifunction and high-performance, which greatly expands the function and application of traditional optical fibers. While great success has been achieved over the past two decades, the future development of multimaterial fibers still faces several challenges: One is that there is still not enough material that can be integrated into the fiber. The most commonly used method for manufacturing multimaterial fibers is still the thermal drawing, which requires all materials to have matching thermal properties, which limits the choice of materials to make multifunctional fibers. Second, the functional integration of multimaterial fiber is still low. In general, most fibers can only sense one physical parameter, while a few fibers can sense two or more physical parameters at the same time. This limitation may depend on the material composition and the structural configuration of the multi-material fibers. Third, the intelligence of multimaterial fiber is still weak. The integration of multimaterial fiber into the next generation of smart textiles can promote the intelligent process of smart textiles. For example, combining smart fabrics composed of multifunctional fibers with artificial intelligence and big data will revolutionize human health monitoring, such as detecting blood pressure, blood sugar, pH, body temperature, heart rate, etc. At the same time, the mechanical information such as bending, torsion, stretching and rotation of multifunctional fibers can be obtained. Based on the above data and intelligent analysis, the health status of the human body can be accurately assessed at any time, which will greatly change the way people live their daily lives.

Significance On account of the unique features including long distance, high spatial resolution, high precision, and anti-electromagnetic interference, optical frequency domain reflectometry (OFDR), having been regarded as one of the major technique in distributed fiber-optic sensing (DFOS), has been widely applied in fields such as optical fiber network, aerospace, traffic facilities, and structural health monitoring. The essence of OFDR lies in the generation of the linearly frequency swept optical frequency-modulated continuous-wave (OFMCW) probe with high coherence, low phase noise, high linearity, and wide sweep range. High performance measurement has demanded the improvement in the performance of OFMCW probe, including the high dynamic coherence, highly linearized frequency sweep, and a broad sweep range. To date, either direct modulation or external modulation based generation techniques suffer from the trade-off between large sweep range and low phase noise, namely, high coherence. Therefore, in order to improve the performance, there is an urgent need to achieve simultaneously the extension of sweep range and enhancement of coherence.Progress This paper aims at the introduction of the current progress in the technologies for the generation of linearly frequency swept OFMCW probe.First, the operational principle for the DFOS based on linearly frequency swept OFMCW has been introduced. Based on the principle, theoretical analysis have been conducted on the factors affecting measurement performance, including the sweep range, phase noise, and sweep nonlinearity. In general, the sweep range directly determines the Fourier transform-limited spatial resolution (Eq.7). Thus, improving spatial resolution hinges on expanding the sweep range. In addition, the measurement distance is largely dictated by the laser phase noise (Fig.3), making it essential to suppress the phase noise and thus to achieve the enhancement of the laser coherence. This is especially critical for long-distance measurement and sensing. Besides, sweep nonlinearity usually leads to the dispersion for the power of the central peak of the beat signal (Fig.4), which affects both the spatial resolution and the sensing accuracy. Hence, in order to realize a high precision, long distance measurements and sensing, it is urgently required for the suppression and compensation of the sweep nonlinearity.Second, this paper systematically reviews the methods for improving the sensing performance, mainly including two aspects: sweep range expansion and phase noise suppression. High-order sidebands sweeping is a common approach to expand the sweep range. Nevertheless, the spectrum aliasing issue of the adjacent sidebands limits further range expansion. To address this limitation, optical injection locking and optical phase-locked techniques have been proposed. Stitching multiple sections of sweeps is a promising strategy, including multi-laser stitching, recirculating frequency-shifting stitching, and multi-comb modes stitching. However, the effective stitching of multi-segment sweeps and the errors introduced by sweeps gaps remain challenging and this requires further investigation. Finally, Fourier-domain mode-locking (FDML) is an emerging technique that has been adopted to implement fast and wide frequency sweeping, but it suffers from poor coherence.Phase and frequency noise suppression is a crucial technique for enhancing coherence and improving the performance of self-coherent detection, including methods such as pre-distortion and optical phase-locked loop (OPLL). Predistortion is typically implemented via different algorithms, which are usually considered compact, straightforward and effective. However, it is limited to correcting fixed nonlinearities. In contrast, OPLL is a dynamic feedback correction technique that facilitates highly coherent and linear frequency sweeps by locking to a stable reference, such as a laser with a narrower linewidth or a stable interferometer based on optical cavity or fiber reference. In this context, the laser phase error extraction based on the unbalanced Mach-Zehnder interferometer (UMZI) has shown prominent advantages in the generation of OFMCW waveforms. Regarding on this technique, several OFMCW generation methods based on delayed self-interferometry OPLL structures have been proposed and employed in OFDR measurements.Conclusions and Prospects In conclusion, this paper summarized the current progress in the generation of OFMCW probe. For future prospect, OFMCW based technique increasingly demonstrates its advantages in the measurement and sensing and it is poised to make a significant contribution in geophysics, scientific research, and other emerging fields.

Significance In modern engineering research and applications, aerodynamic and hydrodynamic testing serves as a critical method for studying fluid mechanics. These tests aim to analyze the forces acting on moving objects in gas or liquid flows and their interactions with the flow field, and providing essential data support for engineering design. Among these, aerodynamic/hydrodynamic force measurement and structural monitoring are key components of aerodynamic and hydrodynamic testing. They are used to obtain hydrodynamic characteristics, predict object performance under various fluid conditions, and ensure structural stability and safety under different loads. As research in aerodynamics and hydrodynamics continues to advance, the techniques for aerodynamic/hydrodynamic force measurement and structural monitoring face increasingly stringent requirements. Strain sensors, as core devices in aerodynamic testing and structural monitoring, directly influence the accuracy and reliability of test data. However, traditional strain sensors often encounter challenges in harsh environments, such as strong electromagnetic interference and high temperatures, resulting in inaccurate measurements. In contrast, optical fiber strain sensors offer significant advantages in aerodynamic/hydrodynamic force measuring and structural monitoring due to their unique sensing capabilities. Therefore, in-depth research on optical fiber strain sensors is not only crucial for advancing testing technology but also provides substantial support for engineering practices in related fields. Progress Firstly, the working principles and common types of optical fiber strain sensors are introduced, including optical fiber Bragg grating (FBG) strain sensors and optical fiber Fabry-Perot (FP) strain sensors. FBG sensors detect strain by measuring changes in the wavelength of light reflected by the grating, have high sensitivity and multiplexing capabilities. FP sensors utilize multi-beam interference principles to achieve high-precision strain measurements. These sensors are particularly suitable for complex fluid environments due to their miniaturization and resistance to interference. Next, based on FBG and FP strain sensors, their applications in force balance system and structure strain monitoring are discussed from two perspectives of aerodynamic/hydrodynamic force measurement and structure monitoring. Leveraging mature optical fiber strain sensing technology, optical fiber force balance can accurately measure the magnitude, direction, and application points of aerodynamic loads (including forces and moments) under complex conditions. At present, many research institutions at home and abroad mainly focus on FBG strain sensor and FP strain sensor two directions, among which the FP strain sensor research is more extensive and in-depth. FP sensors utilize minute optical interference effects within the fiber to sense strain changes, offering advantages such as high sensitivity, precision, and miniaturization, making them highly promising for force balance applications. Extensive research has shown that while optical fiber FP balances performs well in aerodynamic force measurement under high-speed and hypersonic harsh environments, their broadband measurement capabilities and high-precision measurement ranges still have room for improvement to better meet the demands of complex, ultra-fine flow field testing for hypersonic aircraft. In structure monitoring, optical fiber strain sensors are core equipment, due to their potential to provide high-density sensor coverage with minimal weight impact, they are widely regarded by the scientific community and industry as one of the most promising solutions for continuous, real-time structural monitoring in aerodynamic and hydrodynamic testing. Over the years, these sensors have been extensively applied in structural health monitoring by domestic and foreign research institutions and companies, particularly in studies involving FBG strain sensors. FBG strain sensors achieve high-precision strain measurements by detecting Bragg wavelength shifts caused by changes in the grating period and effective refractive index due to external strain. Numerous studies have demonstrated that FBG strain sensors, leveraging their high sensitivity and distributed measurement capabilities, can enable comprehensive, long-term monitoring of stress and deformation in complex structures. However, it is seriously disturbed by temperature and has a large temperature cross sensitivity coefficient.Conclusions and Prospects As an important measurement sensor, optical fiber sensors have been widely and deeply applied in the field of aerodynamic and hydrodynamic testing in recent years, achieving significant progress. Compared to traditional sensing technologies, optical fiber sensors demonstrate unique advantages in aerodynamic force measurement and structural strain monitoring. Their high sensitivity and quasi-distributed measurement capabilities enable comprehensive strain monitoring in complex structures, these difficult to achieve with traditional sensors. In extreme application environments requiring high accuracy, sensitivity, and stability, the advantages of optical fiber sensors are even more pronounced. This study aims to provide valuable insights for the future design and optimization of optical fiber sensors used in aerodynamic and hydrodynamic testing. As optical fiber sensors continue to evolve in terms of sensitivity, stability, and anti-interference capabilities, they will further drive advancements in aerodynamic and hydrodynamic testing technologies.

Significance Femtosecond laser 3D printing has emerged as a revolutionary technology in micro-nano manufacturing, enabling the fabrication of complex 3D structures with sub-diffraction-limit resolution. Inorganic materials, known for their excellent mechanical properties, thermal stability, and chemical resistance, are widely used in optical devices, MEMS sensors, and biomedical applications. However, traditional manufacturing methods for inorganic micro-nano structures face limitations, such as complex processing, low precision, and high costs, which restrict their potential in high-performance device fabrication. The advent of femtosecond laser 3D printing provides a promising solution to overcome these challenges, offering exceptional spatial resolution and material versatility. This survey systematically reviews recent progress in the techniques and applications of femtosecond laser 3D printing for inorganic materials, focusing on organic-inorganic hybrid and pure inorganic systems. The study also explores the challenges and future directions, emphasizing the potential of this technology in advancing micro-nano fabrication.Progress Femtosecond laser 3D printing leverages nonlinear optical effects of two-photon absorption to achieve high-resolution fabrication of complex three-dimensional structures, making it a versatile tool for inorganic material processing. This technology has been employed in two primary approaches: organic-inorganic hybrid systems and pure inorganic material systems. Organic-inorganic hybrid systems combine the flexibility of polymer matrices with the superior properties of inorganic components by incorporating precursors or nanoparticles into light-sensitive resins. The process involves two-photon polymerization for precise structuring, followed by high-temperature sintering to enhance material performance, although challenges such as achieving uniformity and minimizing shrinkage remain. Pure inorganic material systems, on the other hand, bypass organic components entirely by using direct laser-induced reactions, such as photochemical reduction or decomposition, to fabricate high-purity structures with enhanced thermal and chemical stability. These techniques have facilitated advancements in various applications, including the production of optical microdevices like microlenses and photonic crystals, MEMS sensors with integrated 3D microstructures for force and environmental sensing, and life sciences applications such as biocompatible scaffolds and micro-robots for minimally invasive procedures. The ongoing development of femtosecond laser 3D printing is characterized by increasing standardization, compatibility with diverse materials and processes, scalability for industrial applications, and modularity in manufacturing, paving the way for significant technological breakthroughs.Conclusions and Prospects Femtosecond laser 3D printing represents a transformative advance in the fabrication of inorganic micro-nano structures. Despite its potential, several challenges remain, including low processing efficiency, material restrictions, and issues with thermal stability and shrinkage during post-processing. Future developments should focus on enhancing processing throughput through techniques such as parallel laser writing and digital light modulation. Additionally, innovations in material chemistry, including the development of low-shrinkage precursors and advanced hybrid systems, are essential for broader applicability. The integration of artificial intelligence and machine learning for process optimization holds promise for automated, intelligent manufacturing. These advancements are expected to unlock new applications in photonics, MEMS, and biomedical engineering, driving the evolution of femtosecond laser 3D printing into a cornerstone technology for next-generation micro-nano manufacturing.

Significance Phase-sensitive optical time-domain reflectometry (Ф-OTDR) represents a significant branch of distributed fiber sensing technology. This technique facilitates the quantitative analysis of vibration and acoustic signals along the sensing fiber by real-time demodulation of the phase evolution of backscattered Rayleigh light. It is distinguished by its rapid response and high detection sensitivity, making it particularly suitable for applications such as perimeter security, oil and gas pipeline leak detection, and marine monitoring. However, owing to the utilization of a narrow line-width laser light source in the Ф-OTDR system, the Rayleigh scattered light signal generated within the sensing fiber exhibits high coherence. This results in random fluctuations of the Rayleigh scattered light signal along the sensing fiber due to interference between scattered light, thereby creating sensing dead zones at positions with low signal-to-noise ratios. Additionally, polarization mismatch between the Rayleigh backscattered light signal and the local reference light further contributes to these sensing dead zones, leading to phase demodulation distortion. In recent years, suppressing the fading effect in Ф-OTDR systems has emerged as a research hotspot, with significant academic and practical importance.Progress The Ф-OTDR system includes two main types of structures: self-coherent detection and intrinsic heterodyne detection. Common demodulation methods include IQ phase demodulation, Hilbert transform demodulation, PGC phase demodulation, and 3 × 3 coupling demodulation. The existing polarization fading suppression schemes for the Ф-OTDR system primarily encompass three approaches: Ф-OTDR system mainly consist of three approaches: Full polarization maintenance, probe pulse polarization control, and polarization diversity. The full polarization maintenance approach entails significant structural costs and demonstrates low cost-effectiveness in long-distance engineering monitoring applications. The probe pulse polarization control method requires complex modulation of optical signals within the Ф-OTDR system. In contrast, the polarization diversity scheme only necessitates implementing diversity at the receiving-end optical path, effectively mitigating partial fading while offering a simpler structure and lower costs. Therefore, it is currently the most prevalent solution for Polarization fading suppression.The fundamental principle of interference fading suppression involves generating and synthesizing multiple independent signals with low correlation in order to accurately demodulate high-fidelity phase information. The primary approaches encompass wavelength, frequency, or phase diversity, specialized sensing units, and algorithmic enhancements. The frequency, phase, or wavelength diversity scheme realizes interference fading suppression based on the different Rayleigh scattering distribution characteristics of probe pulses with different frequencies, phases or wavelengths, it is a pioneering and most widely adopted interference fading suppression schemes. The special sensor unit scheme uses multi-mode fiber, few-mode fiber, multi-core fiber and scattering enhanced fiber as the sensor units of the Ф-OTDR system to obtain multiple independent signals with low correlation or a stable Rayleigh scattering distribution, which is one of the effective means to realize interference fading suppression without necessitating complex optical path. The algorithmic enhancement scheme does not require complex probe pulse modulation and special sensing units, and only performs signal processing in the digital domain to achieve high-fidelity phase information demodulation. The classical algorithms include spectrum extraction and remixing, phase shift transformation, rotation vector sum method, etc.Conclusions and Prospects Researchers have proposed and validated numerous schemes to suppress polarization and interference fading effects, which significantly mitigate phase demodulation errors caused by fading in the Ф-OTDR system. This has facilitated the application of Ф-OTDR technology in industrial monitoring. With the continuous expansion of the application field of Ф-OTDR technology, end-users are imposing increasingly stringent requirements on this technology. In the future, real-time demodulation of vibration or acoustic information along the sensing fiber will be of critical importance. This is because the machine-OTDR system generates a substantial volume of data during high-frequency signal monitoring or long-distance monitoring. Furthermore, the distributed optical fiber sensing system, which supports multi-parameter hybrid measurement, is anticipated to offer significant advantages in the realm of industrial monitoring in the future.

SignificanceOptical fiber technology has changed the world by enabling global communication through high-speed information transmission and interconnectivity. The associated optical fiber sensing technology has also been rapidly developed. In recent years, with the great progress of optical fiber sensing technology, it has been widely used in many scenarios with its excellent performance, unique flexibility and rich functional characteristics. On the one hand, it is worth paying special attention to that the research of special optical fibers and their devices plays a crucial role in the realization and expansion of optical fiber sensing applications. On the other hand, the expansion of new application areas will, in turn, put forward some new demands, which is an opportunity and a challenge for optical fiber technology. In fact, the unique physical and optical transmission properties of optical fibers make them ideal components for biomedical sensors, and various types of optical fibers are widely used in biosensing instruments used in clinical research and application processes related to life sciences. However, each fiber optic structure has certain advantages and limitations for specific applications in biomedicine. This has become the source of continuous advancement of special fiber technology to meet new needs. Therefore, this paper takes the application and demand of optical fiber technology in the biomedical field as an example, analyzes the development trend of this field, focuses on several typical functional requirements of clinical treatment such as optogenetics, biosensing, drug delivery and neural recording, and gives technical approaches on how to solve new needs in different fields through DIY technology and methods of optical fiber and devices. This paper provides a flexible fiber optic technology DIY solution for how to respond to new requirements in various application fields.ProgressAfter nearly 50 years of academic research and technical development, China's optical fiber sensing technology has formed an exponential accelerated development trend in recent years, for the following reasons: 1) Because optical fiber sensing technology has been widely used in several practical scenarios, especially the increasingly mature distributed optical fiber sensing technology has formed a new industry branch; 2) The development and application of micro-nano technology and material technology have also provided many new methods for optical fiber sensing technology; 3) The rapid development of China's economy not only provides a broad market for the practical application of optical fiber sensing technology, but also boosts the prosperity and progress of basic research in this field. Under the above development background, optical fiber sensing technology is also constantly expanding to other fields, especially in the biomedical field, which is reflected in the increasing expansion of optical fiber wearable health monitoring, interventional in vivo spectral detection, lensless fiber microscopic imaging, optical fiber in optogenetic neurology and other fields. The urgent problems to be solved are: On the one hand, with the expansion of the application field, various new requirements are constantly emerging; On the other hand, the original fiber structure and function are difficult to meet the needs of the growing development. It is well known that accurate, rapid and non-invasive detection and diagnosis of malignant diseases in tissues is an important goal of biomedical research. Interventional optical fiber technology provides infinite possibilities for minimally invasive in situ diagnosis and treatment. With the deepening of the application research of optical fiber technology in the field of biomedicine, the special demand for optical fiber is increasing day by day, from structure to function: in addition to the optical channel that senses and transmits optical signals, it is also necessary to increase the micropore function that can realize the microfluid input and body fluid acquisition. Further, it is also hoped to add electrode functions that can achieve electrical signal acquisition or electrical pulse stimulation. For application purposes, it is also expected that all or part of the above functions will be integrated into a single fiber. Because combining optical fibers with different functional devices, such as microfluidics with optical fiber devices, the characteristics of microfluidics can be integrated, which provides the possibility of sharing light-based sensors and liquid transport systems. However, such a multi-functional fiber also needs to match the corresponding supporting fan-in fan-out devices to achieve their respective independent functions, which has become a new challenge and difficulty in the use of multi-functional fiber. From the perspective of the needs of the biomedical field, the required optical fibers and devices are very different from the traditional optical fiber technology, requiring a variety of new functions to be integrated in the optical fiber, in addition, it is also necessary to solve the connection with the multi-functional integrated fan-in devices (all-fan-in) at the various applications. For this purpose, this paper reports our miniature DIY optical fiber manufacturing system and a series of DIY processing and manufacturing desktop equipment for the preparation of optical fiber devices, and explains its working principle and manufacturing method. These devices are especially suitable for the design and experiment of new optical fibers and devices in the laboratory, which not only opens the space of free imagination for researchers' innovation and creation. Moreover, it provides greater flexibility and convenience of preparation. It provides more possibilities for the realization of various inspirations, and realizes the freedom from ideological innovation to practical creation in the field of optical fiber technology and its application.Conclusions and ProspectsFiber optic technology has been a key technology in shaping the high-speed transmission of information around the world and keeping everywhere interconnected, with enhanced bandwidth, high data transmission speeds and extremely high fidelity. Developments in fiber optic technology have also been crucial to the proliferation of sensing applications. Optical fiber sensor has the advantages of anti-electromagnetic interference, light weight, small size, high sensitivity, large bandwidth, reliable performance, robust, resistant to harsh environment, easy to implement multiplexing or distributed sensors. So far, optical fiber sensors have been widely used in civil engineering, environmental monitoring, agricultural engineering, biomedical engineering and other fields. It is worth noting that the invention and development of specialty fibers have driven a proliferation of many sensing innovations and applications, including fibers with special structures, special materials, and applications that integrate devices with other technologies. In the field of optical fiber technology and its application, the incentive to achieve cross-innovation has three dimensions, dimension 1: application and demand are traction and driving force, that is, demand is the source of innovation; Dimension 2: The function and structure of optical fiber is a concomitant innovation to meet the needs. It is designed through the optimization of function and structure and realized through the preparation of optical fiber. Dimension 3: The optical fiber device is a part of the function that needs to be completed in order to meet the overall function of the optical fiber system, which can be obtained by secondary processing and preparation of the relevant optical fiber. Therefore, in this multi-dimensional space system, manufacturing capacity is the basis and guarantee for the realization of the above innovation.

Significance Distributed Fiber-optic Seismic Sensing (DFSS) represents a cutting-edge seismic observation technology characterized by high spatial resolution, real-time monitoring capabilities, and reduced operational costs. These attributes grant it unparalleled advantages in traditionally challenging environments such as urban areas, boreholes, glaciers, and oceans. By enabling high-density data acquisition and high-resolution shallow structure imaging, DFSS has revolutionized seismic monitoring. Its ability to contribute to fault detection, site-effect analysis, and seismic risk evaluation showcases its transformative potential in disaster prevention, resource exploration, and environmental monitoring. DFSS's utility extends to detecting seismic events, facilitating ground motion estimation, and yielding fine-scale subsurface structural details.ProgressIn seismological research, natural earthquakes and seismic ambient noise are collectively referred to as passive sources. Passive imaging is a method that utilizes continuous seismic signals from natural sources to obtain structural information about the subsurface medium. Ambient noise imaging, focusing on low-amplitude, less distinct fluctuations, is crucial in geophysics. While seismic signals originate from natural events and human activities, ambient noise, categorized by frequency, exhibits smaller amplitudes. Low-frequency noise (<1 Hz) stems from natural sources like ocean waves, while high-frequency noise (>1 Hz) arises from human activities. Since the 1 950 s, researchers have extracted valuable insights from ambient noise, with advancements enabling Green's function calculations for Earth imaging. Compared to traditional seismic methods, ambient noise imaging reduces uncertainties in source characteristics and provides high-resolution structural images, making it a critical tool in seismic exploration and monitoring. DFSS-based ambient noise imaging began in 2017, with early studies validating its feasibility. ZENG et al. extracted high-frequency Rayleigh wave signals using background noise, confirming DFSS’s potential for seismic imaging. Subsequent studies, like DOU et al. 's traffic noise analysis, revealed subsurface variations such as moisture and permafrost changes. Significant advances were achieved using urban fiber optic cables, as demonstrated by AJO-FRANKLIN's high-resolution imaging of shallow velocity structures. Innovations such as frequency-Bessel function imaging and multi-modal dispersion extraction improved DFSS accuracy and applicability, especially in urban and geologically complex areas. DFSS has broad applications in disaster prevention, resource exploration, and environmental monitoring. It enhances fault imaging and site-effect studies, aids seismic hazard assessment, and improves geothermal and oil resource exploration with cost-effective high-resolution imaging. DFSS excels in extreme environments, monitoring glaciers, oceanic sediments, and fault zones. By leveraging submarine cables for seismic and oceanographic studies, DFSS has become indispensable in geosciences, offering high spatial resolution and cost efficiency, with promising future applications in disaster mitigation and environmental research.Conclusions and Prospects DFSS is rapidly emerging as a critical tool for seismic imaging and monitoring. Future developments in data quality enhancement, algorithm optimization, and integration with complementary geophysical methods are expected to address existing challenges and expand its applications. The prospects for DFSS include multi-parameter imaging, dynamic monitoring, and planetary science applications. By enabling detailed seismic risk assessments and precise subsurface imaging, DFSS holds immense potential for contributing to global disaster mitigation, resource management, and environmental studies. It is anticipated to become indispensable in high-resolution seismic research and its interdisciplinary extensions.

Significance Hydrogen energy, with its high energy density and renewability, has emerged as a preferred solution to address environmental challenges and optimize energy structures. However, safety concerns related to hydrogen leakage and abnormal concentration fluctuations remain critical barriers to its widespread adoption. Consequently, the development of reliable technologies for real-time hydrogen monitoring is of paramount importance. Among various detection methods, optical fiber-based hydrogen sensing technology has garnered significant attention due to its exceptional advantages, including intrinsic safety, immunity to electromagnetic interference, compact size, and suitability for remote and distributed monitoring. This paper provides a comprehensive review of recent advancements in optical fiber hydrogen sensing technologies, with a focus on elucidating the interaction mechanisms between hydrogen-sensitive materials and hydrogen molecules, as well as analyzing the structural characteristics and performance metrics of diverse sensor configurations.Progress Significant progress has been made in the design and optimization of hydrogen-sensitive materials such as palladium (Pd) and its alloys, tungsten oxide (WO3), and graphene-based composites. These materials enable hydrogen detection through mechanisms such as refractive index changes due to chemisorption, plasma effects, and others. Fiber optic sensor structures, such as fiber Bragg gratings (FBGs), long-period gratings (LPGs), and Fabry-Perot interferometers, have been extensively explored. For example, palladium-coated fiber Bragg grating sensors exhibit fast response times (less than 30 seconds) and high sensitivity (33 pm/vol%), while fiber grating hydrogen sensors integrating tungsten oxide amplify the hydrogen response sensitivity by a factor of 16.11 using the vernier effect. Recent innovations, such as structural functionalized sensors using palladium nano-arrays and hybrid plasma-photonic sensors, have further increased the detection limit to sub-ppm levels. Despite these advancements, challenges persist in balancing sensitivity, response speed, long-term stability, and environmental adaptability. Material degradation under cyclic hydrogen exposure and cross-sensitivity to temperature fluctuations remain critical issues. Future research directions include the development of novel nanocomposite coatings, advanced signal demodulation algorithms, and multi-parameter sensing platforms. Additionally, the integration of artificial intelligence for real-time data processing and the implementation of sensor networks for large-scale infrastructure monitoring represent promising avenues.Conclusion and Prospects Optical fiber hydrogen sensors exhibit immense potential for ensuring safety in hydrogen-related applications, particularly in scenarios requiring explosion-proof operation, remote monitoring, and distributed sensing across complex systems. As key technical hurdles—such as material durability and system miniaturization—are progressively addressed, these sensors are poised to play a pivotal role in advancing the commercialization and large-scale deployment of hydrogen energy. Their evolution will not only enhance safety protocols but also accelerate the global transition toward sustainable energy ecosystems. This review underscores the transformative impact of optical fiber sensing technologies in realizing a hydrogen-powered future.

Significance Oil and natural gas are important chemical raw materials and strategic resources in industrial society. With the increasing demand for oil and natural gas, it is urgent to increase the efforts of exploration and development. As a new type of geophone, optical fiber vector geophone has many advantages, such as high sensitivity, easy networking and multiplexing, high temperature resistance, vector measurement. It can obtain rich seismic wave signals, and have a wide range of application values in finding remaining oil, improving the success rate of exploration, monitoring reservoir performance in real time, helping to optimize production plans and improving oil and gas recovery. In the future, fiber vector geophone will not only play an important role in the field of resource exploration, but also have broad application prospects in engineering geology, environmental protection and scientific research.Progress Firstly, the model and working principle of inertial vibration sensor are analyzed. According to the relationship between the frequency of the measured vibration signal and the natural frequency of the sensor, the vibration sensor is divided into displacement meter, speedometer and accelerometer. The amplitude-frequency response curves of these three kinds of vibration sensors are given respectively, and their respective working frequency bands are analyzed. Based on the optical principle, optical fiber geophones are subdivided into distributed optical fiber geophones, laser optical fiber geophones, interferometric optical fiber geophones and fiber grating geophones. The working principles of the four types of geophones are introduced respectively, and the research progress of the above four types of optical fiber geophones is reviewed. Distributed optical fiber geophone (DAS) has been applied to some extent because it is easy to use and there is almost no potential safety hazard. The research of DAS vector detection has appeared, but the transmission loss will reduce the number of detectors and shorten the detection distance, which affects the high-density advantage of DAS using the optical fiber itself as the detector. Laser-type and interferometric fiber-optic geophones have obvious sensitivity advantages, but they are greatly affected by environmental temperature, so it is difficult to multiplex and demodulate in real time.Relatively speaking, FBG geophone has a variety of ways to realize vector detection, and has become the mainstream optical fiber solution for seismic wave vector detection.The measuring principle of fiber grating vector geophone is introduced highlighted and the research progress in recent years is reviewed. According to the mechanism of vibration measurement by fiber grating, the optical grating vector geophone is subdivided into simple orthogonal combination FBG vector geophone, cleverly designed common mass block FBG vector geophone, smaller two-dimensional special fiber FBG vector geophone, orthogonal polarization FBG vector geophone and cladding FBG vector geophone, and the field application of some FBG vector geophones is introduced. Multi-core special fiber FBG and cladding fiber FBG have excellent recognition ability of two-dimensional vector acceleration. In the future, with the reduction of the cost of related optical devices and the appearance of specially designed thin cladding fiber and large spacing multi-core fiber, the practical process of FBG vector detector will be further promoted. Of course, the sensitivity of FBG vector detector needs to be improved, and the mechanical structure of special fiber, orthogonal polarization and cladding FBG needs to be improved to improve its natural frequency. Multi-level multiplexing and demodulation methods of geophone need to be further studied. The cladding FBG demodulated by intensity also needs to eliminate the influence of power fluctuation.In addition, when it is used in oil and gas wells, it also needs the support of special pushing equipment and the photoelectric composite cable.Conclusions and Prospects There are many researches on fiber-optic geophones, but with the higher requirements for seismic wave detection, it is inevitable to develop fiber-optic vector geophones that can detect both transverse waves and longitudinal waves. This paper aims to provide some references for the design and optimization of fiber-optic vector geophones in the future through the introduction and analysis of the current situation of fiber-optic vector seismic wave detection, hoping to promote its practical application.

Significance Distributed Optical Fiber Sensing (DOFS) plays a crucial role in acquiring spatially distributed environmental data along a sensing fiber, offering superior advantages such as long-range, high-precision, and large-area monitoring. Among various DOFS technologies, Brillouin-based sensing methods—especially Brillouin Optical Time Domain Analyzer (BOTDA) and Brillouin Optical Time Domain Reflectometer (BOTDR)—have become the focus of both academic and industrial research due to their ability to measure temperature and strain along optical fibers with high accuracy. However, the performance of these systems is significantly influenced by their signal-to-noise ratio (SNR), which in turn affects the measurement accuracy, spatial resolution, and dynamic range. Optimizing SNR is therefore critical to improving the overall performance of these systems for practical deployment in fields like civil engineering, energy, aerospace, and infrastructure monitoring.Progress Recent advances in the SNR modeling and system performance optimization of BOTDA and BOTDR systems are reviewed and synthesized. One of the key advancements in recent research is the development of more accurate SNR models that account for various noise sources. These models help quantify the influence of different noise sources and provide guidelines for optimizing system parameters.What’s more, key parameters that affect the system SNR performance, such as pump pulse peak power and probe power in BOTDA systems, are limited by nonlinear effects and difficult to further enhance. In BOTDA systems, the pump pulse peak power typically needs to be adjusted close to the modulation instability (MI) threshold to maximize SNR. Meanwhile, the probe power is mainly constrained by non-local effects and Stimulated Brillouin Scattering (SBS). Extensive researches have focused on this aspect, providing in-depth analysis and proposing advanced solutions to overcome traditional power limitations. Based on SNR modeling, researchers have also developed parameter optimization schemes for BOTDR systems, suggesting that pump pulse peak power and local oscillator (OLO) power need to be reasonably adjusted according to different sensing scenarios to balance optimal performance with lower energy consumption. The careful tuning of these parameters is essential for achieving the best performance in different sensing scenarios, whether it’s for long-distance monitoring or high-precision measurements. Researchers have also proposed optimization criteria for detection and sampling, indicating that the detector bandwidth must be greater than the signal bandwidth to ensure the signal is not distorted, and the sampling rate must be greater than twice the noise bandwidth to avoid noise aliasing. Based on this, applying digital post-processing filtering can achieve optimal denoising effects.In addition to system-level optimization, much progress has been made in post-processing techniques to further enhance the SNR. Digital filtering, signal averaging, and advanced fitting algorithms are now commonly employed to reduce noise and improve the accuracy of the extracted Brillouin frequency shift (BFS). Several studies have demonstrated that high-order signal processing methods, such as wavelet transform and deep learning-based algorithms, can offer additional performance improvements. However, research also suggests that such advanced signal processing methods like image processing and deep learning may not provide significant performance improvements once the system bandwidth and fitting methods have been optimized.Conclusions and Prospects In conclusion, this paper provides a comprehensive overview of the research progress in optimizing the SNR of BOTDA and BOTDR systems, emphasizing the importance of both system-level and post-processing optimizations. By carefully adjusting system parameters, including pump pulse peak power, probe power, and local oscillator power, and employing advanced signal processing techniques, it is possible to approach the performance limits of these classical Brillouin sensing systems. However, while advanced technologies such as distributed amplification and pulse coding show great promise, their complexity and high cost remain barriers to widespread commercialization. Moving forward, further research into low-cost, miniaturized systems that maintain high performance will be critical for the industrialization of Brillouin sensing technology. Additionally, the exploration of higher-order sensing schemes, such as those incorporating advanced amplification and coding techniques, will continue to be a vital research direction, offering new opportunities to push the limits of distributed Brillouin sensing.

Significance With intensive research for more than three decades, distributed Brillouin fiber sensing has found numerous applications in the past few decades. It relies on the linear relationship between Brillouin frequency shift (BFS) and physical quantities applied to sensing fibers. Since the BFS is an inherent frequency property of the sensing fiber that is only related to fiber material properties as well as temperature and strain applied to the sensing fiber, the Brillouin fiber sensors can provide ultra-high sensing accuracies and stabilities. The Brillouin phase-gain ratio method uses the linear relationship between BFS and Brillouin phase-gain ratio to map BFS directly, which is initially proposed to reduce the frequency scanning steps as well as raw data size. With a deep study of this method, it was found that it can solve many issues relevant for conventional frequency-scanning- and curve-fitting-based BFS estimation approaches, such as non-local effect, pump power and frequency fluctuations. More importantly, it truly combines Brillouin fiber sensing with image denoising and thus enhances the sensing speed considerably. These advances make the Brillouin phase-gain ratio method a suitable choice for the monitoring of both quasi-static and dynamic temperature variations and deformations of large infrastructures, such as bridges, railway tracks, freeways, and high voltage lines.Progress First, the operation principle of the Brillouin phase-gain ratio method is introduced, which shows that both Brillouin gain and phase-shift information are needed for its implementation. Since the direct detection throws away all the phase information, the coherent detection needed to be performed as it can provide both Brillouin gain and phase information simultaneously. The advances of coherent detection schemes are then introduced, including the elimination of the impacts of the fiber group velocity dispersion, fiber group delay fluctuations, the phase noises of RF driving sources, and relative intensity noises transferred from the Raman pump sources during distributed Raman amplification. These advances in Brillouin gain and phase extraction clearly reveal that ultra-precise Brillouin gains and phases can be obtained by differentiating two signals with different Brillouin gains and phases but the same noises. Such a differentiation process can be realized by using coherent detection schemes based on either digital or analog in-phase/quadrature (I/Q) demodulation. The advantages of the Brillouin phase-gain ratio method are further introduced. Benefiting from the division operation of this method, it avoids the impacts of pump depletion, pump power fluctuation, and pump frequency jitter on the estimation of the BFS, and thus considerably enhances the sensing reliability. Besides, the slope of the Brillouin phase-gain ratio spectrum has a much wider linear region compared to the Brillouin gain and phase spectra. A much wider temperature and strain sensing range for a single pump-probe frequency scanning is then enabled via the slope-assisted analysis of the Brillouin phase-gain ratio. A strain sensing range of more than 5000 microstrains can be reached by changing the pump-probe frequency difference rapidly via the frequency-agile technique. Furthermore, various advanced image denoising methods have been used recently for performance enhancements in Brillouin fiber sensors. However, with a deeper understanding of the image denoising methods, it was recently found that the newly-added image denoising action is redundant with the conventional signal processing that is composed of spatial-domain low-pass filtering and spectral-domain spectrum fitting. The slope-assisted analysis of Brillouin phase-gain ratio does not need spectral-domain spectrum fitting for BFS estimation and thus truly links Brillouin fiber sensing with image denoising. This enables a notable sensing speed acceleration of more than 20 times.Conclusions and Prospects Owning to the precise raw differential Brillouin gain and phase information provided by current advanced coherent detection schemes, the notable signal-to-noise enhancements gained from digital signal processing such as image denoising, as well as the advantages of the Brillouin phase-gain ratio method itself, ultra-precise, -reliable, and -fast Brillouin fiber sensing with ultra-wide temperature and strain range has been realized. This makes Brillouin fiber sensing a quite suitable choice for the structural health monitoring of large infrastructures. The successful measurements of both quasi-static and dynamic temperature and strain changes provide valuable information for in-time early-warning of their structural health conditions. The sensing performance of the Brillouin phase-gain ratio method is ultimately determined by residual noises existed in current sensing systems, further efforts may focus on developing new techniques to eliminate those noises.

Significance Distributed Fiber Optic Sensing (DFOS) technology plays a crucial role in ground collapse monitoring, enabling real-time detection of surface settlement, crack development, and underground structural deformation. It provides essential support for geological hazard early warning and safety assessment. Through a systematic elaboration of the fundamental principles of DFOS technology and its performance advantages over traditional monitoring techniques—such as anti-electromagnetic interference, corrosion resistance, high sensitivity, and long-distance continuous monitoring—combined with the development of ground collapse fiber-optic monitoring technologies, research on collapse disaster mechanisms based on measured data, and engineering application studies of ground collapse fiber-optic monitoring, the applicability and innovative value of this technology are demonstrated. Furthermore, this study analyzes the challenges of DFOS in ground collapse monitoring, including sensor adaptability issues, demodulation stability and reliability are limited, and insufficient intelligent data analysis capabilities. Finally, future research hotspots are identified.Progress Significant progress has been made in ground collapse monitoring using Distributed Fiber Optic Sensing (DFOS). Technically, DFOS, based on Rayleigh, Brillouin, and Raman scattering, enables high-precision measurement of temperature, stress, and strain, supporting long-distance, large-scale, and long-term geological monitoring (Fig.3, Fig.5, Fig.6). Professor ZHU Honghu's team at Nanjing University developed a three-stage mechanical model for interfacial stress transfer, revealing the progressive failure and strain evolution of the fiber-soil interface (Fig.8). Various strain-to-displacement models, such as CHEN's BOTDR-based model and Gao's ANN-modified model, have enhanced monitoring accuracy. In application, DFOS is used in karst collapse, pipeline-induced collapse, mining-related collapse, and tunnel-related collapse, aiding disaster early- warning and safety assessment (Fig.10, Fig.13, Fig.15). For instance, in karst collapse monitoring, DFOS captures strain signals during cave expansion, supporting risk warning. In pipeline-induced collapse monitoring, it monitors pipe deformation post-leakage to reflect collapse conditions. In mining-related collapse monitoring, DFOS captures dynamic characteristics of overlying strata deformation and surface subsidence. In tunnel - related collapse monitoring, its high precision and anti-interference capability make it a key tool. Moreover, the integration of DFOS with other monitoring methods in multimodal perception systems improves data accuracy and monitoring range. Intelligent algorithms further optimize disaster identification and early-warning capabilities.Conclusions and Prospects This paper reviews the latest advancements in Distributed Fiber Optic Sensing (DFOS) technology for ground subsidence monitoring, highlighting its ability to capture subtle surface and subsurface changes for high-precision, real-time monitoring and risk warning in scenarios such as karst collapse, pipeline leakage, and mining subsidence. By leveraging optical scattering principles, DFOS has shown significant advantages in long-distance, wide-area monitoring. However, challenges in environmental adaptability, data demodulation accuracy, and intelligent analysis still exist. For example, in harsh underground conditions, maintaining stable performance of fiber-optic sensors is difficult. Data demodulation is easily affected by environmental noise, leading to potential inaccuracies. Looking ahead, advancements in new fiber materials, multimodal monitoring integration, and intelligent algorithm applications are expected to enhance the efficiency and reliability of DFOS systems. The development of robust fiber materials can improve sensor durability in complex environments. Integrating multimodal monitoring methods, such as combining DFOS with InSAR and GNSS, will provide more comprehensive and accurate data. Furthermore, the application of intelligent algorithms based on machine learning and big data analytics can significantly boost data processing efficiency and the accuracy of risk warnings, thus offering more precise and effective support for geological disaster prevention.