Fiber-optic vibration sensors are crucial for detecting subtle disturbances in applications such as ocean seismic monitoring and structural health assessment. Among various configurations, interferometric fiber-optic sensors offer the advantages of long-range measurement, high dynamic range, and seamless integration into existing telecommunication networks. However, their performance is significantly constrained by laser frequency noise, particularly low-frequency drift, which degrades both sensitivity and signal-to-noise ratio (SNR). To address this issue, a Sampling Matched Integral Fitting Difference (SMIFD) scheme is proposed for low-frequency drift compensation in interferometric fiber-optic vibration sensing systems, leveraging intrinsic system properties and hardware component-free. By synchronizing the sampling rate with the fiber's transmission delay, the randomly distributed low-frequency noise is transformed into phase drift with discernible time-frequency characteristics through integration. Subsequently, the laser frequency drift can be predicted and mitigated from the phase drift through polynomial fitting. The experimental demonstration shows that SMIFD achieves significant noise floor suppression of 81.49 dB at 10 mHz and a SNR enhancement of 27.02 dB at 200 mHz over a 50 km single-mode fiber link. Furthermore, the down-sampling strategy is used to overcome the sampling rate limitation imposed by the transmission delay, enabling a broadband detection from millihertz to kilohertz range. Experimental results confirm that our method achieves the measurement accuracy required for ocean monitoring applications, even in high-drift conditions, enabling precise detection of low-frequency activities.

Cardiorespiratory rhythm disorders (CRD) are prevalent yet underdiagnosed conditions with substantial health risks. Current diagnostic methods are limited by invasiveness, intermittent symptom presentation, and low sensitivity during wakefulness. Sleep, by contrast, offers a stable physiological state ideal for early CRD detection. Here, we introduce a fiber-optic vital sign monitoring pillow (VSMP) that aims to realize the vision of home-based nocturnal health management, leveraging natural sleep for the accurate and unobtrusive early detection of CRD. By integrating fiber-optic sensing, biomechanical engineering, and advanced signal processing, the VSMP achieves early CRD detection through heart rate variability analysis and Lorenz scatter plots, while also facilitating blood pressure estimation (<8% error). Thus, we present VSMP as a new paradigm that transforms passive sleep into an active diagnostic session, paving the way for data-driven, personalized health management and proactive, precision medicine directly in the home environment.

We propose a hydroacoustic source localization compensation method using distributed fiber-optic hydrophone array with pseudo-random sequences. The average position error of the distance after compensation is reduced by 58% to 0.71m.

We propose a method to locate the overwater low-altitude source by the underwater distributed hydrophone with spectral subtraction preprocessing. The strongest external noise component is reduced by 10.2dB and maximum localization error is (7.8°, 0.3444m).

Since the superiorities of large-scale and distributed measurement, the fiber-optic distributed acoustic sensing (DAS) technique can collect plenty of distributed acoustic signals. Therefore, it has been gradually applied to perimeter security applications recently. However, the multisource near-field 2-D and 3-D localization and signal enhancement without aliasing noise for large-scale DAS has not been achieved yet, which are highly necessary for large-scale perimeter security applications. So, in this work, the DAS assisted with near-field array signal processing (ASP) is further developed and demonstrated, in which the near-field multiple signal classification (NF-MUSIC) algorithm is developed to achieve the near-field multisource localization. Then, based on the obtained target source position, the minimum variance distortionless response beamformer (MVDR-BF) is further proposed to enhance target signal in DAS, including multisource aliasing noise. In lab test, the maximum localization error no more than 0.03 m and 0.3 degrees is demonstrated by using the proposed ASP-assisted DAS. And the acoustic target tracking is also achieved in underwater field test. Then, the signal to noise ratio (SNR) of the target source in lab and underwater test is averagely enhanced about 9.7 and 16.8 dB by, respectively, using 5 and 20 channels. To the best of our knowledge, it is the first time to achieve the near-field multisource localization and signal enhancement for fiber-optic DAS. Substantially, this work has been greatly verified the proposed scheme that can further expand and promote the large-scale DAS practical applications, including the low-altitude and underwater multitarget detection, localization, and recognition.

Optical fiber vibration sensors play a vital role in plications requiring precise low-frequency disturbance detection, such as ocean seismic monitoring and structural health assessment. However, laser frequency noise, especially frequency drift, severely limits the sensitivity and signal-to-noise ratio (SNR) of conventional long-distance low-frequency sensing systems. In this work, propose a novel forward-transmission fiber-optic vibration sensing technique based on Time Delay Interferometry (TDI), originally developed for space-based gravitational wave detection. By implementing a dual Mach-Zehnder interferometer configuration applying time-delayed linear combinations of phase signals from both interferometers, the system effectively cancels laser frequency drift-induced phase noise. Numerical simulations demonstrate that this approach achieves over 30 dB SNR enhancement and 100 noise floor (NF) suppression for signals in the millihertz to hertz band over distances up to 1000 km. Experimental validation over 600 km standard single-mode fiber link confirms a minimum SNR gain of 31.18 dB and noise suppression of 155.31 dB @ 10 mHz. root addition, the strain resolution reaches 1.5 n epsilon/ Hz @ 10 mHz. Furthermore, the proposed TDI method outperforms conventional differential method in terms of SNR enhancement and NF suppression, and it enables simultaneous, independent detection of signals from both interferometers. This technique significantly advances long-range, ultra-low-frequency fiber-optic vibration sensing holds promise for applications including submarine seismic monitoring and enhanced security in high-risk environments.

A human footsteps localization method based on fiber-optic distributed acoustic sensing is proposed. With the time differences of arrival compensated by Gaussian time offsets, the footsteps localization error is reduced to 15.63 centimeters.

In this paper, a lightweight and fully distributed fiber-optic streamer with small channel spacing based on the distributed acoustic sensing is developed and demonstrated for the marine seismic acquisition. The bendinsensitive backscattering enhanced optical fiber evenly wound around the sensitization material is employed as the sensing fiber, and a rigid mandrel with high tensile strength is applied in the center of the sensitization layer to guarantee the towed work. Through optimizing the structural parameters with the theoretical and simulation analysis, an excellent frequency response from 5 Hz to 2 kHz with high sensitivity around −137 dB re :1 rad/(μPa m) and slight fluctuation less than 4 dB is demonstrated in the experiment. In the sea trial, the seismic profile with a high lateral resolution of 0.5 m along the 20 km survey line is obtained, in which the continuity of the seismic event is good and the reflections of the stratum more than 1.1 s below the seabed are clearly visible. In general, the proposed novel lightweight and fully distributed fiberoptic streamer has shown immense potential to achieve the marine seismic imaging with high lateral resolution, which can provide richer information for the accurate reservoir prediction.

We demonstrate a low-frequency drift compensation method in interferometry fiber-optic vibration sensing system without any additional components and achieve 65.13 dB @ 10mHz phase noise floor suppression and 50.22 dB @ 1Hz SNR improvement.

In recent years, the unmanned aerial vehicle (UAV) has been widely applied in daily life, but the "black flight" incidents happen frequently and even endanger public safety. Although the electrical microphone can cover the low-altitude and non-line-of-sight (NLOS) limited vision areas of radar or camera system, it is quite difficult to achieve large-scale networking and surveillance. In this article, a low-altitude UAV detection and positioning scheme based on fiber-optic distributed acoustic sensing (DAS) is proposed and demonstrated, which has the superiorities of large-scale networking and antielectromagnetic interference in applications. The DAS system assisted with the highly sensitive sensing cable fabricated by uniformly winding backscattering enhanced optical fiber (BEOF) around sensitization cable can collect the acoustic signals induced by low-altitude UAV, and it can be easily deployed with a large scale. The acoustic signals collected by DAS are further divided into continuous frames and denoised for low-altitude UAV positioning and tracking, in which the time difference of arrival (TDOA) algorithm based on the generalized cross-correlation (GCC) method and least-squares estimation (LSE) method is employed. In the field test, the low-altitude UAV positioning and tracking are both demonstrated and achieved with an ultralow position estimation error of less than 0.5 m in low-altitude airspace. It has exhibited that the proposed surveillance scheme has a great potential for low-altitude airspace early warning in large-scale applications such as border and airport. Then, reporting and monitoring intrusive low-altitude UAV instantaneously allow the controller to take quick action to prevent potential danger and damage from happening in key areas.

This paper evaluates the azimuth estimation of an underwater acoustic source using a fully distributed fiber-optic hydrophone cable in Thousand Island Lake. Backscattering-enhanced optic fiber uniformly inscribed with backscattering-enhanced points was selected as the sensing fiber to improve the signal-to-noise ratio of the received signals. The distributed acoustic sensing system based on phase-sensitive optical time domain reflectometry was used to interrogate and demodulate the received signals to obtain the phase change along the cable. Frequency-domain beamforming was selected to process the demodulated signals and estimate the direction of arrival of the received signals. The cable's ability to detect static and moving acoustic sources was evaluated using various cable arrangement patterns. The results indicate that the cable is capable of detecting the acoustic source, showing that the proposed method has a great application potential in the marine field, such as underwater safety.

Significance Information technology is the cornerstone that supports the development and social life of today's world, with its important component of sensing technology. Fiber optic sensing technology utilizes light waves as information carriers and transmission media to achieve the collection and measurement of signals in the environment. As an important branch of fiber optic sensing technology, distributed fiber optic sensing can achieve long-distance, high-resolution, and highly sensitive continuous distributed detection, obtaining two-dimensional spatio-temporal distribution information. Compared to the other two types of scattering distributed sensing, the system based on Rayleigh scattering features higher backscattering power and faster response and is more suitable for detecting dynamic and static signals such as sound waves and strain. With the increasing demands for engineering applications such as resource exploration, structural health monitoring, and underwater exploration, distributed fiber optic sensing has developed rapidly in recent years. At present, most distributed sensing systems usually employ single-mode fiber (SMF) as the sensing medium. However, its Rayleigh backscattering signals are extremely weak, resulting in poor signal-to-noise ratio (SNR) of sensing light, which in turn causes poor SNR of demodulation signals in distributed sensing systems. Additionally, the intensity fading effect induced by high laser coherence can cause sensing blind spots, and the light intensity fading can also result in poor sensing consistency among multi-channels. Meanwhile, due to the influence of optical transmission loss, the sensing SNR of ordinary non-amplification SMF optic systems is limited at long distances. The fully continuous characteristics of backscattering signals in optical SMFs can also result in mutual limitations between the system response bandwidth and sensing distance. Therefore, scattering enhanced special optical fibers are introduced into distributed sensing systems based on Rayleigh scattering. By continuously changing the fiber material and structure, or introducing discrete scattering enhancement mechanisms, the distributed sensing limitations of ordinary optical SMFs are overcome in specific sensing parameters, sensing performance, and other aspects. Thus, in some specific application scenarios that require high-precision detection, scattering enhanced optical fiber has irreplaceable advantages. In recent years, numerous research institutions and researchers have conducted research on scattering enhanced fiber optical distributed sensing systems and obtain significant results. Progress We focus on analyzing the scattering characteristics and noise suppression mechanisms of scattering enhanced microstructured sensing fibers, and elaborate on the types and precision preparation techniques of scattering enhanced fibers. Meanwhile, the performance improvement techniques of DAS and OFDR systems based on scattering enhanced microstructured fiber are summarized (Fig. 5 and Table 1), and the mechanism and typical applications of DAS SNR and sensitivity enhancement are discussed. The research progress of scattering enhanced hydrophone composite cables is elaborated (Table 2), and the construction of highly sensitive distributed hydrophone systems and their hydrophone applications are introduced (Table 3). Additionally, we summarize the high-density grating scattering enhanced microstructured fiber to achieve high-resolution, highly sensitive, and highly reliable fully distributed strain sensing based on optical frequency domain reflectometry (Figs. 10 and 11). Combined with highly reliable reconstruction algorithms, scattering enhanced microstructured spiral multi-core optical fibers are designed to achieve high-precision three-dimensional shape sensing and practical applications (Table 4). Conclusions and Prospects In summary, we study the mechanism of distributed sensing efficiency enhancement from the perspective of scattering enhanced special optical fibers, introduce the automatic precision fully continuous writing technology, and focus on the principles of its optical time domain and optical frequency domain distributed sensing systems. Meanwhile, the research progress of distributed acoustic sensing and optical frequency domain reflection technology based on scattering enhanced microstructured fiber is summarized, and typical engineering applications based on the above two systems are summarized. In the future, distributed sensing technology based on scattering enhanced optical fibers can still be improved and expanded in various aspects. For example, the material and structural parameters of scattering enhanced microstructured fiber can be optimized, and the high-efficiency and stable writing preparation process can be improved. Additionally, the scattering enhancement characteristics of optical fibers can be combined with intelligent AI algorithms to optimize sensing demodulation accuracy. Meanwhile, the high precision 3D shape sensing and scattering enhanced fiber distributed hydrophone can be further extended to various cross applications. As the scattering enhanced special sensing fibers further develop in the future, distributed sensing systems based on scattering enhanced fibers will play an irreplaceable role in most fields.

In this paper, we demonstrate a fully distributed hydroacoustic sensing based on the ultra-highly sensitive and lightweight fiber-optic hydrophone cable assisted with heterodyne phase-sensitive optical time domain reflectom-etry. The proposed lightweight hydrophone cable is an all-solid state multilayer composite cable, which mainly consists of cable core, acoustic sensitization layer, sensing fiber, and protective layer. Based on the theoretical and simulation analysis results, the acoustic sensitization layer of the hydrophone cable is extruded from ther-moplastic polyurethanes to enhance the acoustic sensitivity, and the sensing fiber is uniformly wound around the acoustic sensitization layer with pre-stress. To improve the signal-to-noise ratio of the sensing signal and guar-antee the measurement stability in complex underwater environment, the backscattering enhanced optical fiber inscribed with a series of backscattering enhanced points is employed as the sensing fiber to suppress the coher-ent fading noise. Besides, the differential-polarization-vector-sum algorithm based on the polarization diversity reception is proposed and utilized to suppress the polarization fading noise of the coherent detection system. In the experiments, the results demonstrate an excellent acoustic sensitivity of - 137.2 dB re:1 rad/( ������Pa & BULL;m) with a flat frequency response at the range of 5 Hz & SIM; 2 kHz, which is consistent with the theoretical analysis and simulation result, as well as it can keep stable whether at the water pressure of 0.3 MPa or the axial elongation of 2%. Besides, it is noteworthy that the acoustic sensitivity is higher in the frequency range of 1 Hz & SIM; 5 Hz, which even can reach up to - 125.3 dB re:1 rad/( ������Pa & BULL;m) at 1 Hz. Lastly, the noise equivalent pressure of the proposed system is about 48 dB re:1 ������ Pa/Hz1/2 at 1 kHz, which is lower than the level of the Deep Sea State Zero. In field tests, the lightweight fiber-optic hydrophone cable can be rapidly deployed, and various emergency intrusive targets like boat, frogman, etc. are all detected and tracked based on the characteristics of high sensitivity and broadband response, which has exhibited the proposed system has tremendous potential for the future surface and underwater surveillance applications.

The frequency drift of lasers is one of the most significant source of phase noise in fiber-optic sensing systems. Time delay interferometry (TDI) in gravitational wave detection mission can effectively suppress the frequency noise of laser. We applied TDI to fiber-optic sensing systems and achieved 80 dB noise floor suppression at 10 mHz and the strain resolution reached 2.39 $p\varepsilon/\sqrt{Hz}{@}> 10mHz$ on a 100 km ultra-long SMF link. The signal to noise ratio was improved by 10 dB and 25 dB at 500 mHz and 2 Hz respectively. The proposed method has the potential to significantly improve the low-frequency response of fiber optic sensing systems such as DAS and OFDR.

An accurate and fast recognition of some noticeable threatening events has been proven effective in fiber-optical distributed acoustic sensor (DAS). However, it is still challenging to find an efficient way to realize the accurate potential threats detection and identification. Especially in complicated environments, some weak threatening signals such as manual digging are usually submerged by the strong background noises, in which the influence of these interferences is unavoidable. Unfortunately, these effects of the mixed heavy interferences may cause ignored cases of the alert of potential threats, which even causes significant economic loss. In this work, an accurate and effective multisource signals separation and recognition algorithm is proposed to achieve the identification of the potential threats submerged in multisource noises for fiber optic DAS. First, the overlapping interferences in complicated environments can be effectively denoised by the proposed multisource signals separation algorithm. Then the multiscale features of different signal targets can be automatically extracted and identified by an attention-based multiscale convolution neural network (MS-CNN) model. In the field tests, four types of mixed multisource signals are performed to validate the effectiveness of the proposed algorithm. Finally, the field test results show that the recognition rate of the mixed signals is improved from 53.82% to 95.43% by the proposed algorithm. Besides, the performance of three network models based on the same database is compared. The final results prove that the attention-based MS-CNN model can obtain improved training speed and recognition accuracy, compared with the 1DCNN model and MS-CNN model. The proposed algorithm has an excellent performance for mixed threat identification in various complicated interferences.

Tide observation is an important part of marine engineering survey. We propose and demonstrate a new-type on-line tide monitoring system based on fiber distributed acoustic sensing (DAS), whose field test has shown its great prospect.

We report field test results of surface and underwater surveillance based on fiber-optic distributed acoustic sensing (DAS) and highly sensitive distributed fiber-optic hydrophone (DFOH). Various intrusive targets like boat, frogman and etc. are detected.

Precise control and measurement of the flow are essential for pipeline safety monitoring and economic accounting. Nowadays, the measurement of pipeline flow relies on point measurement schemes such as mechanical flowmeters, which invade the pipeline and can irreversibly damage the fluid pressure, especially when multipoint measurements are required. To overcome these problems, we have explored a novel technique in which the acoustic field properties of pipeline turbulence are reflected by distributed backscattering light field, from which the flow can be measured via acoustic field imaging. The acoustic field information is recorded by a helical deployment scheme using backscattering-enhanced optical fiber (BEOF) encapsulated in the polyethylene anticorrosive layer, thereby achieving highly sensitive, noninvasive, and distributed online monitoring of gas flows. We obtained 24-h flow fluctuation distributions with only one sensing fiber for two operating gas pipelines, and the flow measurement accuracy reached higher than 98% within the error range of −15% to 15%. Moreover, the minimum detectable flow is evaluated and it is demonstrated that the proposed scheme is extremely sensitive to weak flows. We successfully demonstrated noninvasive and distributed online measurement of operational pipeline flow, which will exhibit great potential for pipeline integrity management in the future.

We demonstrate a low-altitude unmanned aerial vehicle detection method based on distributed acoustic sensing, in which the localization estimation is achieved assisted with time difference of arrival algorithm in the field test.

The sensing performance is severely affected by the intensity fading in the phase-sensitive optical time domain reflectometry ( $\varphi $ -OTDR) based distributed acoustic sensing (DAS) system. The intensity fading manifests a stochastic amplitude fluctuation in the scattering signal. To suppress the fading noise, a heterodyne $\varphi $ -OTDR system assisted with dual lasers with independent frequency, polarization, and initial phase is established to generate two pulses responses with different fading components at the same time. The backscattering signals from two individual probe pulses present different fading positions but possess the same phase change rate which is induced by the external disturbance. Through the differential-vector-sum algorithm including vectorization and phase differential, two complex Rayleigh backscattering beat signals from the two laser probes are efficiently synthesized, to simultaneously suppress the fading phenomenon caused by the inner pulse interference, polarization mismatch and phase mismatch. In the experimental validation, the probability of the fading channels in dual-laser scheme was significantly reduced from 9.4% to 1% compared with the single laser system, which is based on the resolution threshold of 2.1n $\varepsilon / \surd $ Hz at 10Hz. Moreover, the dual-laser scheme also proves the polarization insensitivity by inducing a rapid polarization perturbation for two probe lasers. Owing to the fading noise suppression, the dynamic signal's SNR of the dual-laser system was improved more than 20dB in the intensity fading channels, while the temporal and spatial resolution were not deteriorated. The proposed fading suppressed distributed sensing system possesses the excellent performance, which makes it play an important role for practical distributed acoustic sensing.