This paper presents a multi-class vibrational event detection method utilizing distributed acoustic sensing (DAS) technology in conjunction with the You Only Look Once (YOLOv8) algorithm, based on data collected from field tests. The implemented DAS system employs a chirped pulse and direct detection approach, enhanced by noise suppression, achieving a sensitivity of 7.3 n epsilon/ root at a very low frequency of 0.1 Hz. This advancement broadens Hz the frequency detection range of events by improving the sensitivity at low frequency limits while preserving the upper limit, thus enhancing the capability to acquire information across a wide frequency spectrum. Additionally, the YOLOv8 network's performance is augmented by incorporating an attention mechanism that highlights important event-related features while minimizing noise during image processing. The improved YOLOv8 successfully identifies vibration events such as hammer impacts, iron buckles, balls entering the water, and engine vibrations, achieving a comprehensive detection accuracy with an overall mAP of 96.8%, respectively. It achieves a detection speed of 70.4 frames per second, showcasing its ability for real-time identification. This method demonstrates a possibility of wide-frequency-response DAS and enhanced YOLOv8 algorithm to be potentially used for monitoring the security of submarine communication cables.

Monodisperse SiO2 nanospheres (similar to 50-200 nm) were synthesized via a modified Stober method involving hydrolysis and condensation reactions to generate SiO2 templates. The SiO2 surface bears negative charges through silanol (SiOH) groups, while positively charged ammonium ions (NH4+) from the carbon precursor electrostatically self-assemble onto the SiO2 surface, forming organic-coated SiO2 nanospheres. Subsequent high temperature sintering under an argon atmosphere yields SiO2@C composite nanospheres. TEM and XPS confirm the spherical morphology, narrow size distribution, and the presence of a graphitized carbon layer. When applied as fingerprint developing powders on typical forensic substrates knife blades, rusted iron pieces and unpolished silicon wafer, the powders rapidly adsorb fingerprint residues and emit weak fluorescence under multi-band illumination, markedly enhancing the contrast of ridge details. The inherent chemical inertness and low cost of SiO2, combined with the optical scattering and fluorescence properties of the carbon coating, endow these nanocomposite powders with superior latent fingerprint visualization performance across diverse substrates, offering a highly efficient and environmentally friendly material for forensic evidence collection.

Wearable surface-enhanced Raman spectroscopy (SERS) sensors can detect analytes in sweat containing interfering substances, but they face challenges in integrating sampling, sensing, and detection into a single system for remote in situ analysis. To address this, this paper proposes a flexible "hydrogel tentacle" optical fiber (HTOF) SERS sensor, enabling remote in situ detection of analytes in sweat. The sensor uses a hydrogel with excellent water absorption as the flexible SERS substrate, which is in situ crosslinked with Ca2+ at the optical fiber tip to assemble the flexible hydrogel tentacle (HT). The light transmitted through the fiber is coupled into the HT, enabling direct sampling of analytes. The sensor's performance was evaluated using 4-mercaptopyridine (4-Mpy), achieving an enhancement factor (EF) of 9.44 × 1010 and a limit of detection (LOD) of 8.69 × 10-11 M. After 40 days of storage, the sensor maintained 95.24% of its SERS activity, and the relative standard deviation (RSD) between different batches was as low as 2.64%. With the excellent bending and stretching properties of the HT, the sensor can be applied in wearable human devices. When combined with a one-dimensional convolutional neural network (1D-CNN) machine learning model, it can achieve semiquantitative recognition of uric acid, creatinine, and urea with an accuracy of up to 91.05%. The proposed sensor shows promising potential for applications in kidney disease assessment and health monitoring.

Multi-turn optical fiber coils, as the key sensing components of Fiber Optic Disc Accelerometers (FODAs), significantly affect the FODA performances due to their layered structure. This paper explores the strain distribution characteristics of the optical fiber coils by utilizing the inherent properties of the FODA, supported by Finite Element Method (FEM) and high-precision Optical Frequency Domain Reflectometry (OFDR). The results indicate that the simulation outcomes are largely consistent with the OFDR measurements, accurately reflecting the strain distribution within the optical fiber coils of the FODA. This analysis dose not requires additional external force application devices and carries no risk of damaging the FODA. Based on FEM, the simulation model is employed to optimize the existing structure, proposing a multi-turn optical fiber coil fabricated from ultra-fine diameter fiber. By altering the structure of optical fiber coil to make the internal strain distribution trend uniform, the sensitivity of the FODA is increased to approximately 150% of its original value.

Sensors have been instrumental in driving technological progress and enhancing quality of life by delivering precise data and enabling intelligent responses across a multitude of disciplines. In alignment with the advancement of technology and the demands of both industrial production and everyday life, a dual-core PCF-SPR sensor was developed. The sensor, based on D-type photonic crystal fiber and incorporating ITO and Au, was designed for the simultaneous detection of refractive index and temperature. Research results demonstrate that by using the finite element method, the maximum sensitivity of the structure in the refractive index detection range of 1.33-1.39 reached 18,100 nm RIU-1, and the maximum sensitivity in the temperature detection range of 10 degrees C-50 degrees C reached -12.3 nm degrees C-1. The refractive index and temperature detection channels operate independently of each other. This sensor architecture enables simultaneous dual-parameter measurement of refractive index and temperature, is lightweight and highly sensitive, and possesses potential applications in cancer cell detection and tsunami prevention.

We proposed a novel, to the best of our knowledge, chirp-pulse pair phase-sensitive optical time-domain reflectometry (CPP-φOTDR) technique, enhanced by an adaptive filtering algorithm. This technique utilizes a pair of chirp pulses: one with a low chirp rate and another with a high chirp rate, with their Rayleigh backscattering (RBS) processed through a low-pass (LP) electrical filter. The adaptive filtering algorithm effectively preserves the extensive measurement range afforded by the high chirp rate pulse while enhancing the sensitivity provided by the low chirp rate pulse. Consequently, the CPP-φOTDR enables vibration measurements over a wide dynamic range and broad frequency bandwidth without incurring additional acquisition costs. In the experiments, we employed a chirp-pulse pair featuring bandwidths of 500 MHz and 8 GHz, utilizing only the receiver's 500 MHz bandwidth to retrieve the vibrational signal. The dynamic range of the CPP-φOTDR was enhanced by 25.1 dB, with the assistance of adaptive filtering of the acoustic waveform demodulated with RBS of the chirp-pulse pair. The proposed method could be utilized for monitoring the ocean in marine science and for analyzing seismic waves in geophysics.

In this study, we propose a sensing probe architecture based on a 1-D photonic crystal (1DPC) composed of periodically arranged Cytop/Al structures and realize long-range surface plasmon resonance (LRSPR) through prism coupling. This architecture is characterized by high sensitivity, high detection accuracy, and micrometer-level penetration depth (PD). LRSPR is characterized by narrow resonance peaks, and the combination of multiple LRSPR structures results in strong coupling resonance, leading to the splitting of the resonance signal into multiple peaks. We found that the first resonance peak is the most sensitive and can effectively detect changes in the surrounding environment. The results show that this LRSPR sensing structure based on the Cytop/Al 1DPC demonstrates exceptional sensitivity with a periodicity of two layers (S-max = 5846 RIU -1). Compared with conventional surface plasmon resonance (SPR) sensors (S-max = 59 RIU-1), the sensitivity of the proposed LRSPR sensor is increased by about 100 times, achieving an extremely high detection limit of up to 5x10(-7) RIU. In addition, the two-layer structure remains optimal across refractive indices from n(s) = 1.328 to 1.334, underscoring its adaptability. Our research utilizes the 1DPC structure, which, due to its simplicity and stability, exhibits significant potential to enhance both the sensitivity and detection limit of SPR sensors through structural design and simulations. This approach facilitates substantial improvements in sensor sensitivity and resolution within a relatively simple structure, thereby offering potential application value for biomedical and chemical detection.

This paper presents a two-step cross-correlation algorithm to extend the demodulation range of chirped-pulse phase-sensitive optical time-domain reflectometry $(\text{CP} \varphi \text{OTDR})$, of which the shot-to-shot measurement range can reach 2 folds as that of using one-step cross-correlation.

The commonly used mandrel-type or disk-type fiber-optic accelerometers (DT-FOAs) are inherently limited in their capacity to detect low-frequency signals due to their structural characteristics. The fiber-optic coil (FOC), as the core sensing component of an interferometric FOA, plays a crucial role in determining the sensor's sensitivity through its strain distribution characteristics on the transducer plate. This article presents, for the first time, the novel introduction of noncircular FOCs (NC-FOCs), which are employed in the design of a rectangular thin-plate FOA and plate-tpye FOA (PT-FOA) to improve low-frequency detection performance. In this study, the finite element method (FEM) and tensor analysis theory are employed to establish a universal model for PT-FOA constructed with NC-FOCs. Sensors with FOCs of various shapes were designed, fabricated, and tested. Both theoretical and experimental results demonstrate that different shapes of FOCs can improve sensitivity by approximately 14.62%. The underlying mechanisms of these improvements are analyzed by studying the strain tensor distribution on the transducer plate. The proposed PT-FOA addresses the limitations of existing FOAs in low-frequency detection. The sensitivity enhancement method maintains the same resonant frequency and thermal noise levels while significantly improving the signal-to-noise ratio (SNR) and sensitivity of the sensor. This work provides a general modeling approach and valuable practical guidance for the design and optimization of high-sensitivity FOAs, with important implications for both research and application.

Temperature cross-sensitivity, which especially performs complicatedly and severely in fiber-optic sensors composed of various materials, will significantly affect the stability and the accuracy of the parameter to be measured. Consequently, the polarization-multiplexed technology is introduced to separate the temperature and strain parameters, while a piecewise uniform compensation method is further proposed and employed to get rid of the nonlinear temperature effects in the fiber-optic vibration sensing unit (FOVSU). In the sensitive fiber coil (SFC) of the FOVSU, the solution interval of temperature coefficient is subdivided and employed for decoupling in different temperature ranges; then, the data are processed by splicing finally. The experimental results show that a suppression effect of temperature drift noise of 43 dB at 1 mHz is achieved under unidirectional self-cooling temperature conditions and 33 dB at 1 mHz under continuous temperature rise and fall conditions by using the piecewise method, which demonstrates that the performances in both situations are better than the original method using only one set of temperature coefficients. Therefore, the proposed new method could significantly reduce the temperature noise, leading to the testing accuracy and stability enhancement of FOVSUs.

Interferometric optical vector analysis (OVA) technique, employing orthogonal polarization interrogation and polarization diversity detection, provides comprehensive spectral response characterization of optical devices, though with exacting polarization alignment demands. To overcome this intrinsic limitation, we introduce an unbalanced orthogonal polarization-interrogated OVA scheme that operates without active polarization control. Besides, our method achieves performance parity with leading commercial instrument.

We proposed a Rayleigh-Brillouin scattering (RBS) spectrometer based on a virtually imaged phased array (VIPA) for fast measurements of high-gas temperature. We measured the RBS spectra of air in the temperature range of 374 to 1073 K with an acquisition time of 7 s. We used the Tenti S6 model to fit the spectra and retrieve the absolute temperature values. The root mean square errors of spectra fit residual were less than 3.05%, and the absolute error of the retrieved temperature was less than 39 K. This study demonstrated the ability of the RBS spectrometer to realize fast high-temperature measurement and its potential for combustion control applications.

In this paper, a single-frequency erbium-doped fiber laser based on self-injection feedback is reported, which uses a high-reflectivity broadband fiber Bragg grating, an ordinary commercial high-doped erbium fiber and a low-reflectivity broadband fiber Bragg grating to form the main resonant cavity, and a stable narrow linewidth single-frequency laser output is achieved by connecting a single-mode fiber and a low reflectivity narrowband fiber Bragg grating to form self-injection feedback. Then, a saturated double-pass erbium-doped fiber amplifier and a saturable absorber were added to the self-injection optical path, which well suppressed the relative intensity noise and phase noise and the relaxation oscillation intensity was well suppressed by about 18.08 dB, and the phase noise was 36.7dBc·Hz−1 suppression, the edge-mode rejection ratio is increased by 13 dB, and the final laser linewidth is about 270 Hz, and the output power is 8.5 dBm.

A method to improve the performance of distributed temperature sensors with kilometer sensing distance and centimeter spatial resolution is proposed based on polarization-sensitive optical frequency domain reflectometry (OFDR). This approach records the temperature change along polarization maintaining fiber (PMF) from the Rayleigh backscattering (RBS) spectral difference between two orthogonal polarization axes using the distributed autocorrelation algorithm, where PMF is used as the sensing fiber. In addition, a technique to calibrate population birefringence and local birefringence, and a method to calibrate the initial birefringence inhomogeneity of PMF are proposed to enhance the performance of the sensor. The birefringence of PMF is used for temperature sensing, which is distinct from the traditional cross-correlation method of single -mode optical fiber temperature sensing, providing us with an innovative scheme to solve real -world engineering problems. The final sensing performance is achieved, with a sensing distance of 1.5 km, a spatial resolution of 5 cm, and a temperature measurement uncertainty of +/- 0.2 degrees C, subject to environmental noise and system random noise.

Optical frequency domain reflectometry (OFDR) can provide a powerful tool for fiber components and devices diagnosis and characterization. However, apart from laser source phase noise, the impact of the chromatic dispersion effect on the system is severely increased with the measurement length and laser tunable range. We propose a distributed chromatic dispersion compensation method for the device under the test (DUT) with complex structures containing different dispersive media connections in OFDR. Based on the mismatch factor (which is related to the difference in dispersion coefficient between the reference fiber and DUT), chromatic dispersion errors in the signal from different dispersion mediums can be compensated segment by segment by constructing the dispersion phase error signal. The compensation method is evaluated by experiments on DUT with multiple material dispersion (including reduced-cladding single-mode fiber (RC SMF) and dispersion compensation fiber (DCF)) and various measurement distances. Finally, we experimentally analyzed the internal reflection of a multifunctional integrated optical chip (MFIOC). Sweeping the laser source by 160 nm, the input coupling, output coupling, beam splitting, and defect points are individually revealed with a high resolution better than 15 mu m. We believe that this approach enables high-precision quantitative measurements as well as accurate identification and localization of fault diagnosis for optical fibers and devices.

Long-range Brillouin optical time-domain analysis (BOTDA) requires a frequency scanning process and averaging to obtain Brillouin temporal traces to map the distributed Brillouin gain spectrum, which is highly time-consuming. Furthermore, the two-end accessibility of the BOTDA leads to a roundtrip wiring layout, which shortens the sensing distance to half of the fiber length. To overcome these issues, in this study, we proposed a single-end random-access BOTDA. Two pulses, namely, pump and probe pulses, were sequentially generated, one with constant frequency and the other with chirped modulation, between which a stimulated Brillouin scattering interaction occurred with a high signal-to-noise ratio; therefore, a frequency scanning process and averaging were not needed. The pump and probe pulses were sent into the sensing fiber at the near end, and only the probe was reflected by a narrow bandwidth filter placed at the far end. The proposed single-end BOTDA can randomly sense the interested position by adjusting the time delay between the pump and probe pulses. As a result, real-time dynamic acquisition up to 1 kHz can be achieved on any position along the 50-km-long fiber, and distributed measurements can be performed on the second level. In addition, the proposed sensor is immune to the nonlocal effect, demonstrating a higher sensing accuracy at the far end compared with that of a traditional BOTDA.

To simultaneously measure thickness and group refractive index using white light interferometry, it is often required to precisely obtain the interval between interference peaks. There are many demodulation algorithms to calculate the position of a single interference peak. However, these algorithms are rarely adapted for peak interval demodulation. In this article, we propose a method of calculating the interval of the white light interference peaks based on frequency domain analysis (FDA). By subtracting the phase -frequency curves of two interference peaks before linear fitting, the interval of the peaks can be accurately calculated. The demodulation procedure is simplified compared with traditional FDA method. Simulations and experiments show that the noise -induced 2 pi phase jump phenomenon can be effectively suppressed by this method. Simultaneous measurements of physical thickness and group refractive index of specimens of different materials and thicknesses are realized using a home-made fiber-optic white light scanning interferometer. Compared with traditional centroid method, slope FDA method and intercept FDA method, the proposed method shows better robustness to noise experimentally, while the measurement data are in good agreement with the nominal values of thickness. In a well -controlled environment, the standard deviations of repeated measurements reach 3.12 nm and 7.66 x 10 -6 refractive index unit (RIU) for thickness and group refractive index respectively, which have been significantly improved compared with our previous work measuring the same specimen [1].

Sensing devices are key nodes for information detection, processing, and conversion and are widely applied in different fields such as industrial production, environmental monitoring, and defense. However, increasing demand of these devices has complicated the application scenarios and diversified the detection targets thereby promoting the continuous development of sensing materials and detection methods. In recent years, Tin+1CnTx (n = 1, 2, 3) MXenes with outstanding optical, electrical, thermal, and mechanical properties have been developed as ideal candidates of sensing materials to apply in physical, chemical, and biological sensing fields. In this review, depending on optical and electrical sensing signals, we systematically summarize the application of Tin+1CnTx in nine categories of sensors such as strain, gas, and fluorescence sensors. The excellent sensing properties of Tin+1CnTx allow its further development in emerging intelligent and bionic devices, including smart flexible devices, bionic E-skin, neural network coding and learning, bionic soft robot, as well as intelligent artificial eardrum, which are all discussed briefly in this review. Finally, we present a positive outlook on the potential future challenges and perspectives of MXene-based sensors. MXenes have shown a vigorous development momentum in sensing applications and can drive the development of an increasing number of new technologies.

In this paper, we propose and demonstrate a spectral splicing method (SSM) for distributed strain sensing based on optical frequency domain reflectometry (OFDR), which can achieve km level measurement length, µɛ level measurement sensitivity and 104 µɛ level measurement range. Based on the traditional method of cross-correlation demodulation, the SSM replaces the original centralized data processing method with a segmented processing method and achieves precise splicing of the spectrum corresponding to each signal segment by spatial position correction, thus realizing strain demodulation. Segmentation effectively suppresses the phase noise accumulated in the large sweep range over long distances, expands the sweep range that can be processed from the nm level to the 10 nm level, and improves strain sensitivity. Meanwhile, the spatial position correction rectifys the position error in the spatial domain caused by segmentation, which reduces the error from the 10 m level to the mm level, enabling precise splicing of spectra and expanding the spectral range, thus extending the strain range. In our experiments, we achieved a strain sensitivity of ±3.2 µɛ (3σ) over a length of 1 km with a spatial resolution of 1 cm and extended the strain measurement range to 10,000 µɛ. This method provides, what we believe to be, a new solution for achieving high accuracy and wide range OFDR sensing at the km level.

We suggest a distributed birefringence measurement method for Polarization-maintaining fiber using Optical Frequency Domain Reflectometry. Method provides a spatial resolution of 5 cm and an uncertainty of 6.8×10-7 at a test distance of 2257 m.