This PDF file contains the front matter associated with SPIE Proceedings Volume 12968, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The oblique incidence is a crucial metrology method to realize the measurement demand for high aspect ratio multiple elements piezoelectric X-ray bimorph mirror surface by small-aperture Fizeau interferometers in one single shot. Due to the change of optical path difference in oblique incidence, we analyzed how different oblique incidence angles would affect the results of the surface height with numerical simulation and evaluated angular errors when curved mirrors are measured. Then we constructed an oblique incidence measurement system for measuring the 200 mm X-ray bimorph mirror with multiple elements piezoelectric by using a 100 mm clear aperture Fizeau interferometer and return reference mirror based on a preliminary determination of the angle of oblique incidence, and so the precise angle of oblique incidence is calculated from the actual measurement results. The oblique incidence can directly observe the dynamic effects of a high aspect ratio X-ray bimorph mirror surface in response to the applied voltages. With practical interferometric measurements of the multiple elements piezoelectric X-ray bimorph mirror by the constructed oblique incidence measurement system, and we obtained multiple sets of response functions and optimized surface at different voltages. Furthermore, the bimorph mirror surface optimized height profile was experimentally controlled and measured to be 17.13 nm PV (peak to valley) at the centerline, which can lay the foundation for measuring the X-ray bimorph mirror surface with extremely high aspect ratios.

Aiming at the problem that the collimating target recognition algorithm of optical path docking cannot distinguish the adhesion state of double targets, a dual optical target recognition method based on BLOB region feature analysis is proposed in this paper. The experimental results show that: The dual optical target recognition algorithm proposed in this paper can not only realize the recognition of completely separated analog optical targets and main laser targets, the error is less than 3 pixels, the processing time is less than 1 second, but also can realize the recognition of optical targets in the state of adhesion and a single independent optical target. It meets the requirements of adaptability, accuracy, and efficiency of optical path butt collimation image recognition algorithm.

In order to improve optical measurement precision of tooth profile form deviation, a high resolution line spectral confocal method was used. Aiming at the problem that tooth profile information contained in optical measurement results and traditional contact measurement results are inconsistent and couldn't be directly compared, A form feature data filtering method of virtual probe envelope contact points was proposed. This method simulated filtering method of contact probe to filter optical tooth profile data and extract macroscopic form feature data. In order to calculate tooth profile deviation along expanded length direction, firstly, the iterative nearest point method was used to register extracted tooth profile with theoretical involute profile in gear coordinate system to achieve coordinate conversion, and then the deviation value was calculated. In the experiment, the profile form deviation of gear involute master was measured by line spectral confocal method, and compared with verification results of contact method. The results showed that the measured tooth profile data filtered by the proposed envelope filtering method could be directly compared with the results of the contact method. The average value of optical measurement results of tooth profile deviation was 1.07 μm, which was consistent with verification results, and the maximum difference between them was 0.12 μm. Moreover, the form change of tooth profile deviation curve was highly consistent with that of verification curve, which verified the effectiveness of proposed method.

A model for calculating the scalar factor in an interferometric digital closed-loop fiber optic current sensor is derived when the fiber retarder and the sensing fiber are working in non-ideal conditions. The key parameters leading to high current nonlinear errors in fiber optic current sensors are proposed: the alignment angle of the retarder, the phase delay angle of the retarder, and the linear birefringence of sensing fiber. The nonlinear error model of the fiber optic current sensor in the measurement range of 1~1400kA under typical parameters is simulated. A fiber-optic DC high current sensor calibration system was built, and the test results showed that the ambient nonlinear error of the sensor in the range of 16~840kA was 1.2% when the alignment angle of the retarder was 45° and the phase delay angle of the retarder was 98°, which was consistent with the theoretical derivation. The measurement error of the sensor is reduced from 1.2% to 0.15% by compensating for the nonlinear error through 3rd -order polynomial fitting modeling.

Here, a multi-band metamaterial absorber (MMA) is proposed. The MMA (multi-band metamaterial absorber) consists of a symmetrical snowflake resonator layer and an underlying metal plate, separated by a dielectric layer. Furthermore, the snowflake resonator is composed of several branches which leads to multiple resonant modes. The coupling of each resonant mode leads to an absorption enhancement effect. The structure is optimized to achieve perfect absorption at three resonant frequencies. Simulation results show that the MMA achieves three narrow absorption peaks with absorptivity of 94.6%, 98.95%, and 98.82% in the range of 15~36 GHz. The absorption mechanism is analyzed in detail at all three resonant frequencies, and the structure size and resonant frequency are calculated and matched quantitatively. By means of analog circuit, the absorption spectrum of the absorber is verified. In addition, the application of the proposed MMA as liquid sensor is realized by analyzing the frequency shift of the absorption peaks. The sensitivity of the refractive index sensor based on the proposed MMA is 11.64 GHz/RIU, the quality factor is 1236, and the figure of merit (FoM) is 151.33 RIU^-1 at one of the resonant frequencies, when the analyte thickness is 5.5mm and the refractive index range is 1 to 1.48.

Photobiological safety experiments of two typical active lighting units in video surveillance systems, white LED lamp and Xenon lamp, are performed according to the photobiological safety requirements (GB/T 37958 and IEC 62471). The white LED lamp working in stroboscopic mode is in low risk level, while the lamp working in pulse mode shows moderate risk, due to the reason that the radiation energies release in a short time range (~2 ms). The spectrum is located in the visible region with the correlated color temperature (CCT) of ~4000 K and the retinal blue light hazard and the retinal thermal hazard are the dominant mechanisms. The Xenon lamp shows high risk level with an ultra-wide spectrum, of which the pulse duration is ~1 ms and CCT is up to ~22400 K. Thus, six types of hazards all can be generated, where the photochemical ultraviolet hazard ES, near-ultraviolet hazard EUVA and retinal thermal hazard LR exceed moderate risk level and ES is nearly two orders of magnitude larger than the emission limit. When the input power of Xenon lamp is turned down, CCT decreases to ~6000 K. The lamp is in low risk level, and the photochemical ultraviolet hazard ES is too weak to be detected. Therefore, Xenon lamps of high CCT and white LED lamps in pulse mode should be used cautiously in video surveillance systems to avoid possible injury. White LED lamps in stroboscopic mode and Xenon lamps of low CCT can be a priority to apply in video surveillance systems.

Atmospheric aerosols have a significant impact on the earth's environment and climate, directly or indirectly affecting human production and life. The high-precision detection of atmospheric aerosol characteristics plays a fundamental guiding role in systematically studying climate and meteorology. By actively emitting laser light and receiving backscattered signals, the atmospheric parameter profile in the detection direction can be obtained, which gives lidar the advantage of high temporal and spatial resolution. Among the lidars, the high-spectral-resolution lidar (HSRL) has high signal-to-noise ratio, and can realize high-precision day and night observation. It has great scientific potential in studying scientific issues such as aerosol transmission mechanisms, cloud-aerosol interaction process, etc. This paper reports a ground-based HSRL based on iodine molecular absorption cell developed by Zhejiang University. The basic principle and hardware system structure of the HSRL are introduced in the paper. This lidar provides vertical profiles of aerosol scattering ratio together with lidar ratio and particle depolarization ratio at 532 nm. In the field observation experiment carried out in Beijing, the lidar was compared with instruments such as sun photometer and Raman lidar. The field experimental have proved that the observation results of the HSRL developed are in good agreement with the sun photometer, which verifies the accuracy of the HSRL's daytime observations. In the comparative observation experiment with Raman radar, the night data has a high degree of consistency, and the HSRL can obtain a better signal-to-noise ratio in daytime, which verifies the great value of the HSRL in atmospheric detection.

Optical Self-Healing Ring (SHR) offers high level of resilience at low cost, since it is often geographically easy to take multiple paths and link them up into a ring with very little extra optical link length. However, the self-healing performance of optical ring networks is limited, and the limitation of self-healing ability of ring networks becomes a problem to be studied. We proposed a conception of the minimum network and three kinds of minimum networks of the Bi-link undirection multiplexing segment protection ring network are found, and through which, the network fault tolerance rate of the two practical applications of all nodes communication and key nodes communication is derived, which provides a reference for evaluating the finiteness of self-healing performance of the Bi-link network, and it is found that the network fault tolerance rate of key nodes communication is related to the ring network distribution of key nodes, and the more closely distributed the network fault tolerance performance is stronger.

Ocean beacon is a type of beacon, mainly used in ocean submersibles or other underwater equipment. It is used to report the current position to the information management when the equipment floats on the water surface, achieving monitoring and recycling of the equipment. The reliability of its performance and the accuracy of positioning information are the focus of research. Research is being conducted to address issues such as unstable water detection, limited transmission distance, and limited communication coverage of existing marine beacon machines. By studying the backup detection of seawater pressure and adaptive communication mode, a Beidou system and Iridium dual mode adaptive ocean beacon was designed. The beacon selects its working mode based on the seawater pressure backup detection method, and adaptively selects the communication mode with the information management platform based on its own position information and the power intensity of the signal for Beidou system. Through practical testing, this beacon can effectively ensure the reliability of equipment water detection, the accuracy of positioning information, and the integrity of information transmission, effectively improving the robustness of the system.

Phase measuring deflectometry (PMD) is a high-precision and low-cost measurement method for specular surface. The simple system configuration and nanoscale measurement accuracy make it possible to integrate the fabrication and testing systems of ultra-precision components, which is of great significance to the improvement of manufacturing efficiency and reliability. In machining, workpiece clamping is a key step before turning. However, repeated the clamping error of the workpieces will reduce the accuracy of manufacturing. Therefore, an alignment method is needed to obtain the actual position of the remounted workpiece to ensure machining quality. A method is proposed to realize non-contact workpiece self-positioning using PMD. A positioning method is proposed with strong anti-noise ability based on PMD in this paper. By adding different centroid positioning errors to the camera target surface, it is verified that the positioning accuracy of the model can achieve a micron level and strong noise immunity. The method solves the problem of workpiece positioning using PMD without increasing the complexity of the system.

Because of factors such as aberration, processing defects, and beam symmetry loss, the three important performances of traditional autocollimators in terms of measuring range, measurement accuracy and working distance cannot be improved at the same time. In this paper, the Risley prisms normal tracing angle measurement system is built according to the method of "normal tracing", and the angle measurement is realized by tracing the normal angle change of the mirror to be measured. When using this method for angle measurement, by rotating the Risley prisms, the measurement beam is incident on the mirror surface in the near-specular normal direction and reflected, so as to realize the trace measurement of the normal direction of the reflective mirror. The experiment shows that when the normal tracing method is used for angle measurement, the system error related to distance in the autocollimator system is greatly reduced. Within the precision range allowed by the experimental environment, the measured data of the normal tracing angle measurement system based on Risley prisms does not show obvious systematic error characteristics.

The optical fiber distributed vibration/acoustic wave sensing system (DVS/DAS) based on phase-sensitive optical time-domain reflection (Φ-OTDR) technology detects vibration/acoustic wave signals along optical fibers by detecting backward Rayleigh scattered light, which has the advantages of long detection distance, high spatial resolution, and wide detection frequency range compared with traditional electronic monitoring systems. In recent years, the pattern recognition technology of DVS/DAS for the purpose of identifying intrusion signals has received wide attention and has been widely used in national defense technology, aerospace, perimeter security, oil and gas pipeline security monitoring and other fields. This paper analyzes and summarizes the feature extraction methods in intrusion signal recognition from four aspects: time domain, frequency domain, time-frequency domain, and space-time domain, and then summarizes the relevant progress of pattern recognition algorithms in the field of intrusion detection of DVS/DAS in recent years from two aspects: machine learning and deep learning.

To effectively extract the underwater laser target echo signal submerged in backscatter, a seawater backscatter filtering method based on an adaptive filtering algorithm is designed in this article. Firstly, the fundamental of the adaptive filtering algorithm is introduced, the LMS algorithm with simple iterative calculation and fast convergence is selected, and a new variable-step LMS algorithm is designed based on the traditional algorithm according to the signal characteristics. Then, to quantify the filtering effect, the SNR ratio at different distances is calculated and compared. The experimental simulation results indicate the effectiveness of the new algorithm applied to the backscattering cancellation.

The traditional linear-motion-based spotlight synthetic aperture sonar obtains the spatial distribution function of the target by sending out a specific bandwidth signal and using stolt interpolation and two-dimensional inverse Fourier transform technology. The spatial resolution in azimuth is limited both by the transmitted bandwidth and by the platform motion angle. The acoustic reflection tomography imaging sonar receives the cross-sectional echo of the target at 360 degrees. The spatial resolution of the acoustic reflection tomography depends only on the bandwidth of the transmitted signal, thus is better than that of the spotlight synthetic sonar. Based on this, a wavenumber domain imaging method of the acoustic reflection tomography is proposed in the paper. In each sub-band, the spatial spectrum is transformed from polar coordinate format to Cartesian coordinate format. The spatial distribution function can be reconstructed by sub band division and further be fused. It not only overcomes the problem that the target distribution function is torus and the imaging precision is low, but also avoids the estimation error and the calculation complexity caused by the torus interpolation in wavenumber domain.

With the continuous deepening of human exploration of space, space control technology is receiving increasing attention. In space in orbit missions, both autonomous approach and close range operations require measuring the relative pose between targets. At present, there have been a large number of research results and applications of visual pose measurement methods in engineering both domestically and internationally. However, research on visual pose measurement for non-cooperative targets is relatively weak. Not only is there a lack of systematic and complete target pose measurement methods, but the response speed and accuracy in engineering applications also need to be further improved. This article proposes a new spatial object pose measurement method based on the Zynq platform, which utilizes the control algorithm and operating system advantages of the processor, as well as the parallel computing and dynamic configuration advantages of FPGA. Through software and hardware collaborative design, highquality data and image acquisition and precise pose calculation are carried out for non-cooperative targets. The algorithm in this paper is optimized and improved in data format, time sequence control, distortion correction, corner detection, feature extraction, stereo matching, Iterative reconstruction, pose calculation, etc. Finally, the target pose measurement method with clear data flow, hardware and software combination, and industrial application is realized. The experimental results show that the proposed method has fast response and accurate calculation results in visual pose measurement of non-cooperative targets.

A Hartmann-Shack wavefront sensor is designed for adaptive optics confocal scanning laser ophthalmoscopy. The Hartmann-Shack wavefront sensor designed consists of a lenslet array with square configuration, sub-aperture size 0.2mm×0.2mm , focal length 5mm and a CCD camera with pixel size 3.75μm×3.75μm.Thedynamic range and measurement accuracy of the HSWFS are simulated through the software MATLAB. Theresult of simulation indicates that focus dynamic range±14λ (λ=635nm), wavefront measurement accuracyreach RMS λ/100, all these indicators reached the requirements of the system. Finally, experimentsofcalibration by spherical wavefront were done on these indicators.

Fluorescence spectroscopy has received attention in the water quality monitoring, as it is a rapid, reagentless technique that requires no sample preparation prior to analysis. It has been investigated as a monitoring tool for a range of applications including water quality and pollution monitoring in marine, rivers and lakes. The permanganate index (CODmn) can reflect the degree of water damage by organic pollutants, and is an important indicator for water environment protection and marine monitoring. In this paper, we present a compact handheld fluorescence spectrometer for water CODmn measurement. The semiconductor laser, spectroscopic unit and detection unit are integrated in the spectrometer, its actual size is about 100mm×60mm×25mm. The spectrometer covers the wavelength range of 400-760 nm, with spectral resolution of 2 nm. The semiconductor laser emitting at 405 nm is employed as the excitation source, which is controlled by the computer. The laser is incident into the water in test bottle at a certain angle in order to avoid the interference come from the bottle wall. The scattered light, water Raman light and organic matter fluorescence excited by the laser can be received by the spectrometer. The CODmn can be obtained according to the method of intensity ratio of laser Fluorescence-Raman signal proposed by us. In the experiment, water samples collected from 9 different sites of Sow River at Weihai are tested by using our designed handheld fluorescence spectrometer. The CODmn can be achieved by the computer analyzing the spectral curve. In addition, we also measured the 9 water samples by chemical method, and the obtained permanganate index compared very well to the values by using our designed spectrometer: a significant relationship (R²=0.941) is identified between the two methods. The experimental results indicate that our designed spectrometer has the advantages of small size, high sensitivity and high detection speed, which is promising for CODmn monitoring in many water areas.

The increasing use of photogrammetry systems in large-scale field testing has put forward the demand for on-site calibration of the system, and the field calibration system was developed to meet this need. The calibration method of hexahedral and tetrahedral combined in this paper is consistent with laboratory static calibration for on-site evaluation of photogrammetry system performance parameters, which are length measurement error and spatial coordinate measurement repeatability.

In this work, we propose a computational Brillouin optical time-domain reflectometry based on time-domain ghost imaging technique. The spontaneous Brillouin scattering time-domain trace of the sensing fiber can be retrieved through differential ghost imaging protocol, so that the system sampling rate requirement can be reduced significantly compared to conventional method. Preliminary experimental results demonstrate that sensing measurement of 50 km can be achieved.

In order to improve the visual reconnaissance ability of unmanned aerial vehicles (UAVs), a method combaning three cameras, namely “Wide-Field”, “Long-Focus”, and “Infrared” cameras, is proposed, in which the wide-field camera simulates the peripheral vision of the human eye, the long-focus camera simulates the human eye gaze, and the infrared camera ensures the robustness of the system in the degraded visual environment (DVE). First, the target’s location is preliminarily determined according to the visual salient information of the infrared image; then, based on the multi-scale image fusion method, the infrared/visible fusion images of the target area are obtained; finally, YOLOv5 and improved SORT algorithm are utilized to complete target detecting and tracking. To verify the effectiveness of the proposed method, the images are rendered in the Unity3D engine, and the simulation experiments of UAV reconnaissance are carried out. Results show that, the performance of image fusion algorithm proposed in this paper is close to the typical methods like ADF and VSMWLS in the sence of the evaluation indicators such as CE, AG, SD, FPS. However, the proposed method only costs about 1/5 of the calculation time.

We propose and implement a source-sharing configuration based on fiber-optic gyroscopes for differential-mode and common-mode measurements. By adopting suitable values of the modulation parameters, the effect of relative intensity noise and thermal phase noise can be suppressed. Experimental results show that this configuration can achieve a maximum of 5.78 times self-noise suppression. We also show that the deviation of the modulation frequency from the eigenfrequency or its harmonics leads to an increase of the self-noise level. A deviation of 400 Hz can lead to a degradation of the self-noise level by up to 2.38 times. Finally, we prove the ability of suppression the environmental disturbance through an observation with the effect of natural vibrations.

Raman scattering distributed fiber optic temperature sensing technology has been widely used in many fields such as oil and gas transportation, public buildings, and fire monitoring of electrical equipment. However, the measurement performance of this technology is influenced by various factors at extremely low temperatures. In this study, a low-temperature experimental platform was independently built, and a 30-meter fiber optic was wound on a cryogenic cold head. The Raman scattering distributed fiber optic temperature sensing equipment was used to compare with the Cernox temperature sensor, and the low-temperature sensing performance of the fiber optic in the range of 5K to room temperature was examined. The experimental results showed that within a certain temperature range (room temperature to 65K), the system could provide accurate distributed temperature measurement. However, at low temperatures (65K to 5K), the increase in fiber optic stress and the decrease in Raman scattering intensity led to a reduction in signal-to-noise ratio, resulting in a sharp increase in measurement error and a significant impact on the measurement validity of the system. This study provides useful references and insights to further improve the measurement accuracy and stability of the Raman scattering distributed temperature sensing system at extremely low temperatures.

A vibration optic fiber sensing system with tunable sensitivity based on Sagnac interference is proposed and demonstrated. In virtue of the linear Sagnac interference structure, vibration sensitivity adjustment could be realized by changing the delay fiber length. The phase modulation depth on sensing fiber induced by intrusion disturbance is heavily influenced by the delay fiber length, which reflected in the intensity detected by the AC photo-detector, thus enabling the vibration sensitivity adjustment in optical sensing system. The relationship between the amplitude of intrusion disturbance and delay fiber length shows a sinusoidal function in absolute value.

The accurate and adjustable structural model of hollow-core photonic bandgap fibers (HC-PBFs) is crucial for design and fabrication of high-performance HC-PBFs. The traditional modeling method based on air holes or silica struts has non-negligible structural differences and few adjustable parameters degree of freedom. The 2D structural modeling method of HC-PBFs based on feature points location is proposed. The ideal structure and common distorted structure of HC-PBFs are constructed by this method before fiber fabrication. The scale and scope of irregularities and distortions are continuously tunable. Meanwhile, the realistic HC-PBFs structure and homemade HC-PBFs are reconstructed by this method after fiber fabrication. The universality, robustness, and accuracy of this method are verified successfully according to the result of contrast between model and realistic structure. According to the simulated result from this model, the modification could be conducted precisely before and after fiber fabrication. Thus, the overall manufacturing process of HC-PBFs is considerably accelerated.

A mode interferometer coated with polydimethylsiloxane (PDMS) film based on photonic crystal fiber (PCF) is embedded into the fiber ring laser (FRL) sensing system for temperature and relative humidity measurement. High optical signal to noise ratio (OSNR) of ~54 dB, narrow 3 dB bandwidth of ~0.26 nm, and high-quality factor Q of 6×10³ were obtained, which indicate that the designed FRL system has ability to realize the measurement of environmental parameters with high resolution and accuracy and has potential application in remote monitoring of temperature. As the temperature increased from 35 °C to 115 °C, the spectral response has wavelength blue-shifts trend. The average temperature sensitivity of temperature sensor based on PCF is 90 pm/°C with small error bar and excellent linear correlation coefficient of 0.992. The average temperature sensitivity of temperature sensor based on PCF is 89 pm/℃ during the cooling process, revealing the proposed sensor has excellent reversibility. In addition, the sensor is insensitive to the change of relative humidity. Furthermore, the stability of temperature sensor based on PCF was experimentally demonstrated and analyzed. The maximal dip wavelength shifts of the peaks at 45 °C and 95 °C were 0.074 and 0.075 nm, respectively, within 3 hours. Besides, the FRL temperature sensor based on PCF has simple structure, excellent reversibility, repeatability, and stability. Therefore, the proposed FRL temperature sensor based on PCF has a great potential in practical applications where the high measurement precision and accuracy is required such as chemical reaction.

Weak fiber grating sensing technology has started to be applied in various fields, and its introduction into landslide monitoring can realize comprehensive monitoring of points, lines and surfaces. In this paper, on the basis of introducing the principle of weak fiber grating, weak grating displacement sensor, inclination sensor, weak grating optical cable and weak grating demodulator are combined to build a weak grating sensing real-time monitoring system, and finally a weak grating real-time monitoring application test of a landslide is given. The test results show that the weak fiber grating sensing real-time monitoring system can achieve real-time monitoring of landslides, and its experience can be extended to other landslides of weak fiber monitoring.

Theoretical and simulation analyses of fiber optic extrinsic Fabry-Pérot interferometer (EFPI) based on a commercial collimator are studied. Fringe visibility and interference intensity are evaluated quantitatively under different cavity lengths, different tilt angles, and different mirror reflectivities. The analysis can be used to optimize the design of the EFPI.

Fiber optic temperature sensors for small-scale heat sources have an urgent need in many industrial fields. We first propose and demonstrate an ultra-short fiber optic temperature sensor based on a Fabry-Perot (FP) interferometer which combines a fiber Bragg grating (FBG) with Fresnel reflection on the fiber end face. The FBG with a grating length of 50 μm was inscribed by a femtosecond laser line-by-line scanning technology. This FBG exhibits a resonant wavelength of 1548.8 nm and a broad full width at half maximum (FWHM) bandwidth of 12.4 nm. The selected dip wavelength of the FBG-FP cavity sensor changes linearly with the temperature increase, corresponding to the temperature sensitivity of 10.8 pm/°C. Moreover, the FBG-FP cavity sensor can be used to measure the temperature of the small-scale laser spot due to its compact structure. The experimental results show the temperature at different positions of laser exposure area are 138.4 °C, 111.5 °C and 74.4 °C, respectively.

Pipelines play an indispensable and vital role in modern society, facilitating the transportation of gas, liquids and other energy sources. Therefore the safety of pipelines has become a growing concern in recent years. In response to this challenge, we have developed a vibration device based on the microfiber loop resonator (MLR) structure, designed for monitoring pipeline intrusion. The MLR coupling area is securely fixed on an aluminum substrate using PDMS, which is then installed onto the pipeline. When external intrusion occurs and causes pipeline vibration, the suspended microfiber loop rapidly responds to the oscillation, causing its effective refractive index to change. Through detailed analysis of the generated signals, we can accurately identify intrusion situations and categories, providing valuable insights into potential threats to pipeline integrity. Our experimental approach involves subjecting the pipeline to a fixed-frequency vibration using a standard vibration source. Subsequently, we concentrate on studying the signals generated by different types of pipeline intrusions, such as climbing, knocking and shaking. These comprehensive investigations allow us to establish a robust model, facilitating reliable and real-time monitoring of the pipeline's vibration state. The experimental results have been in agreement with the theory, underscoring the method's high reliability and sensitivity in detecting pipeline vibrations accurately.

Fiber optic gyro inertial navigation system (FOGINS), as a kind of navigation equipment with high accuracy, wide dynamic range, and simple mechanical structure, is widely used in moving platforms such as vehicles, aircraft, and ships. For some motion platforms, such as vehicles and aircraft, the FOGINS can be provided with a stationary state so as to realize the initial alignment under static base conditions. However, some motion platforms, such as ships and floating balloons, are always in a swaying state and cannot provide a stationary state for the FOGINS. In such special motion platforms, only the dynamic base initial alignment technique can be implemented. This paper proposes a moving base simulation method, through the simulation, and observed the mainstream initial algorithm effect under different conditions, and categorized to make the relevant analysis, provides a number of engineering guidance strategy, through these strategies, can effectively avoid huge errors of initial alignment, which affect the subsequent positioning accuracy.

Traditional Brillouin optical time domain reflectometer (BOTDR) generally uses frequency scanning to obtain the Brillouin gain spectrum. And the measurement speed of the sweeping frequency (SF) method is slow, usually on the order of minutes. In this paper, the mechanism of rapid BOTDR measurement based on sloped-assisted (SA) technology is analyzed, the measurement frequency of SA-BOTDR is theoretically calculated, and the system scheme is proposed and designed. Through experimental research, the temperature measurement range of the system is 25°C~70°C at the end of the 203m sensing fiber. The temperature measurement accuracy is 1.38°C, the spatial resolution is 1.21 m, and the measurement frequency is 11.49 Hz.

Fiber-optic surface plasmon resonance (SPR) sensors have the advantages of high sensitivity and fast response. Traditional single-channel sensors cannot fulfill practical detection needs, while multi-channel fiber-optic SPR sensors can achieve a diversity of analyte determinations. Therefore, developing a multi-channel fiber-optic SPR detection system is significant for field detection. To solve the issues of single-channel detection and lack of reference channels in fiber-optic SPR, we propose a multi-channel fiber-optic SPR real-time monitoring system. This system uses the LabVIEW platform for system design. It can realize the function of multi-channel dynamic monitoring. The experimental results showed that the four independent channel has similar sensitivity. The result confirmed the multi-channel SPR sensors have excellent stability and repeatability. To further validate the reliability of the multi-channel sensors in bio-detection, the different concentrations of ConA solution were tested. The constructed multi-channel SPR sensing system has an excellent linear response in the 0.05~0.2 mg/mL range. The wavelength shift of a single channel could reach 0.58~1.55 nm in the range of 0.05~0.2 mg/ml with a linearity of 0.98, which verified the biochemical detection capability of the system. Furthermore, the ConA solution of the same concentration was used to test sensitivity for different channels. The experiment shows that the four independent channels have similar resonance wavelength shifts (0.772 nm, 0.814 nm, 0.7968 nm, and 0.799 nm, respectively). The proposed multi-channel fiber-optic SPR sensing system has applications in the field of multi-analyte detection.

In order to monitor the safety of the whole cable in real time and effectively, this study introduces and adopts distributed optical fiber temperature sensing (DTS) technology as the method of cable safety monitoring. A variety of statistical results are used as the basis for judging the running state. Not only does the whole system have no blind zone of safety monitoring, but also provides accurate alarm information and fast response, which enables managers to deal with faults timely and avoid heavy losses. Therefore, the DTS technology has a high promotion value in the field of cable safety monitoring.

This paper proposes a low-power, wide demodulation range solution for Fiber Bragg Gratings (FBG) based on voltage and current dual-wavelength tuning of VCSEL lasers. The paper first explains the principle of the demodulation system and then discusses the impact of driving voltage, current, and temperature on the emission wavelength of the laser. By investigating the wavelength scanning overlap range of the laser at different temperatures, the wavelength range of the FBG sensor is determined to be 4.5nm. The experimental results show that the demodulation system exhibits good wavelength repeatability, with the wavelength repeatability of all three channels within ±3pm. Furthermore, the overall power consumption of the system is only around 189mW, indicating that the demodulation system possesses the low power consumption characteristic and can be powered by portable power sources.

A sensor for dual-parameter sensing of strain and temperature by dual fiber Bragg gratings (FBGs) structure was proposed and demonstrated. The structure with a traditional FBG (FBG-1) and a strain-assisted FBG (FBG-2) and were fabricated by femtosecond laser phase mask technology. The sensor has shown a positive wavelength sensitivity in temperature and strain, whose are 11.39 pm/°C and 0.5824 pm/με for FBG-1, and 11.34 pm/°C and 0.5831 pm/με for FBG-2, respectively. Meanwhile, the reflection intensities of two FBGs have exhibited a linear change in strain, and a sensitivity of 1.7275×10^-4 dB/με and -2.018×10^-4 dB/με are achieved. Moreover, the sensor has merits of simple fabrication, compact size and high reflectivity, which has shown a good prospect in multi-parameter sensing fields.

To meet the high-precision positioning requirements of dam health monitoring,this paper designs a high spatial resolution distributed Brillouin dam health monitoring scheme based on DPP-BOTDA （Differential Pulse Pair Brillouin Optical Time Domain Analysis） technology.Due to the large pulse width and strong pulse energy of the two differential pulses used in the Brillouin system, the high-precision measurement of Brillouin frequency shift can be achieved; Moreover, the difference in pulse width between the differential pulse pair is small, which can achieve high spatial resolution and meet the high-precision positioning and measurement requirements for dam health monitoring. The rise and fall time of the pulse signals is a key factor affecting the spatial resolution of DPP-BOTDA systems. To achieve centimeter level spatial resolution, this paper designs a SOA (semiconductor optical amplifiers) narrow pulse driving circuit that can generate pulse signals with rise and fall times less than 1 ns (ps level pulse edge). An experimental setup for the DPP-BOTDA system is built. The SOA narrow pulse driving circuit mentioned above is used to generate pulse signals with pulse widths of 48ns and 50ns, respectively, for differential Brillouin tests. By analyzing the spatial resolution test curve of the system with the differential pulse signals demodulation, it can be concluded that the system can achieve a spatial resolution of 0.2m. It meets the high spatial resolution requirements for dam health monitoring and is of great significance in the field of distributed Brillouin fiber optic sensing applications.

In inertial confinement fusion (ICF) systems, damage induced by high power lasers is one of the key factors for limiting the further improvement of energy density. Also, there is a close relationship between various defects and damage characteristics of optical components. Usually, the optical components detected in the system have a large-aperture and a variety of surface types. The defects have the characteristics of small scale, low distribution density and wide distribution range, etc. It is a great challenge to achieve multi-faceted detection with high precision and efficiency. In this paper, according to the requirements for micro defects detection on the surface of large-aperture optical components, a multimodal scanning detection method combining the bright-field and dark-field imaging (BDFI) is given. Based on the high sensitivity of dark-field microscopic imaging (DFMI), the ring illumination strategy of collimate light source is studied to locate defects globally. The coaxial illumination method is used to accurately detect the characteristic parameters of defects, which is able to realize the size quantification of multi-morphological defects. At the same time, in order to cope with the detection needs of large-aperture and multi-faceted optical components, the sub-aperture scanning method based on the distribution structure of geodesic sphere is proposed. The multi-aperture scanning path is automatically planned and the spatial distribution of defects is reconstructed with three-dimensional mapping. On this basis, by cooperating with the automatic motion device, automatic focus and high-speed automatic scanning of detection can be realized. The methods proposed in this paper can be accomplished for efficient detection of large-aperture optics with a resolution of 0.5μm for surface defects, which meets the needs of multi-faceted detection. Therefore, it is of great significance to guide the precision machining of optical components and support the process optimization, which has great importance for the final research results of the ICF system.

Distributed fiber optic sensing technology is based on Raman scattering and optical time-domain reflection technologies, and consists of two parts: the demodulation host and the fiber optic cable. The system obtains the structural changes of the building through the sensing fiber optic cable, such as sensing the strain, crack, tilt, settlement and other specific changes of the dam and other buildings. By calculating the structural safety condition of the building, the system determines the structural safety level of the dam and other buildings according to the judgment standard of structural safety of the dam and other buildings and the subsequent treatment and remedial measures. Realize the long-term online monitoring of the structural safety of dams and other buildings, and provide long-term sustainable structural safety guarantee for dams and other buildings.

An antiresonant waveguide interference fiber sensor was created using single-mode fiber and hollow-core fiber. To boost its refractive index sensitivity, femtosecond laser processing was employed. The sensor involves splicing a hollow-core fiber with an inside diameter of 15 μm onto the single-mode fibers. The environmental refractive index was estimated by monitoring the movement of the resonant peak wavelength Dip2 based on the sensitivity of the resonant valley of the sensor transmission spectrum to changes in the external environmental refractive index. When the resonant peak wavelength is near 1450 nm, the maximum refractive index sensitivity is 924.2 nm/RIU, according to the experimental data. To improve the sensitivity of the fiber sensor to the surrounding refractive index, we inscribed nine microscopic grooves on the surface of the hollow-core fiber with a width of 30 μm and a depth of just 10 μm by femtosecond laser processing, with a spacing of 330 μm between each groove. To retain the robustness of the fiber sensor, microscopic grooves with very small width and depth were created. The profile of the initial spectrum remains unchanged after the grooves are etched, while the intensity and position of the resonance peaks alter less. The refractive index was measured again, and the experimental results showed that when the wavelength of the resonant peak was near 1450 nm, the maximum refractive index sensitivity was 3195 nm/RIU, and the sensor's refractive index sensitivity was improved by 3.46 times compared to before the femtosecond laser processing.

In this work, a novel tunable diaphragm structures are designed as sensing diaphragms for ultrasonic sensors. The natural frequency and sensitivity of the new structure diaphragms is simulated by finite element method. The proposed membrane structures is outer-ring clover (ORC) structure. Typically, an increase in membrane sensitivity is accompanied by a decrease in its natural frequency. However, this can be significantly mitigated by the proposed structures, which can exhibit notable structural optimization effects by simultaneously enhancing sensitivity and minimizing the reduction of the natural frequency. The simulation results indicate that, when the outer diameter and thickness remain unchanged, the sensor can achieve a maximum sensitivity improvement of 3.5 times and a reduction of 1.25 times in the natural frequency compared to the circular diaphragms (CD). Furthermore, ORC can be fabricated by micro-electromechanical systems (MEMS) technology, making it feasible for practical manufacturing and production. The sensing membrane is expected to be applicable in detection fields such as ultrasound partial discharge and underwater acoustics.

In this paper, we propose a high fidelity phase-sensitive optical time-domain reflectometer (Φ-OTDR) system based on compact and flexible multi-frequency probe pulse modulation. Single-frequency continuous light is multi-frequencymodulated by a broadband acousto-optic modulator (AOM) loaded with multiple microwave signals of different frequencies, finally, multiple Rayleigh backscattered (RBS) optical signals with different intensity distributions can be obtained by a single acquisition, and the most accurate signal is always selected for phase reconstruction to achievethehigh fidelity Φ-OTDR system, which has the advantages of compact structure, precise control of phase delay, flexible and controllable frequency components, and no sacrificing response bandwidth and spatial resolution, etc. In the experiment Φ-OTDR system, we simultaneously modulate the multi-frequency probe pulse light with a width of 100nsand three non-equidistant frequencies, and inject it into a 2km sensing fiber. The RBS light signal multiplexing results show that the probability of interference fading effect in the system is reduced from 17.541% to 1.123%. And the high-fidelity phase information of a 100Hz simulated vibration signal was extracted on a 3km sensing fiber, corresponding to a strain value of about 11.9nε.

Phase sensitive optical time domain reflectometer (φ-OTDR) can retrieve vibration waveforms based on linear relationship between phase change and external events. Yet, it is difficult to identify different events due to the complexity of working environment. How to accurately determine the type of vibration events and thus reduce false alarm rate is important in many practical engineering applications. The existing deep learning (DL) algorithm can directly extract the original data feature, without manual extraction. Hence, DL is usually used to classify and recognize multiple events in φ-OTDR. In this work, a dual input deep convolutional neural network (Di-DCNN) is applied to distinguish six kinds of actual vibration events (including walking, tapping, blowing and raining, vehicle passing, digging and background noise). The features of these two inputs are extracted, respectively, and fused together to identify six vibration events. For comparison, network models with other five inputs are employed for event recognition, including single input of 1D time-demain or 2D image of phase (amplitude) data, and dual input of 1D time-demain and 2D image of amplitude data. Here, the 2D image denotes the transformation of 1D data by Markov Transition Field (MTF). Experimental results show that the Di-DCNN with 1D time-domain phase waveforms and its 2D MTF image being the two inputs considerably improve the recognition accuracy. The average recognition rate of six kinds of vibration events is higher than 94%.

Phase sensitive optical time domain reflectometer (Φ-OTDR) has the advantages of higher sensitivity and spatial resolution, longer monitoring distance and multi-point positioning, and has been widely used in dynamic sensing fields such as perimeter security, rail transit and pipeline monitoring. Based on the Radial Basis Function (RBF) neural network, we extract, recognize, and classify the various constructed features of fiber optic vibration sensing signal including time domain, statistical domain, frequency domain. We accurately identify and classify the no intrusion events, shaking events, crossing events, and knocking events, with an average recognition rate of 93.85%. Our work points out a new direction for improving the recognition accuracy of Φ-OTDR systems for disturbance signals and decreasing false positive rate.

The linewidth of a single frequency laser plays an important role in improving the detection distance of a distributed optical time-domain reflectometer (OTDR). The commonly used linewidth measurement method is the delayed self heterodyne interferometer (DSHI). However, it is usually difficult to calibrate the linewidth of fiber laser (less than 1 kHz) with DSHI， because which requires the optical path difference of interferometer to be far greater than the coherence length. We accurately characterized the laser linewidth based on the sine-cosine optical frequency analysis of the 3×3 coupler Michelson interferometer. First, we accurately and rapidly detected the change of optical frequency under the stable state of laser, and then analyzed and accurately obtained the linewidth through the power spectral density (PSD) of frequency noise. The linewidth of the 1550 nm single frequency fiber laser is accurately detected and with 0.8 kHz, which is close to the technical parameter table with the linewidth of 1 kHz. Our work is important for selecting the light sources in φ-OTDR and other applications and points to a standard and convenient method in the field of the linewidth measuring.

To mitigate the failure of a Hilbert transform-based fixed-threshold multi-peak demodulation algorithm in distributed Fiber Bragg Grating (FBG) sensing systems with light noise, an adaptive multi-peak demodulation algorithm based on the Hilbert transform is proposed in this paper. The algorithm adopts the thought of Divide and Conquer, including reflection spectrum segmentation and single peak detection. Multiple sub-spectra of the FBG multi-peak spectrum is segmented by the Hilbert transform and calculating adaptive threshold method, and then the Gaussian fitting algorithm is used to achieve the precise positioning of the spectral peaks. Theoretical analyses and simulation results show that the algorithm using an adaptive threshold as a threshold of splitter can greatly improve the portability of the algorithm in complex situations, and improve the algorithm’s noise resistance, demodulation speed, and accuracy. It shows that the algorithm provides a new way of demodulation for distributed FBG sensing systems.

A Fiber Bragg Grating (FBG) based force sensor with high-sensitivity and temperature compensation capability. It can be used to accurately measure the contact force (CF) between medical equipment and tissues or organs in minimally invasive surgery (MIS). To achieve temperature compensation, a novel design incorporating dual elastomer-based dual FBGs is employed, and its effectiveness is verified through finite element simulation. After temperature compensation, the developed sensor has a sensitivity of 54 pm/N, a resolution of 0.019 N, and a root-mean-square error (RMSE) of 0.064 N. The utilization of temperature disturbances for measuring distal contact force demonstrates a substantial enhancement compared to direct measurement methods. Additionally, in vitro experiments are conducted using a silicone vascular model, demonstrating the sensor’s capability to detect certain alterations in vascular pathways. Finally, a simulated palpation liver experiment confirmed that the designed force sensor can preliminary detect of the relative hardness of the tissue based on the magnitude of the CF. In vitro experiments have proved that the designed FBG force sensor has a practical value in MIS.

To address the issue of massive data in distributed acoustic sensing system, we proposed using the ultra-low sampling resolution and undersampling techniques to reduce the data volume last year. However, the fading noise of the 1-bit undersampled signal becomes difficult to be completely removed with conventional denoising methods. In this paper, we proposed an integrated fading suppression algorithm to solve this problem. Experimental results show that the proposed approach can completely eliminate the fading noise of the 1-bit undersampled signal, while the vibration signal-to-noise ratio is also enhanced by 7.72 dB.

Broadband and multi-wavelength laser is an emerging research direction with promising future applications. The optical parametric oscillator has the characteristic of wide tunable wavelength and the zinc sulfide polycrystal doped with Cr²⁺ ions have a wide absorption spectrum. Combining the advantages of both output and input, a unique cascade structure using optical parametric oscillator as pump source is constructed for Cr²⁺:ZnS laser resonator. The output advantages of the cascade laser are studied by analyzing the characteristics of the continuous wave and pulse laser output from the experimental device. The experimental system successfully outputs a laser with a wavelength of 2330 nm and an average power of over 510 mW. The experimental system realizes the laser output of four wavelengths over 2000 nm in the three spectral regions of near infrared, short wave infrared and medium wave infrared. The experimental results provide a new idea and reference for differential absorption measurement and multi-component material measurement in toxic gas molecular detection and optical measurement.

OPGW power overhead fiber optic cable suffers from fiber core aging, ice-covering dance and stress damage in long-term operation due to laying method and geographical environment. The monitoring of multi-fiber parameters such as vibration, strain, temperature and attenuation based on distributed fiber optic sensing can realize early warning of malfunction. In order to solve the problems of multiple deployment devices, occupying multiple core resources and high operation and maintenance costs of existing multiparameter measurement technologies, the fusion sensing technology based on ΦOTDR, BOTDR and OTDR is studied. The system employs multiplexing of light sources, two-way coherent reception of Brillouin scattered light and Rayleigh scattered light, and joint demodulation of phase and intensity to achieve distributed measurements of vibration, temperature, strain and long-range attenuation measurement on a single fiber. The system achieves linear reduction of 100Hz perturbation signal with 10m spatial resolution and 80kmsensingdistance, and the signal-to-noise ratio of demodulated vibration signal is about 35dB, and the accuracy of temperature measurement is about ±1°C. For the practical application of OPGW overhead optical fiber cable, the mathematical model of the inherent vibration frequency of the cable and the over-ice tension as well as the stress calibration method of the pole and tower joints are studied to realize the monitoring and early warning of the over-ice thickness of optical fiber cable and the decoupling measurement of BOTDR temperature/strain. The error between the demodulated ice thickness and the actual ice observation result is less than 10%, and the decoupling accuracy of temperature and strain is ±2°C and ±40με respectively.

The demodulation parameter optimization of fiber optic acoustic sensor for broadband large signals is studied in this paper. Firstly, the performance of fiber optic acoustic sensors in demodulating broadband large signals is introduced. The low-frequency energy of the demodulation result is significantly increased and completely submerge the effective signal. By deducing and analyzing the demodulation algorithm of fiber optic acoustic sensors, the reason why it is impossible to demodulate broadband large signals is theoretically explained. An arctangent demodulation algorithm is realized in MATLAB. White noise signals with limited frequency bands are used as simulation signal sources for demodulation testing. Through simulation analysis, the main criteria for determining the upper limit of the system's dynamic range under a certain set of demodulation parameters are provided. Finally, the arctangent demodulation algorithm is implemented in FPGA. Detection experiments on the noise signal of a certain equipment are carried out using the fiber optic microphone system developed by our institute. The experimental results prove that the dynamic range analysis of the demodulation algorithm in this paper is accurate. The fiber optic microphone system designed through parameter optimization can correctly demodulate broadband large signals.

The optical frequency domain reflectometry (OFDR) demodulation technology based on DBR laser and spectral splicing was proposed in our previously studied. Spectral splicing solves the problem of laser mode hopping that is not allowed in the original OFDR demodulation technology. However the detailed analysis of the influence of spectral splicing on the spatial resolution of OFDR didn’t been provided. Based on previous research, the influence of spectral splicing on the spatial resolution of OFDR was analyzed in detail. The effects of single-segment wavelength range, number of spectral splicing segments, and splicing error on spatial resolution of OFDR were simulated. The results indicate that the spatial resolution of OFDR after spectral splicing is determined by the single-segment wavelength range, and it has the same spatial resolution as demodulating only one section of the spectrum. Meanwhile, the spatial resolution is independent of the number of spectral splicing segments and splicing errors (within 10 pm). Therefore, in order to improve the spatial resolution of OFDR, the single-segment wavelength range should be expanded as much as possible. Finally, experimental testing was conducted to verify the effectiveness. 35 spectral segments were achieved using a DBR laser, with each segment having a wavelength range of 1.2nm. By identifying the peak width of the fiber optic connector, it was verified that the spatial resolution of multi-segment spectral splicing and single-segment spectral is the same. However, experiments have also shown that spectral splicing weakens the measurement noise of the system, which is one of its advantages.

Wavelength-tuning phase-shifting interferometry has advantages over conventional hardware phase-shifting for the simultaneous detection of all surfaces of the transparent parallel flats. The Fourier transform and its derivative methods can be used to characterize the signal based on the optical path difference and the phase-shifting value of the corresponding interferometric harmonic signals on each measured surface and to solve for the phase accordingly. Therefore, considering the influence of the sampling length and the choice of the window function in the Fourier transform on the measurement results, based on the established multi-surface interference model, an adequate simulation analysis is carried out, and the corresponding explanations and discussions are given subsequently. In practical measurements, the appropriate window function and sampling length should be selected according to the corresponding measurement conditions, so that reliable measurements can be guaranteed.

The Optical Frequency Domain Reflectometry (OFDR) is widely used in fields such as aviation temperature detection and integrated circuit processing and manufacturing. This paper proposes a large range OFDR distributed temperature sensing method based on the frequency sampling method and the reference spectral moving method. The frequency sampling method is employed to compensate for the tuning nonlinearity effect of the tunable laser source in the OFDR system, ultimately achieving a spatial resolution of 0.2mm on a fiber under test of 11.1m. However, when the temperature of the fiber under test exceeds the effective range of the system, the phenomenon of the traditional cross-correlation method fails to correctly demodulate the temperature occurs. Therefore, demodulating the spectral shifts by moving the reference Raleigh scattering spectrum. Experimental result shows that by using a narrow sweep range of 4nm, the reference spectral shifting method can stably and reliably measure a temperature range of 50°C to 340°C with a high spatial resolution. In comparison, the traditional cross-correlation method can only test the temperature range of 50°C to 150°C. Therefore, the reference spectral moving method extends the effective temperature measurement range by 2.9 times.

Traditional production logging techniques are greatly affected by intermittent liquid production in low flow rate and unstable production situations. And the commonly used turbine structure testing instruments are impracticable in complex conditions such as small diameter casings and wellbore fluids. In response to those challenges, a new optical fiber testing system combines traditional instruments with distributed optical fibers was developed. The system integrated fiber and power cores into an optical-electrical composite logging cable and features a sealed connector to achieve high-pressure insulation and separation of the optical and power units in the cable. Moreover, Technique to mitigate the effects of Fresnel reflection was used at the end of small-diameter optical fibers. The signal loss or distortion zone caused by the reflection was reduced to 3m. In addition, a testing process for optical and electrical integration system in casing tubing annulus was developed, allowing for simultaneous traditional logging and optical data acquisition in a single trip into the well. The new integrated logging technique has been applied; the accuracy of the optical fiber depth correction is less than 1.5m by using magnetic positioning curve to ensure accurate depth. Low-frequency Distributed fiber Acoustic Sensing (DAS) data was used to evaluate production profile of the well. The results from optic fiber were validate via traditional production logging data, indicating that fiber optic monitoring technology is feasible for evaluating production profiles of low-yield oil and water wells.

In the process of aerial refueling test flight and autonomous aerial refueling, it is necessary to measure the high-precision motion parameters of the Taper sleeve relative to the oil-receiving probe to provide data for the docking process. In this paper, aiming at the problems of intelligent identification and tracking of aerial refueling targets and high-precision stereo vision positioning, a multi-layer convolutional neural network with visual characteristics was constructed by deep learning theory, and the recognition results of Taper sleeve were corrected by using frame regression algorithm, so as to improve the Taper sleeve positioning accuracy from three dimensions: identification and tracking, optical calibration and measurement and solution. In this paper, combined with the test and flight verification, the solution accuracy is better than 0.09%, the identification success rate is better than 98%, and the Taper sleeve positioning accuracy is better than2cm+0.15%*L, which accords with the positioning accuracy of the refueling taper sleeve in the flight test.

Aiming at the need of detecting trace dissolved characteristic gases in transformer oil, technology of dissolved gas analysis (DGA) in oil based on a T-type photoacoustic (PA) cell with a small capacity was proposed. The capacity of the T-type PA cell was reduced to 30 mL by optimizing the parameters. This resulted in a significant reduction in sampling volume of transformer oil. A laser photoacoustic spectroscopy (PAS) system for DGA in oil was built combined with headspace degassing method. The T-type resonant PA cell was used to enhance PA signals. The second-harmonic wavelength modulation spectroscopy (2f-WMS) technology was used for detection of escaped gases. The experimental results show that the detection limit of acetylene (C2H2) gas dissolved in oil is 0.5 μL/L when the oil volume is 50 mL. The technical scheme has the advantages of small oil extraction volume, high precision and fast response speed.

The electromigration problem in flip chip becomes one of the focuses of the microelectronic device durability. Temperature is a key parameter for the electromigration life evaluation, due to the occurrence of material structure defect will be accelerated with the rise of temperature. But it is difficult to measure the temperature inside the flip chip packaging structure of large-scale integrated circuits directly with traditional test means. In this paper, a distributed temperature profile test method in interconnect solder joints flip chip has been present, which is measured by the optical-frequency-domain reflectometry (OFDR) with telecom single model fiber. The most distinguishing feature of this method is that the thin flexible optical fiber can directly penetrate into the flip chip from the position of the interconnection solder joint to realize the distributed sensing of the temperature field of the solder joint inside the chip. The modulated linear sweep light directly injected into optical fiber and transmitted forward, and a certain interference pattern formed by back Rayleigh scattering is generated. When the temperature environment of the optical fiber changed, the interference pattern formed by back Rayleigh scattering will change accordingly, which will cause the wavelength shift of the interference pattern, that is similar to the fiber grating effect. Thus, the distributed temperature change can be demodulated from the wavelength shift. The experimental results show that this method can realize the distributed measurement of the internal temperature of flip chip directly, and provides a novel solution for more accurate evaluation of electromigration effect.

To identify the object edges and corners, silicone rubber embedded with a fiber Bragg grating (FBG) sensor is used as the manipulator skin. The Bragg wavelength-shift of FBG appears under axial strain. The strain distribution in the skin is analyzed by the simulation of edge and corner contact. The contact situation is determined by the observation of the wavelength-shifts and power changes of the FBG. This method can help identify and distinguish object edges and corners. This method has a wide range of potential applications and can be used for robot tactile sensing. It is beneficial to the advancement of bionics.

Aiming at improving the performance of distributed sensing fiber, a distributed strain sensing optical cable based on ultra-weak fiber gratings is proposed. The optical cable is prepared by pretreating an ultra-weak fiber grating array with a pitch of 0.2 m, compounding it with glass fiber to make an FRP sensing fiber with a diameter of 1.0 mm, and then twisting with a steel cable and extruding the sheath. The multilayer structure strain transfer model of steel cable-FRP-optical fiber is established, and the temperature characteristics of the optical cable were tested by the water bath method, the tensile test of the optical cable was carried out by the tensile machine. The results show that the temperature sensitivity of the optical cable is 23.8 pm/°C, the strain sensitivity is 1.72 pm/N, there is no "strain dead zone" during strain tension, and the strain transfer effect is good, which provides a potential solution for high-resolution distributed strain sensing.

In view of the high cost, slow response and insufficient detection limit of traditional detection methods, this paperproposes a high sensitive fiber sensor based on magnetic ion imprinting technology for the detection of lowconcentration of Cd²⁺. First, the evanescent field and waist cone diameter are analyzed by beam propagation method(BPM), and the sensing unit is prepared. Second, γ-Fe₂O₃@MAA(M-IIP) magnetic ion printing nanomaterials wereprepared by solid-liquid separation and combined the magnetic blomaterial with the sensing unit by silanlation. Finally, the sensor enables a low-concentration detection of Cd²⁺. The detection sensitivity was up to 2691.5nm/μMin the rangeof 0-1 μM. The limit of detection (LOD) reached 0.45 nM and the response time was 37s at the lowest concentration. At the same time, the sensor also has good results in terms of structural compactness and production cost.

Some single-mode fiber and its elements are used in low-polarization and polarization-maintaining hybrid path of fiber optic gyroscope. As the development of the miniaturization of interferometric fiber gyroscope, in some cases during the fiber optic gyroscope (FOG) light path assemble, the bending radius of optical device pigtails has to be bend with a small radius because of the compact structure requirement, particularly in single-mode pigtails. In single-mode optical fiber, the transmitted core-guided mode light and whispering gallery mode light will interfere with each other, and present influence on the transmitted light. In the precious investigations, the wavelength is fixed to 1550nm to measure the power loss with different bending radius of single-mode fiber. Thus, in this paper, a three-in-one integrated light source with the wavelength 1310nm is used for the investigation. The effect of single-mode fiber with bending radius from 4mm to 20mm and winding number from 10-50 will be investigated. It shows that single-mode fiber bending can lead to the transmitted light mean wavelength drifting and oscillating as the temperature ramping.

The impact of attitude deformation of towed linear array and the fitting error of point attitude sensors on beamforming. We studied All-fiber acoustic-posture integrated towed linear array based on distributed feedback fiber laser (DFB-FL) hydrophone and optical frequency domain reflectometry (OFDR) distributed attitude measurement. Analyzed the principle of towed array direction finding, and the influence of towed array attitude deformation on directional error was analyzed. Designed an 8-element hydrophone towed linear array structure with a diameter of 16mm, effectively combining fiber laser hydrophones with attitude sensing multi-core optical fiber. Finally, the effectiveness of attitude correction was verified through static direction finding experiments in anechoic tank, and the accuracy of direction finding was better than 2° under the extreme "S-bend" shape conditions of the drag array attitude. It can effectively improve the accuracy of direction finding.

Distributed Acoustic Sensing (DAS) based on backscattered Rayleigh scattering in optical fibers is a rapidly developing technology in recent years. It utilizes the backscattered light from Rayleigh scattering in optical fibers for sensing purposes. By demodulating the phase information of the backscattered light, it enables highly sensitive distributed detection of external vibrations. DAS can provide real-time monitoring of small vibration signals near the optical fiber, showcasing extensive applications in the field of marine fishery monitoring. However, there is currently a lack of theoretical knowledge regarding the acoustic response characteristics of DAS to fish sounds. In order to provide theoretical guidance for future cable laying and fish sound monitoring work, this study considers the marine environment as an isotropic homogeneous medium. It investigates the acoustic response characteristics of the DAS system under different fish sounds and cable deployment methods in the marine environment. The feasibility of using a distributed optical fiber acoustic sensing system to monitor fish vocalizations is theoretically demonstrated, further promoting diversification in passive monitoring approaches for marine fisheries.

Magnetic sensitive material, giant magnetostrictive powder composite (GMPC), coated fiber bragg grating (FBG) cascaded extrinsic Fabry Pérot interferometer (EFPI) multi-parameter optical fiber sensor is proposed. Which enables the simultaneous detection of temperature, magnetic field and strain. The proposed sensor uses a double FBG cascade EFPI structure including GMPC coated FBG with a wavelength of 1550 nm to achieve magnetic field sensing. The results show that the sensor has a maximum temperature sensitivity of 175.4 pm/℃, a maximum magnetic field sensitivity of -1.54 pm/mT and a maximum stress sensitivity of -9.55 pm/με. The proposed multi-parameter optical fiber sensor solves the multi-parameter cross-sensitivity problem, while the sensor has the characteristics of low cost, interference resistance, compact structure, simple preparation process and long distance transmission. The sensor has promising applications in areas such as biomedicine, environmental detection and military applications, and oil well survey, industrial production, power systems and so on.

In recent years, due to the construction of large storage tanks and gas pipelines, there is an urgent demand for gas remote sensors with high sensitivity and fast response in the fields of petrochemical, gas, coal mine and electric power, and gas remote sensors carried by vehicles, drones and intelligent robots have become a hot spot in industrial applications. At present, the gas remote sensor based on TDLAS is still limited to methane. The near infrared absorption lines of ethane are weak, the absorption spectra of propane and butane are bands not characteristic peaks, so it is difficult to achieve the high sensitivity of alkane gas except for methane. Therefore, the actual industrial applications mostly use Fourier transform spectroscopy (FTIR) or photoionization detector (PID).The emergence of inter band cascade lasers (ICL) makes it possible to detect alkane and olefin gases with high sensitivity. Most propane detection schemes mostly use the absorption peak of 3370nm. Considering avoiding the influence of water vapor and carbon dioxide in the air, the absorption peak of 3364nm for propane is selected in this paper. In this band, the ICL laser is used to measure methane, ethane, propane and n-butane at the same time. A TDLAS measurement system with open optical path is constructed by placing a hollow reflector at a distance of 50 meters. The return light is collected and concentrated through the lens into the detector, and the absorption signal of alkane gas within 50 meters is obtained. In this paper, the absorption signals and harmonic signals of four kinds of gases at 15 meters and 50 meters are given respectively. According to the harmonic signal and noise, the detection limit of the gas to be measured is better than 50ppm∙m. According to the different gases to be measured in gas and petrochemical fields, a method of mixture identification is proposed to measure the recognition effect of mixture. Looking forward to the future, the response sensitivity of mid-infrared detectors continues to be improved, and alkane gas sensors without reflector at the end will become possible.

Wavelet transform can suppress the noise of multimode fiber optic micro-vibration sensing signal, but still seriously affected by the existing threshold function. In this paper, we proposed an improved wavelet threshold function based upon hyperbolic tangent function to perform wavelet denoising on heartbeat signals to verify its effectiveness. The results show that under the input signal-to-noise ratio (SNR) of 5dB, the output SNR of each decomposition layer obtained by the improved threshold function is higher than those of the existing methods, while the root mean square error (RMSE) is smaller. It is worth noting that when the signal is cleaner, less noise can be denoise but higher SNR can be achieved, indicating that the proposed threshold function greatly alleviates the pseudo-Gibbs and constant bias. This research improves multimode fiber micro-vibration sensing performance and facilitates health diagnostics.

A wind turbine tower damage detection method based on optical frequency domain reflection (OFDR) technology is proposed in this paper, which can detect structural damage position and damage degree of wind turbine towers. The damage location of the wind turbine tower structure can be accurately detected by the method with a spatial resolution of 1cm, the detection error of damage degree is less than 2.2%. This method provides a candidate solution for real-time detection of wind turbine tower damage.

Interferometric integrated optic gyroscope usually applies a square wave phase modulation signal, namely two state modulation or four state modulation, between two back-propagation light waves. The amplitude of square wave phase modulation is π/2, and the modulation frequency is the intrinsic frequency of the sensitive loop. Based on this unique demodulation method, it is inevitable to generate a spike pulse signal at the output signal end of the detector. This spike pulse signal can cause transient saturation in the front or rear amplifier of the detector, and signal distortion will occur when the detector recovers from this overload state; The asymmetry of peak pulses can generate odd harmonic interference coupled into useful signals, resulting in errors during demodulation and increasing the output noise of the gyroscope. Therefore, it is necessary to process the spike pulse. This article theoretically analyzes the reasons for the output noise of the gyroscope caused by spike pulses, and adopts a method to avoid the impact of spike pulses. Simulation shows that this method not only eliminates spike pulses, but also eliminates odd harmonic interference induced by spike pulses. The experimental comparison shows that this method significantly improves the accuracy of the gyroscope.

During the vibration process, the output of the gyroscope will add a non-zero offset small amount based on the Earth's rotational speed component, which is the output error term of the gyroscope vibration. When the gyroscope adopts two different closed-loop models, the error magnitude of the same gyroscope is different. Therefore, selecting an optimal closed-loop model while adjusting appropriate closed-loop parameters is crucial for solving the vibration problem of gyroscopes. Firstly, compare the differences and stability conditions between the two closed-loop models; Secondly, analyze the reasons for the error terms generated during the vibration process of the gyroscope and the reasons for the different sizes of error terms generated by different models; Then simulate and analyze the output of gyroscope vibration under different closed-loop models. The simulation and specific experimental results indicate that selecting an optimal closed-loop model shortens the closed-loop integration delay time of the model, and maximizes the closed-loop gain as much as possible while ensuring that there are no resonance peaks in the closed-loop amplitude frequency characteristic curve. This can reduce the output error term of the gyroscope and improve its dynamic performance.

High-precision phase reconstruction is crucial for beam analysis and control. In a previous work, we proposed a phase retrieval method based on the Monge-Ampère equation (MAE). Similar to the commonly-used transport of intensity equation (TIE), the MAE is also a partial differential equation (PDE) that describes the relationship between irradiance and phase. However, in the framework of geometric optics, the MAE could be a more accurate and general method for solving the phase retrieval problem compared to the TIE. In this paper, we apply the MAE to reconstruct the field of a laser beam. By capturing two irradiance distributions at two different planes that are perpedicular to the propagation direction, the complex amplitude distribution at the first plane can be retrieved based on the MAE method. The angular spectrum method is employed to determine the complex amplitude distributions at different positions along the beam propagation direction. The factor of the laser beam is calculated based on the reconstructed beam field. Results show that the factor values computed based on the MAE method have small deviations from that tested with a commercial beam profiler.

Methane, as the main component of gas and natural gas, its flammable and explosive characteristics to industrial production, urban safety and daily life has brought great potential hazards. Accurately measuring the concentration of methane gas is of great significance for urban development and environmental protection. At present, tunable semiconductor laser absorption spectroscopy (TDLAS) has become one of the important technologies for methane sensor concentration measurement. However, this technology is easily affected by environmental temperature, and the relative error of concentration measurement caused by environmental temperature changes is greater than 50%. Therefore, this article proposes a temperature compensation calibration method for methane sensors based on TDLAS technology. This article first analyzes the effect of temperature on the absorption intensity of methane gas, and studies the relationship between its concentration value and temperature. Then, based on theoretical analysis, the negative temperature coefficient variation between the concentration value and temperature is achieved through power normalization processing algorithm. At the same time, a temperature compensation system is established to compensate for the concentration value and suppress the interference caused by environmental temperature to the detection. Finally, the feasibility of the temperature compensation calibration method was verified through experiments. The experimental results show that under the same testing conditions, the relative error of the measured concentration after temperature compensation can be reduced to less than 5%, significantly improving the detection accuracy of the methane sensor.

With the development of micro and small aircraft, unmanned systems, autonomous driving, wearable human navigation technology, missile guidance, and other fields. Accuracy is no longer the only core indicator of inertial navigation systems. Low cost, size, quality, and power (CSWaP) have become the key to enhancing the competitiveness of inertial technology products. Due to the large size and independent packaging of optical components, it is difficult to improve the CSWaP comprehensive performance of fiber optic gyroscopes (FOGs) under traditional solutions. Currently, using integrated optical technology to realize the integration of FOGs is the main way to reduce the comprehensive indicators of CSWaP of FOGs. Integrated FOGs refer to FOGs that use integrated optical chips to partially or completely replace optical devices of traditional FOGs. In this article, we propose a design scheme for a specialized integrated device of three-axis FOGs, which can achieve the integration of the main optical components of the interferometric fiber optic gyroscope optical path. It is of great significance for promoting the development process of miniaturization, lightweight, integration, low power consumption, and low cost of FOGs.

In this paper, a wind turbine blade surface shape monitoring system based on Optical Frequency Domain Reflection technology (OFDR) and Multi-Core Optical Fiber (MCOF) shape sensing is designed, and theoretical simulation and analysis are conducted on it. Firstly, a blade model with the airfoil of NACA-4412 is built based on Glauert vortex theory. Secondly, Finite Element Analysis (FEA) is used to calculate the blade’s deformation under aerodynamic forces. Then, the Rotation Minimizing Frame (RMF) is used to calculate the theoretical strain results of the outer cores. Finally, the shape of MCOF is computed using the strain results according to Parallel Transport Frames (PTF) and is compared with FEA results. The shape of MCOF reflected the shape of the blade surface under aerodynamic forces. This paper theoretically verifies the feasibility of reconstructing the blade’s surface shape using OFDR distributed strain measurement and MCOF.

This paper proposed a fast dynamic cavity length demodulation technique for signal demodulation of optical fiber Extrinsic Fabry-Perot Interferometer (EFPI) sensors. The proposed technique averages the five-step phase shift demodulation phase signals at multiple wavelengths to reduce noise and enhance signal demodulation stability. The reflected spectrum is obtained using white light interferometry (WLI) technology, and Ns sets of five-step phase shift signals are extracted from the reflected spectrum. The Ns sets of five-step phase shift signals are individually processed and their corresponding demodulated phase signals are averaged using FPGA, resulting in a dynamically demodulated cavity length signal. A corresponding testing system is established to analyze the noise characteristics of the demodulation technique. Experimental results demonstrate that compared to the traditional single-wavelength five-step phase shifting demodulation technique, the proposed demodulation technique effectively reduces noise, and improves noise stability. The demodulation scheme presented in this paper enables low-noise and high-precision signal demodulation in the weak signal detection field of EFPI sensors and holds significant value for the signal demodulation of EFPI sensor arrays.

In recent years, the large volume, high cost, and high power consumption of fiber optic gyroscopes composed of various discrete devices have limited their application in various micro inertial navigation systems. This paper proposes a fiber optic gyroscope based on integrated optoelectronic devices. Compared with fiber optic gyroscopes based on discrete components, fiber optic gyroscopes based on integrated optoelectronic devices significantly reduce their volume despite limited accuracy degradation, and are expected to be applied in integrated fiber optic gyroscopes.

In this paper, a single-mode watertight 4 channels fiber optic expander beam connectors are developed based on the ball lens. The design principle, the overall structure and sealing structure design and performance test is introduced in detail. After the test, the insertion loss indexes reach the design requirements, and the resistance to water pressure indexes meet the demand of 6000 m depth of underwater use. The watertight fiber optic connector can be used in underwater optical transmission and sensing system, marine oil and gas exploration and exploitation and other fields, and it has wide application prospect.

Hydrogen has a wide range of applications in various industries, including fuel cells, chemical processing, and electronics. However, hydrogen is highly flammable and explosive at a wide concentration range (4%~75%), which poses a significant safety concern. Hydrogen sensors with higher sensitivity, better selectivity, faster response, and wider dynamic range are of increasing importance in connection with the development. In this paper, a scheme of four-channel hydrogen sensor is proposed which is based on stimulated Raman spectroscopy and nanofiber. The tightly confined evanescent field associated with the nanofiber enhances the Raman gain per unit length by a factor of more than 104 over free-space beams. Time division multiplexing technology is used to implement multichannel detection. The whole sensing system consists of commercial optical modules such as DFB laser, EDFA and optical switch, and several homemade circuits instead of the commercial instruments for signal processing and controlling channel switching. Every channel can connect one probe which is based on nanofiber and packaged by special metal structure and method. The switching time can be millisecond order so that the signal period would not be influenced. The four-channel hydrogen sensor is realized in the experiment that demonstrates hydrogen detection from hundreds parts per million to 100% with each channel. The reported sensor could be used in the field of new energy, electric power and aerospace for detection of hydrogen leakage or monitoring of transformer health conditions with advantages of low cost, compact size and high resolution.

In the phase generation carrier (PGC) arctan demodulation schemes, carrier phase delay (CPD) and phase modulation depth (C) fluctuation might lead to incorrect demodulated results. This paper realizes a high-stability PGC-extended Kalman filter (EKF) demodulation algorithm based on a Field-Programmable Gate Array (FPGA). EKF-based ellipse fitting algorithm is used to eliminate nonlinear errors in the two original quadrature signals. The function and logic correctness of the design are verified by simulation. The simulation results show that the algorithm can calculate the ellipse parameters accurately and quickly. In experimental comparisons with the traditional PGC-Arctan demodulation algorithm, the PGC-EKF algorithm demonstrates significant suppression of nonlinear distortion in the demodulated results. As C varies from 1.5rad to 3.5rad, the algorithm achieves a mean SINAD of 38.457 dB with a standard deviation of 0.312 dB. Similarly, as the CPD varies from 0 to , the mean SINAD remains at 38.033 dB with a standard deviation of 0.777 dB. In addition, the PGC-EKF algorithm exhibits high stability during long-term operation on FPGA boards. This is evidenced by the significantly lower SINAD standard deviation (1.319dB) compared to the PGC-Arctan algorithm (3.296dB) when C is 2.63rad and CPD is 0 degrees. The proposed PGC-EKF demodulation algorithm based on FPGA has broad application prospects in fiber optic interferometric sensors.