This PDF file contains the front matter associated with SPIE Proceedings Volume 11436, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

We propose a variational mode decomposition (VMD)-based endpoint detection method for distributed fiber interferometric vibration sensing systems. First, the interference signal is decomposed into two number of modes (intrinsic mode functions (IMF1 and IMF2)). Then, the time moment corresponding to the disturbance starting point can be obtained using threshold judgment to IMF2. Finally, an experiment using a dual-laser source Mach-Zehnder interferometers (DSMZI)-based system is performed. Experimental results demonstrate that the error of the proposed scheme is 2 orders of magnitude lower than the conventional zero-crossing ratio (ZCR)-based method at the sensing length of 85 km. The mean processing time is 0.167 s, which is less than the sampling time of 0.3 s. Therefore, this high- efficiency endpoint detection method has potential practical applications in distributed fiber interferometric vibration sensing systems.

Phase-sensitive optical time-domain reflectometry (Ф-OTDR) has been widely used to interrogate multi-point vibration events in the health monitoring and damage detection of civil infrastructures, mechanical processes and aerospace transportation. Due to the different lengths of the events, the spatial resolution (SR) requirements would also be different. This paper presents and demonstrates two kinds of Φ-OTDR sensing systems with multi-spatial resolution (MSR) analysis characteristics. Frequency division multiplexing (FDM) and multi-scale matching filtering schemes were introduced to provide MSR ability for the phase-discrimination and amplitude-discrimination Φ-OTDR systems respectively. The proposed methods could restore vibration events of different scales with optimum signal-to-noise ratio (SNR) in merely a single measurement while maintaining the detectable frequency range, which offered promising solutions for the performance optimization of Φ-OTDR sensing systems.

In this paper, we propose an improved feature extraction based multiple events recognition scheme for fiber optic perimeter security system. In the scheme, four common types of security sensing events, namely, background noises, waggling the fence, cutting the fence and climbing the fence are collected based on a dual Mach-Zehnder interferometry vibration sensor. Variational mode decomposition in frequency domain, sample entropy in irregularity and zero crossing rate in time domain are considered as the feature description of the given security sensing events. A series of experiments have been implemented by a radial basis foundation neural network, which shows that the proposed recognition scheme can accurately discriminate the three kinds of man-made intrusions from the background noises. The average identification rates of 98.42% and 100% are achieved for the three types of intrusions and background noises, respectively, which can fully satisfy the field application requirements, the recognition response time is also good of real time performance, which can be controlled less than 1.6 s. Therefore, the proposed events recognition scheme can provide a quite promising field application prospect in the fiber optic perimeter security system.

Silicon and sapphire crystal materials have excellent thermal stability and heat transfer characteristics, making them widely used in the field of high temperature sensing. Based on the optical properties of silicon and sapphire crystals, we have fabricated two different kinds of extrinsic optical fiber Fabry-Perot high temperature sensors and matching signal transmission waveguides to investigate the effects of different temperature-sensitive materials on the response speed of the high temperature sensors. The first kind of sensor uses a C-plane double-sided polished sapphire wafer as the temperature sensing element. Heterogeneous fiber splicing between sapphire fiber and multimode silica fiber is realized for long-distance transmission of interference signals. The second kind of sensor uses a single-crystal silicon wafer as the temperature sensing element. Single-mode optical fiber of silicon dioxide is used as transmission waveguide. A series of high temperature assault experiments for heating and cooling processes from room temperature to 800°C, were performed on the two kinds of sensors to investigate their difference on the temperature response speed. In the experiment, the response time of the sapphire fiber high temperature sensor in the heating section is 38s, and the response time in the cooling section is 31.6s. The response time of the silicon-based fiber high temperature sensor in the heating section is 35.8s, and the response time in the cooling section is 28.2s. Due to the higher thermal conductivity of silicon, the silicon-based fiber sensor responded 5.78% faster than the sapphire fiber sensor in the temperature rise experiment and 10.85% faster than the sapphire fiber sensor in the temperature drop experiment

An algorithm of Rayleigh noise compensation in dual-end configured distributed temperature sensor (DE-DTS) is proposed in this paper. A 2 km long multi-mode fiber is used to calculate the attenuation of the light within Anti-Stokes and Stokes bandwidth, and figure out the isolation of the wavelength division multiplex through both Anti-Stokes channel and Stokes channel. Experiments are taken out to validate the proposed Rayleigh compensation algorithm. As a result, the Rayleigh noise in both Anti-Stokes component and Stokes component is compensated, and the temperature error of the dual-end configured distributed temperature sensor is revised. With the help of the proposed algorithm, a dual-end DTS can reach to the absolute accuracy of 1.09°C (RMSE) between 180°C to 300°C, which significantly monishes the temperature error compared to the sensor without Rayleigh noise compensation.

In this work, we employed the Empirical mode decomposition (EMD) algorithm into the demodulation spectrum of the long period fiber grating sensing experiment. After data processing by the EMD-based filtering algorithm, the disordered noise of the fiber Bragg fiber grating (LPFG)’s spectrum caused by the laser source and mode coupling can be effectively eliminated so that the single valley wavelength of LPG can be much more precisely located. As a consequence, based on the proposed algorithm, the related sensitivity of refractive index solution detection can be reached 11 nm/RIU, 18 nm/RIU, 61.3 nm/RIU and 241 nm/RIU in 1.33-1.36, 1.37-1.39, 1.40-1.43, and 1.43-1.45 refractive index region, respectively.

When the ground-based GNSS station is observed, the accurate atmospheric Weighted Mean Temperature (Tm) cannot be obtained, which limits the application of GNSS in remote sensing water vapor. So this paper makes use of the Tm grid data provided by GGOS Atmosphere to interpolate at eight stations (Dalian, Qingdao, Shanghai, Xiamen, Shantou, Taizhou, Haikou and Xisha Islands) in coastal areas of China and evaluate the applicability of GPT2w in coastal areas of China. The results show that: 1) Tm obtained by interpolating four methods achieves the accuracy requirement of ground-based GNSS remote sensing water vapor; 2) the GPT2w model has good applicability in eightareas in the coastal areas of China.

Personal safety in public places has become the primary demand of modern people's social life, and the detection of dangerous liquids is a key technology in the field of security inspection. The spectral drop analysis method is used to study the identification of flammable liquids in this paper. The spectral droplet analysis system is constructed with the fiber-capacitance drop sensor and spectrometer, and through the combination of signal processing circuit and the acquisition software, the experimental platform is designed. Experiments with typical liquid samples are completed and the three-dimensional fingerprints of the samples are obtained. And the characteristic values of liquid samples are extracted after data processing, the methods of rapid identification of flammable liquids based on spectral and drop fingerprint information are researched. With the spectral information, the characteristic wavelength points are selected to extract the characteristic parameters of the sample using the principal component analysis method. And the discriminant prediction models are established by distance discrimination, Bayes discriminant and Fisher discriminant. With the drop fingerprint data, the characteristic parameters are extracted with waveform analysis method, and then the extreme learning machine algorithm is used to build classification and identification model. The experimental results show that it is feasible to identify flammable liquids by spectral droplet analysis method.

White light interferometry is used to measure distributed polarization coupling in high-birefringence polarization-maintaining fibers (PMFs). When there are multiple intrinsic coupling points (CPs), multiple-order ghost CPs will produce. The quantity of multiple-order coupling modes can be calculated recursively. A graphical method is proposed to distinguish the multiple-order ghost CPs from intrinsic CPs. Overlapping of CPs is affected by the positions of disturbance points. Experiments on two and three disturbance points imposed on PMFs are performed. Experimental results show the positions of multiple-order CPs are agree well with graphical analysis. Additionally, the graphical method can be used to analyze other polarization-sensitive optical devices.

This study proposes a novel interferometric fiber optic SPR sensor which is able to realize both the temperature and the strain sensing with high sensitivity. Firstly, it is fabricated by splicing a no core fiber between two single-mode fibers to form the multimode interference structure. Then, half-length of the no core fiber is deposited by the gold film layer to form the classic three-layer SPR sensor. Finally, PDMS is covered onto the whole surface of the no core fiber, which performs as a medium to convert the external temperature changes into its refractive index variations. When compared with the other fiber optic sensors, the proposed sensor realizes the temperature and strain measurements based on two independent sensing mechanisms, particularly speaking, the temperature sensing is realized based on the SPR mechanism, and the strain sensing is realized based on the multimode interference. Hence, its inherent cross-sensitivity can be largely suppressed since the two sensing mechanisms are independent of each other. Besides, the proposed sensor shows both high-temperature sensitivity (1.13 nm/℃) and strain sensitivity (54.1 pm/με), which is suitable for the dual-parameter measurements.

It is difficult for traditional quartz fiber to meet the requirements of multi-parameters (temperature, pressure, etc.) testing in such harsh environments as high temperature, high pressure, strong corrosion and strong electromagnetic interference. In this paper, an optical fiber growth system based on the CO₂ laser- heated pedestal growth method is designed. The system is composed of optical system, high-precision control system and software system. At the same time, the influence of several factors on the growth of high-quality fibers is analyzed, which includes single-crystal fiber growth speeds, diameters and so on. The system can provide uniform and stable heat source, and has successfully grown sapphire, spinel and other ultra-high temperature optical fibers. Some of which have been applied in engineering practice. In the future, the system can also be used to grow magnesium oxide, zirconia, hafnium oxide and other ultra-high temperature optical fibers. So that it can potentially solve the application problems of optical fibers in the field of ultra-high temperature sensing such as aerospace loading and ultra-high-speed flight.

A fiber-optic distributed acoustic sensing method based on phase-sensitive optical time domain reflectometry combined with dual-chirped pulse and micro-reflective fiber Bragg grating (FBG) is proposed. The sensing fiber consists of a micro-reflective FBG with uniform spatial interval. The micro-reflective FBG help to gain a high signalto-noise ratio light signal, comparing with the Rayleigh backscattering (RBS) light of optical fiber itself. The dualchirped pulses are generated by a time delay, whose corresponding spatial interval approximately equal to twice the spatial interval of adjacent micro-reflective FBG. A beating signal is generated due to the interference of the two identical chirped pulses reflected by the micro-reflective FBG array. Acoustic disturbance between the microreflective fiber gratings will change the phase of the beating signal and the interference waveform will shift. Quantitative measurement can be achieved by directly demodulating the beating signal through using a crosscorrelation algorithm. By using such a method to perform the sensing for the micro-reflective FBG array, distributed quantitative measurement can be realized with only direct detection scheme and simple demodulation algorithm. Experiment are carried out with 2km fiber and PZT vibration simulation and the results verified the effectiveness of our method.

We demonstrate a method for fabricating a fiber sensor which is based on the Mach-Zehnder interference principle and used to the curvature sensing. By using of the CO₂ laser fusion, a standard single-mode fiber surface forms a collapse, which is asymmetrical in the normal direction. When the core mode of the transmitted light enters the fusion region, a portion of the light will enter the cladding of the fiber and transport as cladding modes. The cladding modes are sensitive to the external environment and the deformations of the structure. When the cladding modes leave the fusion region, they are re-coupled into the core. Mach-Zehnder interference occurs between the cladding modes and the core mode. The transmission spectrum of the sensor at 1460 nm changes with the curvature. The average sensitivities of the sensor to curvature sensing on the two directions are up to 4.941nm/m^-1 and -1.933nm/m^-1. This characteristic of the sensor could be used to sense the degree and direction of the curvature of constructions. The curvature sensor we proposed is simple, reproducible and low-cost. It offers a promising applications in construction health monitoring.

We report a novel device that the reflected intensity of Bragg grating (FBG) inscribed in a polarization-maintaing fewmode fiber (PM-FMF) is tuned by temperature from 56°C to 156°C with a wavelength shift of 1nm and intensity decrease of ~10dBm. The reflection spectrum and corresponding mode coupling are analyzed, the calcultated birefringence B for LP_o1, LP_11a and LP_11b polarized modes are 4.3×10^-4 , 4.6×10^-4 and 4.7×10^-4 , respectively, which agree with the specified birefringence of the PM-FMF, B ~5.0×10^-4 .

A weak double-peak fiber Bragg grating (FBG) temperature sensor is proposed and demonstrated. Wavelength-swept tunable laser is regarded as one of the most popular demodulation methods for fiber Bragg grating (FBG) sensors. However, due to the limitations of the existing tunable laser technologies, a fast, compact, stable and low-cost tunable laser for FBG sensors is still unavailable, which will become one of the major barriers for more widespread applications of FBG sensors. To further improve the efficiency and accuracy of the FBG interrogation system, a FBG temperature sensor is proposed and demonstrated by using tunable laser and a weak double-peak FBG. Since the reflection of the weak double-peak FBG has two main reflection peaks and relatively wide bandwidth, it is convenient to track the two characteristic peaks to accurately obtain the wavelength shift during the alteration of ambience temperature. A proof-ofconcept experiment is also conducted to verify the theory. By demodulating a weak double-peak FBG in the temperature experiment, a sensor sensitivity of 10.17 pm/ °C is measured for the proposed interrogation system.

The performance of an optic fiber SPR sensor is affected by the optical property of the metal layer. The optical property of the metal layer is affected by the surface morphology and the grain structure of itself. In this paper, the optical property of metal film based on the Drude-Lorentz model is improved by employing the Fuchs-Sondheimer theory and the Mayadas-Shatzkes theory. In the new law, the surface roughness and the grain structure are taken into account to describe the permittivity of metal layer theoretically. Then the improved model is used to analyze the performance of the optical fiber SPR sensor. The results shows that the surface roughness and the grain structure of metal layer significantly affect the resonance properties of the sensor. Specifically, the surface roughness of metal film can change the resonance wavelength of the SPR sensor. When the surface becomes smooth, resonance wavelength of SPR will decrease. In addition, with the increasing of grain size, the depth of the SPR curve increases obviously, while the full width at half maximum of the sensor decreases a lot. It means that, for a certain metal layer, the effects of the surface roughness and the grain structure should be considered in SPR technique.

This study proposes a novel fiber-optic interferometer based on the single-mode fiber-no core fiber-single mode fiber structure with the coated polydimethylsiloxane (PDMS). The no core fiber is the key carrier to excite the modal interference, and the PDMS that covers onto the fiber cascade structure is able to enhance the sensor sensitivity and also protect the sensor structure. The experimental results verify that the proposed sensor owns a high temperature sensitivity and the strain sensitivity, and it is particularly suitable for multi-parameter measurements.

For the remote interferometric optical fiber sensing system with forward pumped fiber Raman amplifier (FFRA), the phase noise is one of the most significant factors that affect its performance. Several factors have impacts on the phase noise of the system, including intensity noise and spectral broadening. Phase modulation is adopted to suppress the intensity noise induced by stimulated Brillouin scattering (SBS), which can introduce some additional phase noise into the system due to the spectral broadening in the meantime. It is observed in the study that the minimal phase noise level that the system can get with the phase modulation conducted is related to the input signal power, while an optimal input signal is existent with a specific Raman pump power to get the lowest phase noise level. The research results provide a guidance when determining the pump power of the forward pumped fiber Raman amplifier in the remote interferometric optical fiber sensing system with phase modulation.

Four-wave mixing created by pump-probe structure with small frequency spacing and enough high power is demonstrated and discussed by introducing single optical beam into semiconductor laser with external cavity, in which the injected beam is called as probe wave whose frequency detuning is controlled within ±50 GHz, and lasing resonance mode is selected as pump wave. Based on the nonlinear interactions of pump wave, probe wave and conjugate signals, microwave signals with different frequencies are simultaneously achieved. The frequency of microwave signal is tunable by adjusting the spacing between pump and probe wave.

Monitoring the generation and expansion of fatigue cracks in mechanical structures is critical to structural safety. To solve this problem, an optical sensing method for identifying crack propagation in mechanical structures is proposed. On-line monitoring of crack location, length, and expansion direction during crack propagation is achieved by combining micro-cavity array (MCA) fiber and optical frequency domain reflection (OFDR) system. Two adjacent ultra-short FBGs are used as a micro-cavity (MC) sensing element to obtain the strain distribution near the crack tip through a high spatial resolution distributed strain detection system. The crack state is obtained by combining the classical theoretical model, and a near real-time detection is achieved. Thereby, the system can perform an online monitoring and timely alarms on cracks. In this paper, we show the monitoring of the crack state during the process of preset crack length of 20 mm and crack propagation to 50 mm. An MCA fiber with 2542 MC elements with a spatial resolution of 1 mm is densely laid perpendicular to the crack tip direction. The crack propagation process is realized by using fatigue machine to apply cyclic load on aluminum alloy specimen, the distribution of non-uniform strain field of aluminum alloy specimen is obtained by detecting the wavelength drift of each MC element in the MCA fiber. In the test result, the distribution of the non-uniform strain field of the aluminum alloy specimen measured by the MC element is consistent with the simulation results. Consistently, the location of the crack tip and the detection of the crack length can be realized according to the distribution of the non-uniform strain field, and the feasibility of the aluminum alloy crack extension recognition system based on the MCA fiber is verified.

Previous photoacoustic flow measurements have been demonstrated with various modalities including Doppler frequency shift measurement from intensity modulated continuous laser wave and pulsed photoacoustic Doppler cross- correlation from nanosecond laser pulse. Here, we propose and demonstrate a new type of photoacoustic Doppler velocimetry based on a hybrid approach: we applied a supercontinnum laser source with picosecond pulse and 40 MHz repetition rate, and then use electo-optic modulator to modulate the intensity at 1 MHz. This method can potentially improve the signal to noise ratio due to pulse excitation of photoacoustic signal, and at the same time to achieve the high resolution of Doppler frequency shift due to the intensity modulation. With this method, we measured the moving speeds of various samples at the range of 0.1-1.6mm/s and obtained accurate results. The extra advantage of using supercontinnum laser pulses is that the spectral range can be chosen by using spectral filter thus to realize the multispectral measurement. To demonstrate this, we set the spectral filter working at five ranges at 500nm, 550nm, 600nm, 650nm, 700nm center wavelengths all with 100nm spectral width and measured different materials as moving targets fixed on the translation stage, including such as graphene thin layer, black tape, red and black dyed microspheres.

We proposed and experimentally demonstrated a distributed optic-fiber sensor based on the Raman loop configuration and fiber loss characteristic for detecting the temperature and structure's crack. Among them, the Raman loop configuration with reference fiber is proposed to detect the temperature profile along the sensing fiber. It can eliminate the influence of external physical perturbation on the temperature measurement results, and don't require pre-calibration process before measurement. This proposed method improves the engineering applicability of optic-fiber sensors. In addition, the information of crack is detected by using the fiber loss characteristics based on OTDR technology.

With the rapid development of modern society, the advent of the era of big data makes the exchange of information increasingly frequent and important. The distributed fiber Raman temperature measurement system is a brand-new sensing technology that has rapidly developed in recent years. As a transmission medium, it uses spontaneous Raman scattering to acquire back Raman scattering signals through a high-speed acquisition card. Since this signal carries temperature information, this signal is amplified and denoised and then demodulated to obtain a curve with temperature information. In this paper, we study the optical fiber temperature sensing scheme based on Raman scattering and analyze its working principle. By analyzing and comparing the demodulation method of sensor temperature, a method of demodulating temperature at different temperatures by using anti-Stokes fiber temperature as a reference channel is proposed. The relationship between the Raman ratio and the distance is demodulated. Because the traditional calibration scheme fails to take the environmental temperature value into account, this paper adopts a novel dynamic multi-segment fiber temperature calibration method, and verifies the feasibility of the calibration scheme through experiments. The result shows that with the change of the external environment temperature, the temperature of the sensing fiber can be accurately demodulated, the temperature demodulation result is more accurate, the measurement error is less than 1°C. The resulting system is more stable and can adapt to complex environmental changes. Since the light will become soft under high temperature conditions, this paper calculates the relationship between fiber loss coefficient and temperature. It is found that the continuous summation method can better solve the loss problem, thereby effectively improving the system signal-to-noise ratio.

A novel dual-parameter sensor created by fusion splicing single mode fiber (SMF) and hollow core silica tube (HCST) is experimentally demonstrated for simultaneous strain and temperature measurement based on hybrid structured fiberoptic Fabry-Perot interferometer (FPI) via phase demodulation. By the phase demodulation in the spatial frequency domain, strain sensitivities of -0.002rad/με and 0.0006rad/με are respectively obtained for air-cavity and the silica-cavity with the strain range from 0με to 1000με, and the temperature sensitivities of -0.0002rad/°C and -0.01rad/°C are respectively for air-cavity and silica-cavity with the temperature range from 50°C to 450°C. Therefore, the different sensing characteristics of silica-cavity and air-cavity to strain and temperature show that this structure can be further developed to concurrently sense strain and temperature.

The distributed measurement of salinity is experimentally demonstrated by Brillouin dynamic grating (BDG) The position-resolved salinity change can be measured by mapping the birefringence change through the BDG, where the salinity altering can induce the swelling or shrinking actions of the PI coating through absorbing or releasing water. Four sensors with the coating thickness of 5 μm, 8 μm, 15 μm, and 20 μm were fabricated as demos to characterize the salinity sensitivity of the sensors, with 30.1 MHz/(mol/L) for 5 μm, 75.4 MHz/(mol/L) for 8 μm, 119.2 MHz/(mol/L) for 15 μm, and 139.6 MHz/(mol/L) for 20 μm, respectively. The proposed sensor shows a linear and reproducible response to the salinity change, and its measurement uncertainty is less than 10 MHz, corresponding to the obtained maximum salinity accuracy of 0.072 mol/L.

In order to ensure the safe driving, autonomous vehicles under dynamic environments must accurately identify important targets that may change or move, especially pedestrians. Because the cameras are vulnerable to weather, light and other environmental factors, the real-time and accuracy of target detection and recognition may be poor. In this paper, we propose a method for pedestrian target recognition based on Lidar. It consists of three parts: ground point cloud removal, target object segmentation and pedestrian target recognition. Firstly, due to the huge amount of ground point cloud data, this paper uses a range image based on angle threshold algorithm to rapidly remove ground point cloud. With the mapping relationship between the disordered 3D point cloud and the ordered range image, the ground point cloud is marked and removed by the angle threshold on the range image rather than on the 3D point cloud. The ground point cloud removal is convenient for subsequent target object segmentation and recognition. Secondly, because the traditional clustering segmentation algorithm can not meet the real-time requirement of point cloud segmentation, we use improved Eps parameter DBSCAN algorithm combined with range image to segment point cloud target in this paper. For the problem of non-uniformity of 3D point cloud data density, we improve the setting of Eps parameter. For the undersegmentation between adjacent targets, we consider both Euclidean space distance and the angular distance which can distinguish the adjacent targets well in the space. Finally, considering the sparseness and disorder of the 3D point cloud and the lack of local feature extraction of the traditional PointNet network, a multi-scale feature fusion PointNet network is proposed, which combines the multi-scale local features and global feature of point cloud. The network also uses the spatial information and reflection intensity of Lidar point cloud to complete the pedestrian recognition. The experimental result shows that our proposed method can rapidly remove the ground point cloud and works well in segmentation between adjacent targets. The multi-scale feature fusion PointNet network performances well on 3D point cloud, its Area Under the Curve of ROC value reaches 0.92.

An optical fiber temperature and strain sensing system based on phase-derived optical fiber time delay (OFTD) measurement is proposed. By detecting the phase different between probe and intrinsic microwave signals, high accuracy delay measurement can be obtained. As the OFTD in single mode fiber (SMF) changes linearly with the strain or temperature in a certain range, strain and temperature sensing can be achieved by measuring the change of OFTD with high accuracy. In the experiments, the delay measurement accuracy can reach 0.06 ps when the accuracy of phase detector is ±0.1 degree while the detecting frequency is 4.5 GHz. In addition, an accuracy of ±11 με for strain sensing with 1.5 m fiber and a resolution better than 0.24 °C for temperature sensing with 10.66 m fiber are achieved.

In this paper we propose a method for analyzing the operation health of a large number of all-fiber optical current transformers used in UHV converter stations by studying the correlations of their operating parameters. This method firstly realizes automatic data sampling of operation parameters of all-fiber optical current transformers by building a monitoring network. Then the method calculates the deviations of the operating conditions from their ideal values and the correlations between the relative parameters, thus achieves the operation health condition of the main functional modules in optical current transformers. Finally, through big data analysis, the comprehensive health index and service life of each optical current transformer are predicted. The proposed method can realize the health monitoring of all optical current transformers, which will guarantee their reliable operations and decrease the potential faults.

Shield tunneling machine is a kind of special engineering equipment used for underground construction, which plays an important role on city development. Automatic segment assembly can significantly improve the operation efficiency and ensure the safety of workers. In this paper we present an automatic segment assembly method of shield tunneling machine based on multiple optoelectronic sensors, which includes laser displacement sensors (LDSs) and smart cameras. The LDSs are used to get the level difference information by measuring the distance between the erector and the corresponding segments, and the gap of the segments is achieved through sense the mating surfaces of both the under assembly and previously assembled segments by the smart cameras. Experiments were conducted to confirm the performance of the proposed method. The results demonstrated the feasibility and effectiveness of the method.

SQUIDs can be used as amplifiers for superconducting Transition Edge Sensor (TES) photon counting detectors. According to the physical process of photon counting, a TES signal simulator was developed to evaluate the performance of SQUID amplifier for TES detectors. A triangle simulation signal was determined to be a close match to that from a real TES photon counter. The dynamic range for the output voltage versus bias current was first characterized and then the current signals were incrementally coupled to circuit to determine the highest sensitivity of a series-array SQUID amplifier. The sensitivities are obtained by changing the triangle signal amplitude added to the TES signal simulator. The impedance changes can be deduced and further obtain the temperature changes of TES devices. When knowing the photon energy, TES design parameters can be known according to the thermal characteristics. In conclusion, the TES simulator can be well applied to guide the device design for effective readout.

In this paper we present the implementation of a multi-point fiber optic sensor for monitoring different concentrations of contaminants in water. The experimental set-up consists in two fiber structures built each one by splicing of Single Mode-Multimode-Single Mode (SMS) segments. Both structures are operating in a multimode interference regime and collocated in parallel geometry. The experimental results show the simultaneous detection in transmission configuration for water and salt (0% - 30% w/w) and water-glycerin (0% - 40% w/w) blends. The advantage of the proposed system has a simple construction, low cost, linear response and each sensing point works independently one of other.

Single-particle image spot on star map of star sensor can be mistaken for real star image and incur fault star identification, how to treat space single-particle properly in a star sensor is an emerging serious problem. Analysis and modeling are conducted for two types of single-particle image which are of spot shape and stripe shape. By observing two extreme cases of center location (pixel center or corner vertex) of a Gaussian star image and after examining of their Gaussian gray distribution characteristics, the discriminant criterion of single-particle image spot is established with a form of gray interval which is different with the pixel number of connected domains. Another discriminant criterion is also set up according to morphology, which is a limit value calculated from an equation expressing the slenderness of a star image. Simulating test results indicate that the involvement of the discriminant criterion can effectively identify single-particle images from real star ones, which helps to promote the reliability of attitude determination of star sensor.

Aiming at improving the signal processing capability of polarization optical time domain reflectometry (POTDR), a recognition method mainly based on the feature extraction and supported vector machine (SVM) is proposed. Apart from locating the certain place of interruptions, this method can help us identify different kinds of intrusion events. Firstly, we preprocess the signal by using an average filter and setting a proper threshold for it. Secondly, the signal is transformed into various kinds of time-domain features and frequency-domain features for the subsequent classification. Finally, the SVM of the system is trained with initial signals so it can discriminate events represented by new signal accurately. Our experiment results show the effectiveness of this method, and it can work well with high accuracy, fast response speed and low cost.

The linear birefringence is not easy to remove, since the spun fiber is used in optical fiber current sensor. We discuss the birefringence properties of a spun fiber around conductor with electrical currents, including the nonlinearity of linear birefringence and linearity of circular birefringence of the spun fiber. Theoretical results show that with the increase of load current, the linear birefringence of spun fiber will increase rapidly. By analyzing the impact of different polarization incident angle on the Poincaré sphere it is possible to obtain the influence of Faraday effect on spun fiber around conductor with electical currents in a high current generating system environment. We believe that the properties presented in this paper will be beneficial for reducing the influence of linear birefringence, as well as high accuracy fiber optic current sensors.

A separation technique for phase-shifted fiber Bragg grating(PS-FBG) physical parameters and spatial stresses based on Random-Elite Differential Evolution(RE-DE) algorithm was proposed. The cross-sensitivity problem of parameter to be measured was transformed into parameter identification of non-linear system. The specific methods adopted were as follows: measured the PS-FBG reflection spectrum under the free state as well as under different loads; constructed a theoretical PS-FBG reflection spectrum under these two states according to transfer matrix theory; established an identification model based on the difference degree between theoretical spectrum and experimental one and use RE-DE algorithm to solve the problem. Experiments showed that the proposed method had relative high identification accuracy and provided an convenient method for optical fiber parameter recognition and spatial stresses decoupling of PS-FBG.

The radiation effect of gamma rays on optical fibers is introduced. The experiments of measuring the induced loss of optical fibers by steady-state and transient gamma rays are carried out. The electron density, collision frequency and plasma oscillation frequency of optical fibers under high dose rate pulsed gamma radiation are calculated. A transient radiation-induced loss model based on low temperature plasma absorption is proposed. The transient radiation-induced loss of optical fibers at 600-1600 nm is calculated and measured. The results show that: (1) At the wavelength of 1310 and 1550 nm, the results calculated on plasma absorption method and measured results of transient radiation-induced loss of optical fibers are in the same order of magnitude, and the experimental results verify the applicability of the plasma absorption model; (2) Plasma absorption and atomic defect absorption coexist when pulsed gamma rays act on optical fibers. The transient radiation-induced loss of optical fibers is the result of the interaction of two mechanisms; (3) In the range of 600-1600 nm, with the increase of detection wavelength, the radiation-induced loss of optical fibers is dominated by plasma absorption.

In this paper, we propose and investigate a novel long period fiber grating (LPFG) refractive index (RI) sensor, which is inscribed in a two-mode fiber and coated with the zinc oxide (ZnO) thin film. According to the coupled mode theory, the resonant wavelength, which appears at approximately 1550 nm, originates from the mode coupling between the LP₁₁ core mode and the sixth order cladding mode. As a comparison, single-mode LPFGs (LPFG-SMFs) with and without ZnO thin film are fabricated and they are formed by coupling light from the LP₀₁ core mode into the sixth order cladding mode. The sensing performance is researched by observing the shift of resonant wavelength with the increasing of surrounding refractive index (SRI) in the range from 1.3300 to 1.4577. The experimental results demonstrate that LPFG inscribed in a two-mode fiber (LPFG-TMF) has a higher RI sensitivity than the LPFG-SMF. And the LPFGs with coated ZnO thin film can achieve a higher RI sensitivity than the bare LPFGs in the mode transition region. The highest sensitivity of LPFG-TMF coated with ZnO thin film reaches 7578.94nm/RIU in the RI region between 1.4558 and 1.4577, which is 23.90 and 38.69 times higher than the bare LPFG-TMF and LPFG-SMF coated with ZnO thin film, respectively. The proposed sensor offers a promising platform to achieve a higher sensitivity for SRI.