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In this paper, we demonstrate a simple structure, ultra-narrow linewidth, extremely high stability and high optical signal to noise ratio single-longitudinal-mode (SLM) erbium-doped fiber laser (EDFL). Nested Fiber Ring (NFR), nested in the ring cavity of EDFL, is employed to serve as a high quality mode filter to eliminate the dense longitudinal mode of the EDFL. The mode-selection capability of the RIR is investigated and discussed theoretically and experimentally. Saturable absorber, which composes of a section of unpumped polarization maintaining erbium-doped fiber and a Fabry–Perot fiber Bragg grating (FP-FBG), is employed to produce a dynamic narrowband grating to ensure the SLM laser output. Saturable absorber, consisted of a section of unpumped polarization maintaining erbium-doped fiber, produces a dynamic narrowband grating to ensure the SLM laser output. We achieve a ultra-narrow linewidth SLM laser output of 550 Hz. The laser has an optical signal-to-noise ratio (OSNR) of more than 59 dB. In addition, the stability of the wavelength and power fluctuations below 0.03 nm and 0.02 dB, respectively. The single-longitudinal-mode fiber laser has a compact structure, ultra-narrow linewidth, and excellent performance, and is expected to be used in the fields of coherent communications, precision metrology, and so on. • We propose a new structure of ultra-narrow linewidth single-longitudinal-mode erbium-doped fiber laser based on Nested Fiber Ring structure and Saturable Absorber. • The new structure has the advantages of simple structure, easy preparation and high stability. • The output performance of the proposed fiber laser is excellent, the 3-dB linewidth is 550 Hz and the optical signal-to-noise ratio is 59 dB. • The filtering function of the Nested Fiber Ring was theoretically derived and its filtering effect was analyzed. [ABSTRACT FROM AUTHOR]

• Multiparameter sensing system using a single FBG with a polarization-assisted analysis. • Strain, torsion and temperature for the analysis of polarization and spectral components. • The relative errors are below 0.5% for each mechanical load condition. • A root mean squared error of 2.91 °C is obtained for the temperature analysis. This paper presents the development of a multiparameter sensing system using a single fiber Bragg grating (FBG) with a polarization-assisted analysis. In this case, the FBG sensor is subjected to axial strain, torsion angle and temperature for the analysis of polarization components and spectral features variations. In addition, the responses at two orthogonal polarization states are compared to evaluate the feasibility of developing a polarization-based analysis for multifeature regression. Results show the differences in the sensors responses considering the full spectrum and the polarization components, where non uniform strain components lead to differences in the polarization states, which are used on the simultaneous analysis of different mechanical loadings combinations. Furthermore, the simultaneous estimation of axial strain, torsion angle and temperature resulted in small relative errors when the whole range of the tests are considered. The relative errors are below 0.5 % for each mechanical load condition, whereas a root mean squared error of 2.91 °C is obtained for the temperature analysis. Thus, the proposed approach resulted in a suitable alternative for three parameters sensing using only one FBG in standard single mode silica optical fiber without any structural modification, which can lead to a new approach on spectrally efficient sensor systems for practical applications. [ABSTRACT FROM AUTHOR]

• Machine learning approach for automatic estimation of responses using different TFBGs. • Estimation based on the optical spectral characteristics of TFBGs as a function of RI. • Root mean squared errors as small as 0.008 RIU for the trained samples. • The approach can be used to enhance the reproducibility of large set of TFBG sensors. This paper presents the development of an approach for automatic estimation of the regression coefficients as a function of the refractive index (RI) in Tilted Fiber Bragg Gratings (TFBGs) based on their optical spectral characteristics. The methodology is based on machine learning approach, where different samples are analyzed in various test conditions to obtain a dataset for the training of a kNN regression algorithm. In this case, the spectral characteristics related to the wavelength spacing between adjacent cladding mode resonances, number of valleys and amplitude of the normalized spectra are correlated with the linear regression algorithm for RI estimation using the machine learning approach. The analysis are performed as a function of the wavelength shift and optical power, considering two regions on the transmitted spectrum and the envelope of the curve. Furthermore, the number of valleys and the area of the envelope curve are also analyzed as a function of the refractive index, which result in six analyzed features. The Principal Components Analysis (PCA) is used to reduce the number of features to two principal components. The results of the proposed algorithm are obtained with different trained and untrained samples, which indicates a root mean squared error (RMSE) of around 0.003 RIU for the trained samples and 0.008 RIU for the untrained ones. Therefore, the results indicate the usefulness of the proposed approach, which can be used to enhance the reproducibility of large set of TFBG sensors in practical applications. [ABSTRACT FROM AUTHOR]

• Over the years 2 µm mode-locked fiber laser has been widely developed as an important light source for various significant applications, such as ultrafast mid-infrared spectroscopy. Besides, as the solutions of CGLE governed by the composite balance among dispersion, nonlinearity, gain and loss in dissipative systems, the field of solitons has become subject to extensive study. • Worth to note that, the rise of time-stretched dispersive Fourier transform (TS-DFT) technique has significantly enhanced the understanding of soliton dynamics, including bound solitons, optical rogue waves and others. However, due to the lack of appropriate dispersive components at 2 µm band with low optical transmission loss for TS-DFT measurement, direct real-time observations of ultrafast soliton dynamical phenomena in the 2 µm mode-locked fiber lasers is still limited. • In the present paper, we utilize DFT method to investigate the ultrafast nonlinear soliton dynamics in a 2 µm Tm-doped mode-locked fiber laser. The real-time observation of the nonlinear dynamics of the soliton explosions and pulsations are successively investigated. Moreover, the real-time observation of the soliton explosions associated with soliton pulsations is demonstrated in 2 µm wavelengths for the first time. In addition, soliton rains with dual-wavelength operation are also observed. Our work will benefit the development of 2 µm ultrafast light source, as well as enhance a comprehensive understanding of the soliton nonlinear dynamics in mode-locked fiber lasers. Over the years, 2 μm mode-locked fiber laser has been widely developed as an important light source for various significant applications. Worth to note that, with the help of real-time spectroscopy technique, i.e., time-stretched dispersive Fourier transformation (TS-DFT), various complex nonlinear soliton dynamics have been revealed, such as soliton molecules, soliton explosions and soliton pulsations, enhancing our understanding of the ultrafast physics. However, due to the lack of appropriate dispersive components at 2 μm band with low optical transmission loss for TS-DFT measurement, the investigation of ultrafast soliton dynamical phenomena in the 2 μm mode-locked fiber lasers is still limited. In this work, we demonstrate the direct real-time observation of soliton explosion and pulsation phenomena at 2 μm band mode-locked fiber laser, for the first time as far as we know, by using specially designed linearly chirped fiber Bragg gratings as dispersive component for TS-DFT measurement. For explosions, we observe slight explosion and drastic explosion, respectively. Particularly, the soliton pulsating explosions are also characterized in real time where the explosions occur in the process of soliton pulsation. In addition, dual-wavelength soliton rains are also observed by carefully adjusting the cavity parameters. Our work provides supplements to the current investigation of 2 μm band ultrafast science, and might enrich our understanding of soliton dynamics. [ABSTRACT FROM AUTHOR]

• Aiming at the demand of pressure monitoring of flammable and explosive gases in industrial production, a diaphragm-type FBG high-pressure gas sensor is designed to realize long-term high accuracy pressure measurement in the range of 0–50 MPa, with good static characteristics under experimental conditions, which verifies the feasibility of the pressure sensing model. • The use of glass welding encapsulation instead of the traditional adhesive encapsulation to produce FBG pressure sensors improves the durability and long-term stability of FBG pressure sensors, and provides the necessary data support and reference for the subsequent use of glass welding encapsulation of FBG sensors. • The use of glass welding encapsulation helps to maintain the long-term high accuracy measurement of the FBG pressure sensor. Large quantities of flammable and explosive gases are often used in various production processes in the chemical and energy industries. these gases, if leaked or improperly operated, can cause serious production accidents. Therefore, in order to meet the requirement for miniaturized, highly accurate, long-term stable and large-range high pressure sensors for flammable and explosive gases in industrial production, a highly accurate diaphragm fiber Bragg grating (FBG) high pressure gas sensor is designed in this paper. In this work, the working principle of the sensor is first analyzed theoretically, the circular elastic metal diaphragm of the sensor will strain under the action of pressure load, and then the strain will be transmitted to FBG. The wavelength drift of FBG is measured to achieve the measurement of the pressure load. Then finite element simulation was carried out for the diaphragm strain of the sensor to verify the feasibility of the sensor design scheme. The traditional adhesive encapsulation is then replaced with a glass welded encapsulation to improve the durability and long-term stability of the sensor. Finally, calibration experiments was conducted on the sensor, and the experimental results were compatible with the theoretical simulation analysis. The results show that the sensor realizes long-term highly accurate measurement of pressure in a large range of 0–50 MPa, and has good static characteristics under experimental conditions. [ABSTRACT FROM AUTHOR]

• We designed and manufactured a 40-channel array waveguide grating (AWG) based on a silicon dioxide planar light wave circuit (PLC). • Using hybrid integration technology to couple AWG and photodetector arrays together. • The hybrid integrated demodulation chip achieves precise wavelength demodulation of C-band FBG sensors. • The designed AWG has a 3 dB bandwidth of 1.98 nm, insertion loss of approximately 3.5–4.1 dB, and adjacent channel crosstalk of approximately −7.84 dB. • The wavelength resolution of the demodulation system is 1 pm, and the demodulation accuracy is 2.11 pm. Fiber Bragg grating (FBG) sensors have been widely used in various fields. To develop a miniaturized, high-precision, and high-response FBG interrogation system, arrayed waveguide grating (AWG) has been widely used as a key device. In this paper, a 40-channel arrayed waveguide grating based on silica planar lightwave circuits (PLCs) is designed fabricated, coupled to a photodetector array using hybrid integration techniques. The hybrid-integrated demodulation chip enables precise demodulation of the FBG sensor wavelength in the C-band. The test results show that the AWG has a good transmission spectrum with a 3-dB bandwidth of 1.98 nm, insertion loss of about 3.5–4.1 dB, and adjacent channel crosstalk of about −7.84 dB. The established FBG interrogation system can realize continuous interrogation in the dynamic range of 1514.6–1578.6 nm, the wavelength resolution is 1 pm and the demodulation precision is 2.11 pm in the dynamic range of 1555.3–1556.1 nm. [ABSTRACT FROM AUTHOR]

• We build a system to sense the micro-deformation of connecting rods. • We design a connecting rod sensing network based on FBG. It not only detects the deformation of the connecting rod after compression but also detects the deformation of the connecting rod in the tensile state. • The work provides a new detection scheme to ensure the machining quality of connecting rods. As a key component of the ship power equipment, the deformation of the connecting rod directly affects its performance and life. Therefore, the deformation detection of the connecting rod plays an important role in ensuring the service life of the power equipment. In this paper, we propose a deformation perception and reconstruction method based on fiber Bragg grating (FBG) for the connecting rod of the marine diesel engine. According to the deformation characteristics under loads of different sizes and directions obtained from the finite element, a sensing distribution FBG-based is designed for the connecting rod. On this basis, we establish the FBG-based deformation sensing system and develop a three-dimensional deformation reconstruction model, which can realize the real-time monitoring of the deformation during the processing of the connecting rod. The experiments show that the deformation trends obtained by FBG and finite element are consistent. Therefore, the FBG-based deformation perception and reconstruction method is feasible. It provides a new and accurate detection scheme to ensure the processing quality of the connecting rod and a new idea for the deformation detection of other key parts of the ship power equipment. [ABSTRACT FROM AUTHOR]

• In this paper, we propose a distorted pulse detection method based on odd function for pulse decomposition, to solve the problem of optical path mismatch in UWFBG array. • In theory, we build a mathematical model of the distortion pulse and derive the corresponding odd function. • Then, we analyze the peak trajectory obtained by the difference curve by simulation, and determine the optimal interference position of the distorted pulse. • Finally, we use the vibrating liquid column method to carry out underwater acoustic sensing experiments, under different conditions of optical path mismatch. • The results show that, when the distance between adjacent UWFBGs is 50 m and pulse width with 100 ns, the gain of harmonic SNR and average PSD is improved from 2.3 dB and 1.8 dB to 11.2 dB and 10.0 dB respectively, under different circumstances with length difference for 1 m, 1.5 m, and 2 m respectively. In this paper, we propose a distorted pulse detection method based on odd function for pulse decomposition, to solve the problem of optical path mismatch in UWFBG array. In theory, we build a mathematical model of the distortion pulse and derive the corresponding odd function. Then, we analyze the peak trajectory obtained by the difference curve by simulation, and determine the optimal interference position of the distorted pulse. Finally, we use the vibrating liquid column method to carry out underwater acoustic sensing experiments, under different conditions of optical path mismatch. The results show that, when the distance between adjacent UWFBGs is 50 m and pulse width with 100 ns, the gain of harmonic SNR and average PSD is improved from 2.3 dB and 1.8 dB to 11.2 dB and 10.0 dB respectively, under different circumstances with length difference for 1 m, 1.5 m, and 2 m respectively. So, the algorithm is more robust to length mismatch, and verifies the effectiveness of the algorithm, and has potential significance for reducing the difficulty of designing the UWFBG array interferometer. [ABSTRACT FROM AUTHOR]

• In view of the lack of online damage detection of CFRP driveshaft, this paper aims to establish a damage detection system for CFRP driveshaft and establishes a damage identification model for CFRP driveshaft based on strain and damage mechanism. • Considering the shortcomings of the existing research, this paper makes full use of the unique advantages of FBG sensors, combined with GWO-BP neural network, constructs a CFRP driveshaft damage identification system, and realizes the damage identification and localization of the CFRP driveshaft. • Combining theoretical methods, simulation calculations and experimental studies, torsion experiments were carried out on CFRP driveshafts, and damage identification was carried out by using the identification and localization system constructed in this paper to compare the actual damage and damage identification. Carbon Fiber Reinforced Plastic (CFRP) has the advantages of light weight, high strength, small coefficient of thermal expansion, good vibration damping performance, etc., the use of its production drive shaft can reduce weight, improve the intrinsic frequency, transmission efficiency and precision, and has been used in helicopters, heavy duty machine tools, wind turbines, ships and other high-end equipment. Once the damage occurs, it will cause adverse effects such as reduction of mechanical efficiency of the system, degradation of transmission system performance, and decrease of reliability and safety. The existing non-destructive testing technology for carbon fiber reinforced plastic has limitations such as inconvenient use, complex equipment, and no real-time online monitoring. To address this problem, this paper adopts Fiber Bragg Gratings (FBG) sensors to obtain real-time strain field data of CFRP drive shaft, and uses the linear and nonlinear mapping ability of Back Propagation Neural Network (BP neural network) to identify and locate the damage of CFRP drive shaft. At the same time, the GWO optimization algorithm is used to optimize the BP neural network to construct a CFRP drive shaft damage identification and localization system based on FBG sensing network and GWO-BP neural network. The experiments show that the CFRP drive shaft damage identification and localization system constructed in this paper can obtain the CFRP drive shaft strain field information in real time and accurately identify the damage and its location. [ABSTRACT FROM AUTHOR]

• In this paper, a new feature extraction and recognition method for perimeter intrusion vibration signals is proposed, using msCNN to extract multi-scale structural features of the signal, and combining features of different scales at the end of each convolutional block. • Before entering into the next convolutional block, the attention mechanism is used to recalibrate the combined features to enhance the expression ability of the features. • Finally, the time dependence is analyzed by LSTM, so that the characteristics of the signal are richer and more comprehensive, so as to obtain a higher recognition rate. To improve the recognition rate of ultra-weak fiber Bragg grating (UWFBG) arrays in perimeter security monitoring, this paper proposes a msCNN-LSTM combinatorial model recognition method, which combines an improved multi-scale convolutional neural network (msCNN) and a long short-term memory neural network (LSTM) to identify intrusion behaviors more accurately by synchronously extracting the multi-scale structural features and time-dependent relationships of vibration signals. Using msCNN to cross-learn multi-layer features, the hidden information between features at different scales can be obtained. The attention mechanism was applied to recalibrate the combined features extracted from the shallower layers, which enhanced the expression ability of features. Finally, the time dependence is analyzed by LSTM. Experimental results show that the scheme can effectively distinguish five typical intrusion behaviors, and the average recognition rate can reach 97.84%. [ABSTRACT FROM AUTHOR]

• In linear erbium-doped fiber laser cavity, dual-wavelength lasing performances are compared with 3 dB FLM-FBG cavity and FBG-only cavity. • In the dual-wavelength EDFL with FBG-only cavity, the lasing performances are compared when the reflectivities of the output-mirror FBGs are the same or different, and the effect of the center wavelength spacing between the two pairs of FBGs on lasing performances is studied. There are few reports in the field of linear cavity dual-wavelength fiber lasers about the laser output characteristics when 3-dB fiber loop mirror (FLM) and Fiber Bragg grating (FBG) are used as full reflective mirrors, respectively. Additionally, there is scarce research on further tuning of the FBG parameters (reflectivity and center wavelength interval) to study dual-wavelength fiber lasers. In this paper, we compare the performances of dual-wavelength lasing outputs between the 3-dB FLM-FBG cavity and the FBG-only cavity in a linear-cavity erbium-doped fiber laser (EDFL) based on the polarization hole burning effect. The results show that the optical signal-to-noise ratios (OSNRs) of the dual-wavelength lasing outputs with the FBG-only cavity are approximately 4 ∼ 5 dB higher than those with the 3-dB FLM-FBG cavity. Furthermore, the lasing output performances based on the FBG-only cavity are compared when the reflectivities of the output-mirror FBGs are the same or different. It is observed that the dual-wavelength laser with the same output reflectivity leads to higher OSNR, slope efficiency, and stability. Moreover, by varying the center wavelength interval of pairs of FBGs in the FBG-only cavity to values of 4 nm, 8 nm, and 12 nm respectively. It is shown that wider intervals result in higher OSNRs but lower laser slope efficiency. Finally, we demonstrate a dual- wavelength linear cavity EDFL operating at room temperature with two lasing wavelengths of 1550 nm and 1562 nm. The laser results in OSNRs around 50.62 dB and 50.93 dB; center wavelength variations of less than 0.030 nm and 0.035 nm; output power fluctuations of less than 0.061 mW and 0.059 mW, respectively. [ABSTRACT FROM AUTHOR]

• This paper proposes an erbium-doped fiber laser (EDFL) system based on intrinsic Fabry-Perot interferometer (IFPI) structure for simultaneous measurement of temperature and strain. IFPI is composed of two fiber Bragg grating (FBGs) inscribed by femtosecond laser and used as the frequency selective device in fiber laser. • FBG can be inscribed on fiber core through fiber coating and cladding, its mechanical strength is greater. Moreover, FBG inscribed by femtosecond laser is more resistant to high temperature. • The changes of temperature and strain parameters can be sensed by the drifts of the laser spectrum, the most accurate spectral drifts for high-precision parameter sensitivities can be monitored due to the advantages of fiber lasers, such as narrow linewidth, high signal-to-noise ratio (SNR), high stability. In this paper, an erbium-doped fiber laser (EDFL) system based on intrinsic Fabry-Perot interferometer (IFPI) structure is proposed for simultaneous measurement of temperature and strain. IFPI with a central wavelength of 1546.2 nm, an interference fringe interval of 1.8 nm is composed of two FBGs inscribed by femtosecond laser and used as the frequency selective device in fiber laser. The signal-to-noise ratio (SNR) and 3-dB linewidth of 1546.2 nm fiber laser are 27.735 dB and 0.21 nm, respectively. For 1546.2 nm-laser stability, the maximum fluctuation of laser peak wavelength and output power is 0.08 nm and 0.655 dBm, respectively. Experimental results show that the proposed EDFL sensing system exhibits a temperature sensitivity of 9.16 pm/℃, the linearity R2 is 0.99965 within the temperature range of 100 °C-800 °C and a strain sensitivity of 7.45 pm/με, the linearity R2 is 0.99985 within the strain range of 0 με–1050 με at 800 °C. The stability of both maximum temperature and maximum strain was monitored within 30 min, and the maximum fluctuation of laser peak wavelength and output power was 0.12 nm and 0.372 dBm, 0.05 nm and 0.667 dBm, respectively. [ABSTRACT FROM AUTHOR]

This paper presents the development, signal analysis and application of an FBG array embedded smart rubber carpet for 2D shape reconstruction. The proposed sensor system is based on the optical fiber integration between two rubber layers, where the optical fibers have 7 FBGs for the quasi-distributed force assessment. The sensor system analysis is performed at room temperature in difference set of tests, namely the stability tests (without any applied force), the force characterization tests (forces applied directly on the FBG region) and the shape reconstruction tests (different forces applied at different positions in the carpet). The stability tests indicated a maximum variation below 0.02 nm considering all 7 FBGs, whereas the force characterization tests indicated a mean sensitivity of around 80 N/nm. However, it is worth to mention that there is a variation on the sensitivities considering all FBGs, which is related to minor thickness differences in the rubber layer that directly effects the strain transmission from the rubber layer to the optical fiber beneath it. Thereafter, the shape reconstruction occurs in two steps, the first one is the cubic interpolation between the FBGs, considering the boundary conditions on the carpet following the beam theory. Then, the k-nearest neighbors regression algorithm is used on the force distribution assessment with a dataset divided into training and testing with a 70/30 proportion. The shape reconstruction results indicate the feasibility of the proposed approach, where root mean squared errors below 0.65 N are obtained in the range of applied force between 3 N and 35 N. Therefore, the proposed smart carpet as a feasible option for different 2D shape reconstruction applications ranging from biomechanics to structural health monitoring. • Force sensor array using Fiber Bragg Gratings embedded in rubber carpet. • 2D Shape reconstruction using machine learning algorithms. • Relative errors on the shape reconstruction below 3%. [ABSTRACT FROM AUTHOR]

• The experiment were carried out by temperature compensation of the central wavelength offset collected during the charging and discharging process of Li-ion batteries, BP neural network and optimized NGO- BP neural network were used to compare the prediction effects. • In terms of regression model performance and improvement of FBG non-electrical parameters on model performance, the various indicators predicted by NGO-BP are stronger than those predicted by BP. • The joint accurate estimation of lithium battery SOC by electrical measurement characteristics and FBG non-electrical parameters, which has a certain value for the design and development of battery management system. Accurately estimating of the state of charge (SOC) of lithium batteries is of great significance for improving the service life and utilization efficiency of lithium batteries. In this paper, fiber Bragg grating (FBG) sensors are used to capture the offset of the FBG central wavelength caused by strain and temperature during the charging and discharging process of lithium batteries, and the current state of charge of lithium batteries is obtained through the combination of electrical parameters and FBG non-electrical parameters.A battery monitoring experimental system was established, and the comparison of wavelength offset collected by PDMS packaged FBG and bare FBG during the charging and discharging process of lithium batteries was discussed. The prediction results were compared by temperature compensating the central wavelength offset collected during the charging and discharging of lithium batteries, then introduced BP neural network and optimized NGO- BP neural network regression models for training to compare the prediction effects, the results of which point to the fact that in terms of regression model performance and improvement of FBG non-electrical parameters on model performance, the various indicators predicted by NGO-BP are stronger than those predicted by BP. After adding FBG non-electrical parameters, both the BP regression model and the NGO-BP regression model showed remarkable improvement in RMSE, MAE, and R2 compared to the purely electrical parameters, and the best prediction method for the NGO-BP non-purely electrical parameters, with RMSE of 0.0201 and MAE of 0.0154, which decreased by 77.94% and 73.12% respectively compared to the BP pure electrical parameter prediction method, and R2 of 0.9988, which increased by 5.88%, thus the joint accurate estimation of lithium battery SOC by electrical measurement characteristics and FBG non-electrical parameters is basically realized, which brings values for the design and development of battery management system. [ABSTRACT FROM AUTHOR]

• The FBG sensitivity is amplified by a special spring-moving mass mechanism. The maximum tilt sensitivity is 427.3 pm/°. • Special mechanism avoids temperature strain sensitivity of FBG. • The proposed sensor has low hysteresis and extremely good linearity and repeatability. This paper presents a temperature self-compensating, high-sensitivity fiber Bragg grating (FBG) inclination sensor. A sliding mass block and a spring are selected as the inclination-force conversion components to induce strain in the substrate where the FBGs are located, resulting in a change in the center wavelength. Both sides of the sliding mass block are arranged with one FBG on each side to play the role of temperature self-compensating. By calculating the variation of the center wavelength of the two FBGs, the variation of the inclination angle can be obtained. The detailed mechanical construction and sensing mechanism of the sensor are introduced, and the theoretical sensitivity factor formula of the sensor is given. The calibration test and static characteristics analysis of the sensor are carried out with the inclination angle change as the parameter. The experimental results show that there is a good linear relationship between the inclination angle and the FBG central wavelength, and the sensor possesses good repeatability and creep resistance. The maximum angular sensitivity is 427.3 pm/°, avoiding the cross-sensitivity between temperature and strain of the FBG. The sensitivity of the sensor can be easily adjusted by replacing the sliding mass block and the spring. [ABSTRACT FROM AUTHOR]

• This accelerometer principally consists of a pair of identical mass blocks, a pair of identical inclined cantilever beams, a pair of identical fixing blocks, a pair of identical columns, and a base. • The natural frequency of this accelerometer is 46 Hz, which has good flatness in the frequency range of 0.1 to 28 Hz. • Static stress analysis and modal simulation analysis using ANSYS. • Built a low-frequency vibration testing system consisting of a low-frequency vibration system and an optical fiber sensing system. • The signal of the time series is transformed into a fast Fourier transform using the periodogram method. The low-frequency vibration measurement technology is significant in energy exploration, aerospace, geophysical activities, health monitoring of large engineering structures, etc. A low-frequency FBG accelerometer based on a dual oblique wing is proposed to address the issue of difficulty in accurately measuring low-frequency vibration signals in current fiber Bragg grating accelerometers. Firstly, the structure of this accelerometer was theoretically analyzed and dimension-optimized. Secondly, the static stress analysis and modal simulation analysis are carried out, and the accelerometer is developed. Finally, it is fixed on the vibration table, and the low-frequency vibration test system is built to test the performance. The experimental results show that the natural frequency of the accelerometer is 46 Hz, the working frequency band is 0.1–28 Hz, the sensitivity is 55.6 pm/g, the dynamic range is 48.94 dB, and the resistance to transverse interference of less than 5.62 %. The accelerometer designed in this paper provides a new idea for low-frequency vibration signal measurement. [ABSTRACT FROM AUTHOR]

• A overlapping spectral demodulation method based on OTDR and 1-DDCNN was proposed. • Real time monitoring of small fire sources in subway tunnels is realized. • The determination of peak wavelengths of four highly overlapping sensing signals realizes that the RMSE is less than 1.5 pm and the average demodulation time is less than 30 ms. To improve the rapidity and reliability of the subway tunnel fire monitoring system and realize the monitoring of small fire sources, the method of regional subsection demodulation is used to group several adjacent nearly identical ultra-weak fiber Bragg gratings (UWFBGs) sensors into a group, and the 1-D dilated convolutional neural network (1-DDCNN) is used to demodulate the same group of highly overlapping sensing signals. The well-trained 1-DDCNN model can achieve extremely low signal demodulation error. The experiment shows that the UWFBG demodulation scheme proposed in this paper improves the detection accuracy of the subway tunnel fire monitoring system and shortens the detection time. The root-mean-square error (RMSE) of four highly overlapping peak wavelengths of sensing signals is less than 1.5 pm, and the average demodulation time is less than 30 ms. [ABSTRACT FROM AUTHOR]

• The localization of low velocity impact on CFRP laminate was conducted by using a normalized error outlier-based algorithm. • An experimental system for low-velocity impact monitoring of CFRP laminates based on FBG sensing network was constructed. • A reference database of impact location-impact response normalized characteristics quantities was constructed. • The low velocity impact on CFRP plate was successfully located with an average localization error of 8.19 mm. • The algorithm used in this paper is better than that of the PCA and BP neural network-based localization algorithms. This study conducted the localization of low velocity impact on Carbon Fiber Reinforced Plastic (CFRP) laminate using a normalized error outlier-based algorithm cooperating with Daubechies3 (Db3)-wavelet threshold noise reduction. The monitoring of CFRP plate impact was conducted by multiplex Fiber Bragg Grating (FBG) sensors connected to its upper surface, and the impact response signal was collected using an interrogator at a frequency of 2 kHz. Results showed that the Db3-wavelet threshold noise reduction algorithm could improve the impact localization performance of CFRP plate structure. By setting an appropriate outlier threshold and using a normalized error outlier-based algorithm, the low velocity impact on CFRP plate was successfully located with a maximum localization error of 21.35 mm and an average localization error of 8.19 mm. [ABSTRACT FROM AUTHOR]

• Compared with ordinary cantilever beam structures with equal cross-section, the lever amplification structure can further increase the axial strain of the FBG, thereby improving the response sensitivity of the accelerometer; • While ensuring the high sensitivity of the accelerometer, the thickness of the cantilever beam has been increased to improve its lateral anti-interference ability. • Its structure is simple and easy to package, and changing the thickness of the beam or the length of the rod can further expand the measurement range of the accelerometer. Combined with the lever amplification principle, this paper designed a fiber Bragg grating (FBG) accelerometer based on the structure of an equal-section cantilever beam, which can weaken the contradiction between the lateral immunity and sensitivity of the equal-section cantilever beam. Increasing the thickness of the beam increases its stiffness, thus improving the lateral immunity of the beam; at the same time, the lever structure increases the axial strain of the FBG, further enhancing the sensitivity of the accelerometer. The experimental study and theoretical analysis of the accelerometer show that the accelerometer response sensitivity is 239.12 pm/G with a good fit of R2 = 0.995 for frequencies varying from 5 to 90 Hz and accelerations varying from 0.1 to 1G; the lateral anti-interference degree is less than 6.27%. It shows that the accelerometer has good sensing performance. And the structure is simple and easy to package. The thickness of the beam or the length of the lever can be changed to further extend the measurement range of the accelerometer. [ABSTRACT FROM AUTHOR]

• A novel FBG-based sensor for vector displacement measurement has been presented. • The sensor shows high linearity with the R2 of 0.998, large measurement range of 140 mm, high sensitivity of 4.362 pm/mm, maximum hysteresis error of 3.25% and maximum repeatability error of 6.62%, respectively. • A series of sensor calibrations and performance tests of the sensor are conducted. This paper presents a novel fiber Bragg grating (FBG)-based displacement sensor with simple construction for vector deformation monitoring of slope surface. The interesting structure design and sensing principle of determining the displacement direction and amplitude simultaneously are reported. Calibration tests of six different auxiliary structures (namely core sensing elements) of the sensor are reported, which demonstrates that they are characterized by a superb linearity with an R2 of over 0.998, large measurement range of 140 mm, high sensitivity of 4.362 pm/mm, maximum hysteresis error of 3.25% and maximum repeatability error of 6.62%, respectively. A simulated rock slope test of the sensor is conducted to investigate the performance for determining the progressive sliding process of sliding mass. Additionally, a homogeneous soil slope model test is further conducted and the vector deformation of slope surface is accurately captured by three proposed FBG-based displacement sensors. This capability in identifying two-dimensional displacement can provide a bright application prospect in intelligent monitoring field. [ABSTRACT FROM AUTHOR]

• A novel wide dynamic range of temperature sensors based on a Fiber Bragg Grating (FBG) is presented and demonstrated. • The FBG sensor head is reinforced with borosilicate glass wool fiber (BGWF) and housed in a 4 mm quartz tube. • BGWF-reinforced FBG sensors with a dynamic temperature range of 1270 °C from −170 °C to 1100 °C have been demonstrated. • In the 12 Hr stability test, BGWF-reinforced FBG sensors show excellent stability. A novel wide dynamic range of temperature sensors based on a Fiber Bragg Grating (FBG) is presented and experimentally demonstrated. The FBG sensor head is reinforced with borosilicate glass wool fiber (BGWF) and housed in a 4 mm quartz tube. BGWF-reinforced FBG sensors with a dynamic temperature range of 1270 °C from −170 °C to 1100 °C have been demonstrated. Further, the thermal expansion and correlation coefficients are calculated from high-temperature measurements to be 11.46 pm/°C and 0.99677, respectively. In the 12 Hr stability test, BGWF-reinforced FBG sensors show excellent stability, the same as thermocouples. We believe this paper will lay a foundation for researching and designing new FBG sensors to measure extreme temperature conditions. [ABSTRACT FROM AUTHOR]

• Optimized surface bonding configuration for FBG sensors in extreme environment. • Excellent signal is observed in FemtoSecond Grating with polyimide coating at cryogenic temperatures. • Bonding FBG using cryogenic bonding agent Stycast and vacuum grease Apiezon N have been investigated. Optical fibre Bragg grating (FBG) sensors have been used for cryogenic temperature and strain measurements in a variety of applications, e.g., hotspot detection in high-temperature superconductor magnets. In such application, the sensitivity of the FBG and its reflection spectrum strongly depend on the methods of application, chiefly the adhesion and the elasticity of the bond, and the birefringence effect. To investigate those features, Draw Tower Gratings (DTG®) with Ormocer coating and FemtoSecond Gratings (FSG) with polyimide coating are mounted in the V-shaped grooves and on the surface of copper substrates using Stycast and Apiezon N. Their performance is investigated between 293 K and 77 K. The FBG sensors mounted in the V-grooves can achieve higher temperature sensitivities than those on the surface of the copper due to enhanced thermal strain transfer. It is found that although there is no obvious difference between the temperature sensitivity of the Ormocer and polyimide coated FBGs at cryogenic temperatures, the polyimide coated FBGs show superior reflection spectra, whose peak intensity remains strong at 77 K. Cryogenic vacuum grease Apiezon N is shown to bond the fibre well to the copper at cryogenic temperatures, which also reduces the birefringence effect induced from the non-uniform strain across the FBG. This paper provides practical knowledge of bonding FBG sensors in cryogenic environments for temperature and/or strain measurements. [ABSTRACT FROM AUTHOR]

• A transverse pressure sensor based on Sagnac cascaded FBG is proposed. • Based on PMF birefringence effect. • FBG structure is introduced to improve sensitivity. • Finite element method combined with experiment. In the field of engineering, the deformation and lateral stress in structural health detection have attracted extensive attention to obtain the structural health information of building objects in time. Based on the birefringence effect of polarization maintaining fiber (PMF), a transverse pressure sensor based on Sagnac cascade fiber bragg grating (FBG) was proposed and prepared. The performance of the sensor is analyzed by the combination of the finite element method and experiment. The simulation results show that the cross section of PMF is deformed under the pressure along the fast axis and the slow axis and then modulates the refractive index. The experimental results show that by adding FBG structure, the lateral pressure sensitivity of the sensor is 408.7 pm/N, and the linear fit is 98.3%. The lateral pressure sensitivity of the structure without FBG is 281.7 pm/N, and the linear fit is 99.3%. That is to say, the FBG structure increases the sensitivity of the sensor by 45.1%. The experimental results are in full agreement with the theoretical derivation. The optical fiber lateral pressure sensor proposed in this paper has the characteristics of simple fabrication, low cost, and high repeatability. It plays an important role in the field of structural stress monitoring. [ABSTRACT FROM AUTHOR]

• The high-temperature resistance wFBGA based on PI-wFBGA fabricated online by drawing tower. • Post hydrogen-loading and low-temperature annealing to improve its high-temperature resistance. • Optimal annealing of post-hydrogen-loaded PI-wFBG at 200℃ for 120 min. • Operating at 350℃ for more than 40 days and have good temperature consistency. Polyimide coated weak fiber Bragg grating array (PI-wFBGA) fabricated online by drawing tower overcomes the temperature limitation of conventional acrylate coating, and has broad application prospects in high-temperature environments. However, the periodic refractive index modulation of PI-wFBG is weak and rapidly decays or even erases in high temperature, indicating poor high-temperature resistance. In this paper, we report the design of a high-temperature resistance wFBGA based on PI-wFBGA fabricated online by drawing tower, which uses post hydrogen-loading and low-temperature annealing to improve its high-temperature resistance. Activated by thermal energy, hydrogen molecules react chemically with the germanium or silicon-related defects in the glass structure of the optical fiber to produce hydroxyl groups. When two hydroxyl groups combine, they form molecular water, which is a key factor in enhancing the high-temperature resistance of PI-wFBG due to its high stability and low diffusion. After annealing the post-hydrogen-loaded PI-wFBGA at 200℃ for 180 min and then removing the residual hydrogen molecules inside the fiber, the mechanical strength of the fiber remains unchanged. The high-temperature resistance PI-wFBG can operate at 350℃ for more than 40 days and have good temperature consistency in the high and low temperature cycle at 400℃. Changing the temperature and time of annealing, the experimental results show that the annealing of post-hydrogen-loaded PI-wFBG at 200℃ for 120 min is an optimal annealing method considering both the high-temperature resistance and mechanical property of high-temperature resistance PI-wFBG, which can realize the reliable use of PI-wFBGA in the temperature resistance range of polyimide coating. [ABSTRACT FROM AUTHOR]

Three-component geophone rotates randomly during oil and gas exploration, which is difficult to orient. There is a severe impact on the subsequent stratigraphic inversion. By fixing the fiber Bragg grating (FBG) sensor to the three-component geophone, the twist angle is the same. The rotation angle of the three-component geophone can be obtained by measuring the twist angle of the FBG sensor. However, the three-component geophone below the FBG sensor stretches the FBG sensor. Therefore, avoiding the effect of tensile strain on the torsional strain, requires simultaneous inversion of the twist angle and stretch length of the FBG sensor. This paper introduces an FBG sensor with simultaneous inversion of torsion angle and tensile length. The sensor consists of two FBGs embedded in the spiral groove of the compliant cylinder. Firstly, the sensing principle of the FBG sensor is analyzed, and developed FBG sensing model for tensile–torsional strain, including FBG by tilting arrangement, the extension of the tensile strain range of the FBG sensor was realized. Secondly, it describes the fabrication process of the FBG sensor, including loading the FBG with pre-stress, making the FBG sensor realize the extension of the torsion angle range. Next, the convenience of inversion is realized by averaging the calibrated strain transfer coefficients. Finally, to verify the effectiveness of this bidirectional helical fiber design and sensing technology, simultaneous torsion and tension tests were conducted on FBG sensors. The experimental results show that the sensor achieves simultaneous torsion angle and tensile length inversion. The maximum torsion angle of the inversion is 180°. The maximum stretch length of the inversion is 10 mm. • The first application of FBG sensor based on a bi-directional helix structure for identification of three-component geophone orientation, and the simultaneous inversion of FBG sensor torsion angle and tensile length is realized. • The strain range is 6000 μ ϵ for axially arranged FBG. In tilt arrangement FBG, where the helical angle is 45°, the tilted FBG strain range is 4 times higher than the axial FBG strain range, and the extension of the tensile strain range has been realized. • The twist angle measured by the four-core FBG sensor is 78° (Modes et al., 2020). By loading pre-stress, the FBG sensor based on a bi-directional helical arrangement measures a torsion angle of 180°. In contrast, an expansion of the torsional angle range has been achieved. [ABSTRACT FROM AUTHOR]

• Dual parameters sensing of temperature and strain is realized by cascading two π–phase-shifted fiber Bragg grating(π-PSFBG) with different characteristic peaks. • The characteristic peak of π-PSFBG has a very narrow reverse peak, which can be accurately located. • 1 μm thick Indium Tin Oxide(ITO) film was deposited on the surface of π-PSFBG by pulsed laser deposition(PLD) method. • Compared with the sensing performance before and after coating, the ITO film enhances the temperature sensing sensitivity of π-PSFBG and reduces the strain sensing sensitivity. • By wrapping the heat shrinkable tube, the strain sensing sensitivity of π-PSFBG is greatly reduced and the temperature sensing sensitivity is greatly increased. • The two modification methods can not only enhance the sensitivity of the sensor, but also protect the sensor, and are economical. This paper investigates the dual-parameter sensing of temperature and strain using π-phase-shifted fiber Bragg gratings (π-PSFBGs). In many practical engineering applications, temperature and strain are often present simultaneously, which can interfere with each other's sensing. To achieve dual-parameter sensing of temperature and strain, two π-PSFBGs are modified with phase-shifted peaks of 1547 nm and 1550 nm, respectively. The π-PSFBG of 1547 nm is wrapped with a heat shrinkable tube to increase its temperature sensing sensitivity by 185.6%, while its strain sensing sensitivity is reduced by 91.083%. The π-PSFBG of 1550 nm is coated with indium tin oxide (ITO) film using the pulsed laser deposition (PLD) method to increase its temperature sensing sensitivity by 22.3%, while its strain sensing sensitivity is reduced by 16.03%. Finally, the two modified π-PSFBGs are cascaded to realize dual-parameter sensing of temperature and strain. The ITO film coating on the surface and heat shrinkable tube wrapping can reduce the interference of external light on the signal light and protect the fiber grating. By setting the optical spectrum analyzer (OSA) accuracy to 0.02 nm, the sensing information can be extracted by scanning within a few nanometers, and the sharp phase-shifted peak of the π-PSFBG helps accurately locate the signal wavelength. The results demonstrate the potential for the practical application of the proposed dual-parameter sensing method. [ABSTRACT FROM AUTHOR]

• In this paper, an online steam pipeline leakage monitoring method, based on a weak-reflectivity optical fiber Bragg grating (wFBG) array for temperature–vibration hybrid sensing, is presented. • This method meets the requirements of long-distance, large capacity, low false positive rate (FPR), and low false negative rate (FNR). The drawing tower online-writing FBG technology was used to write the wFBG (-40 dB), which has the advantages of high strength and good consistency. • A large-capacity sensor network is realized using mixed time-division multiplexing and wavelength-division multiplexing technology. A novel temperature–vibration composite optical cable structure was designed to realize multi-parameter pipeline leakage monitoring. • A chirped wFBG array with a spatial resolution of 3 m and a narrowband wFBG array with a spatial resolution of 1 m were combined in the same optical cable to simultaneously monitor the vibration and temperature of the pipeline. Leakage of the steam pipeline is accompanied by changes in the temperature and vibration signals. The leakage and its location can be identified more accurately by changes in these two parameters in the wFBG wavelength and interferometric phase. • The field experiment results show that this temperature–vibration two-parameter detection system can accurately identify and locate steam pipeline leaks and reduce the FPR. This validates the feasibility of the wFBG array-based temperature–vibration hybrid sensing technology for monitoring steam pipeline leakage. In this paper, an online steam pipeline leakage monitoring method, based on a weak-reflectivity optical fiber Bragg grating (wFBG) array for temperature–vibration hybrid sensing, is presented. This method meets the requirements of long-distance, large capacity, low false positive rate (FPR), and low false negative rate (FNR). The drawing tower online-writing FBG technology was used to write the wFBG (−40 dB), which has the advantages of high strength and good consistency. A large-capacity sensor network is realized using mixed time-division multiplexing and wavelength-division multiplexing technology. A novel temperature–vibration composite optical cable structure was designed to realize multi-parameter pipeline leakage monitoring. A chirped wFBG array with a spatial resolution of 3 m and a narrowband wFBG array with a spatial resolution of 1 m were combined in the same optical cable to simultaneously monitor the vibration and temperature of the pipeline. Leakage of the steam pipeline is accompanied by changes in the temperature and vibration signals. The leakage and its location can be identified more accurately by changes in these two parameters in the wFBG wavelength and interferometric phase. The field experiment results show that this temperature–vibration two-parameter detection system can accurately identify and locate steam pipeline leaks and reduce the FPR. This validates the feasibility of the wFBG array-based temperature–vibration hybrid sensing technology for monitoring steam pipeline leakage. [ABSTRACT FROM AUTHOR]

Over the last 37 years, particular interest has been directed towards exploring the characteristics of the optical environment for sensing, giving rise to what would now be one of the largest applications of well-known optical fibers, typically employed to transmit data at high rates. Sensing temperature, pressure, liquid level, deformation, and other physical parameters utilizing optical fibers has become a growing branch of research and a business competing with well-established electrical sensors in the industry. Optical fiber sensors have all the inherent characteristics of a fiber optic cable, such as electromagnetic immunity, small size and weight, multiplexing, and so on. These exclusive features have made fiber sensors so versatile as to become a transformative technology by enabling several industrial processes to be carried out with higher reliability. Nowadays, there are several optical sensors, including fiber Bragg grating, interferometric, polarimetric, polymer fiber, distributed, and several others. Specifically, Raman-based distributed temperature sensor (RDTS) is a class of fiber optic sensors broadly employed in temperature measurement of large structures such as oil and gas wells, tunnels, and pipelines. Since 1985, many techniques have been proposed to break through the barriers of exploring Raman scattering as a distributed temperature measurement method. Range, spatial and temperature resolutions have been the most investigated parameters. In this perspective, this paper presents a comprehensive review focused on the progress of the RDTS technology over the past 37 years (1985–2022), covering an analysis of over 500 journal papers. First, a brief introduction to fiber optic sensor technology is presented as a theoretical basis, discussing the emergence of distributed sensors. Subsequently, Raman scattering in optical fibers is introduced, as well as how this nonlinear effect can be used to build temperature sensors. Next, RDTS technology is detailed, followed by a discussion of its applications and evolution over nearly four decades of development. Lastly, future perspectives are addressed in this review for the advancements in distributed temperature sensor technologies. • A review of the Raman scattering-based distributed temperature sensors is done. • A comprehensive literature analysis over the past 37 years is addressed. • Timelines, applications, as well as future perspectives, are also discussed. [ABSTRACT FROM AUTHOR]

• This paper presents a cross-type fiber layout method. • This method can achieve deformation sensitivity of 1000 pm/mm. • This method can realize continuous small deformation monitoring of structures. Deformation monitoring is an important part of structural health monitoring. A high-precision continuous deformation monitoring method based on ultra-weak fiber Bragg grating (FBG) array sensing technology is proposed in this paper. The ultra-weak FBG array sensing fiber is continuously arranged by cross-type. The wavelength shift difference of the corresponding two ultra-weak FBGs is monitored in real-time to obtain the relative deformation between the monitoring nodes, so as to obtain the absolute deformation of all monitoring nodes. The mathematical model of continuous deformation based on cross-type is established and verified by experiments. The results show that this method has the advantages of good linearity, high-sensitivity, and temperature-insensitive. In the continuous deformation simulation experiment, the tiny deformation of the monitoring node is accurately obtained, and the error between the monitoring results and the actual deformation results is less than 5%. The proposed monitoring method is particularly suitable for continuous small deformation monitoring of large-scale metal structures. [ABSTRACT FROM AUTHOR]

• This paper proposes a strain sensing system based on an erbium-doped fiber laser in liquid nitrogen environment. • The FBG is inscribed through polyimide coating and cladding, realized by a femtosecond laser point-by-point inscribing method. • The strain sensitivity of the sensing system is 0.890 pm/με and the linearity is 99.72 % in liquid nitrogen environment. In this paper, a strain sensing system based on an erbium-doped fiber laser in liquid nitrogen environment is proposed and experimentally verified. A fiber Bragg grating (FBG) and a Sagnac loop are used as cavity mirrors of a fiber laser. The FBG with a central wavelength of 1532.39 nm, a 3-dB linewidth of 0.61 nm, and a signal-to-noise ratio (SNR) of 9.06 dB is inscribed through polyimide coating and cladding, realized by a femtosecond laser point-by-point inscribing method. The 3-dB linewidth and SNR of the fiber laser are 0.06 nm and 36.53 dB, respectively. The measurement results show that the linear range of the measurement is from 1,000 με to 3,000 με, the strain sensitivity of the system is 0.890 pm/με and the linearity is 99.72 % in liquid nitrogen environment. [ABSTRACT FROM AUTHOR]

• SPR-based optical fiber sensor for salinity measurement. • FBG sensors for salinity assessment through thermal responses. • Combination of SPR and FBG sensors for higher accuracy in salinity estimation. This paper presents a novel optical fiber sensor system for salinity monitoring using data fusion in different optical fiber sensors approaches. In this case, a surface Plasmon resonance (SPR) sensor in a D-shaped optical fiber is developed in conjunction with a fiber Bragg grating (FBG). The SPR sensors are able to detect the liquid sample salinities through the refractive index variation of the different samples. In contrast, the FBG sensors estimate the salinity through the thermal dynamics of the samples. Results show a sensitivity of 164.0 nm/% and 12.5 pm/°C for the SPR and FBG sensors, respectively. Thus, the combination between the SPR and FBG sensors result in an improvement of the salinity monitoring of different samples, where a root mean squared error of 0.001% is obtained. [ABSTRACT FROM AUTHOR]

• 1:3 scale laboratory train model. • Rail track stability, rail-wheel contact stress in laboratory rail test rig. • Vertical force of wheel to axial strain on the rail measurement using optical sensors. • Train axle peak detection, rolling contact fatigue analysis using FBG in laboratory train test rig. • Fiber Bragg grating sensing technology in rail transport. Railway and train condition monitoring are two critical components in assuring passenger safety and comfort during their journey. In this work, a new laboratory downscale rail test rig has been designed and developed for exploring rail-wheel interaction and axle peak detection using fiber Bragg grating (FBG) optical sensors. This paper describes the laboratory setup's development process which includes numerical analysis of rail model, review of different experimental techniques, followed by 1:3 scaled model, mechanical and functional analysis of the railway setup with the grating sensor. By employing various data processing techniques axle peak detection of train was done and strain/stress transfer was calculated by the train passage to the rail track which was ∼100.86 µɛ. Total wavelength shit in the FBG sensor was 0.133 nm due to wheel load and sensitivity value was calculated approximately 1.36 pm/µɛ for the wavelength/strain changes. Characteristics of 1:3 laboratory rail test rig such as track retention, number of wheels, axle peak detection, rail-wheel rolling contact, stress/strain track deformation can be obtained in real time with fiber Bragg grating sensors. [ABSTRACT FROM AUTHOR]

• Measurement of large surface displacements is crucial for SHM. • Characterization of an FBG displacement sensor based on curvature sensing is presented. • Bidirectional displacement measurement is possible with high sensitivity. • Temperature compensation technique reduces contributions due to fast temperature changes. Measurement of displacement, deformation, curvature, and other physical parameters is of great importance for adequate Structural Health Monitoring (SHM) of large surfaces. Fiber Bragg Grating (FBG) sensors have become a focus of research in academia and engineering for structural monitoring applications. This paper presents the study and characterization of a displacement FBG sensor based on curvature sensing of an array of 4 FBGs , as well as possible applications for structural monitoring, developing the sensing part, and analyzing the signals obtained to develop a sensor with good sensitivity and resolution, capable of measuring bidirectional displacements. The sensor prototype is presented to measure displacements in a range of ±30 mm with a single grating, with a sensitivity of up to 74 pm/mm, linearity to R 2 = 0. 999, and hysteresis error up to 1.27 %, with temperature compensation for displacement monitoring in case of temperature variations. [ABSTRACT FROM AUTHOR]

• This article presents an FBG-based method for submarine cable shape monitoring. • The Frenet framework is used in the paper to achieve the continuity of discrete curvature. • When the monitoring system from the article was used on the undersea landing cable, it discovered potential dangers. Submarine cables are crucial in the transmission of electricity between coastal islands. Compared with terrestrial cables, submarine cables operate under harsher environments and thus their probability of failure is higher. Therefore, the effective monitoring of submarine cables is necessary to prevent related accidents. Quasi-distributed sensing technology based on fiber Bragg gratings converts the sensing information into discrete curvature information according to the pure bending theory and subsequently reconstructs the shape of the submarine cable using discrete curvature interpolation and curve reconstruction algorithms. The fiber Bragg grating sensor is placed on the landing cable section to analyze the reconstruction effect and error, demonstrating the feasibility of the method for submarine cable monitoring. Finally, the monitoring results in extreme weather reveal potential safety hazards in the submarine landing cable. [ABSTRACT FROM AUTHOR]

In this paper, we present a thorough study on an all-fiber narrow-linewidth FBG-based laser oscillator. Having established a theoretical model, the spectrum broadening characteristics of the narrow-linewidth FBG-based oscillator were analyzed based on multiple factors. During the experiment, using optimized structural parameters according to theoretical analyses, a backward pumped laser oscillator achieved an output power of 192.4 W and a spectral 3-dB bandwidth of 33.7 pm, corresponding to a power spectral density of 5.7 W/pm. To the best of our knowledge, this is the highest PSD reported to date by an all-fiber FBG-based laser oscillator. • In this paper, we present a thorough study on an all-fiber narrow-linewidth FBG-based laser oscillator. • Having established a theoretical model, the spectrum broadening characteristics of the narrow-linewidth FBG-based oscillator were analyzed based on multiple factors. • During the experiment, using optimized structural parameters according to theoretical analyses, a backward pumped laser oscillator achieved an output power of 192.4 W and a spectral 3-dB bandwidth of 33.7 pm, corresponding to a power spectral density of 5.7 W/pm. • To the best of our knowledge, this is the highest PSD reported to date by an all-fiber FBG-based laser oscillator. [ABSTRACT FROM AUTHOR]

• A flow sensor based on fiber Bragg grating is designed, which has the advantages of small size, wide range, and high accuracy. • Dynamic performance indexes were analyzed, and it was concluded that the flow sensor has a better Nonlinearity, Hysteresis and Repeatability. • This work is based on theoretical research, simulation and numerous experimental verifications. In this paper, the limitations of traditional electronic-type gas flow sensors for flow measurements in practical engineering applications such as flammable and explosive gas environments are discussed. Further, the importance and uncertainty of gas flow measurements in critical parts of industrial equipment are also discussed. In this work, a high-precision differential-pressure FBG gas flow and a temperature sensor with an embedded nozzle were designed using a long-diameter nozzle-type throttle tube with a fiber optic Bragg grating and an elastic-sensitive diaphragm composite strain mechanism. It was observed that the pressure difference between the two sides of the nozzle leads to the deformation of the elastic-sensitive diaphragm, and it results in a wavelength drift of the strain grating that is attached to the elastic diaphragm. Further, it also enables real-time monitoring of the gas flow value. Moreover, the temperature compensation grating which was attached to the side wall of the gas chamber was used to eliminate the cross-sensitivity of the strain grating as well as to improve measurement accuracy while implementing a temperature monitoring system. The experimental results showed that the Bragg wavelength drift in the fiber grating gas flow sensor acts as a quadratic curve with respect to the gas flow, and the experimental results were found to be highly compatible with the results of the theoretical analysis. [ABSTRACT FROM AUTHOR]

• An amplitude temperature sensor based on chirped fiber Bragg gratings is presented. • An accuracy is 0.48 °C and a sensitivity is 0.20 µW/°C in the range of 30–70 °C. • The power of signal can be scaled by increasing the number of pairs of CFBGs. In this paper we study an original technique for creating an amplitude temperature sensor based on n -pairs of chirped fiber Bragg gratings (CFBGs). The scalable design of the sensor allows to increase the range of the measured signal. The use of the amplitude interrogation technique ensures high speed, elementary nature of the scheme and simple operating conditions. The application of superimposed CFBGs as a sensitive element of an amplitude temperature sensor is investigated. As a result of the work, a method for measuring temperature with a fiber-optic sensitive element based on n -pairs of CFBGs is tested. Two superimposed CFBGs are inscribed in a standard telecommunication single-mode optical fiber using a KrF excimer laser system and Talbot interferometer. One of the superimposed structures is used as a reference element of the sensor and the second – as a sensitive element and is placed in a brass tube, where it is fixed on one side with an adhesive joint. The sensor has an accuracy of 0.48 °C and a sensitivity of 0.20 µW/°C for temperature measurement in the range of 30–70 °C. The results obtained are of great importance in the development of fiber-optic temperature control systems. Implementation of the sensor based on n -pairs of CFBGs allows scaling the power of signal, increasing the ability to detect a slight change in ambient temperature. The created sensor works according to the amplitude polling technique, which is characterized by high accuracy and speed of measurement, minimized scheme, simplified use and easy processing of the detected signal. In addition, the implemented interrogation method corresponds to the trend of developing low-cost fiber-optic systems without losing their main advantages. [ABSTRACT FROM AUTHOR]

• All parasitic components analyzed for the FBG-assisted Phase-OTDR system. • A complete (original) SNR definition provided. • Performance parameters analysed when MRC (multi-reflection crosstalk) is present. • 1st- and 2nd- order MRC considered, single- double- pulse configurations compared. • The interrelated MRC and SSC (spectral shadowing) effects are demonstrated. In this paper, the parasitic components (i.e., multi-reflections, Rayleigh scattering, photodetector noise, and phase variations due to external perturbations) are analysed and based on this analysis, a new signal to noise ratio (SNR) definition is provided suitable for the FBG-assisted Phase-OTDR system. A detailed analysis of performance parameters in the presence of multi reflection crosstalk (including its first- and second-order components) and spectral shadowing crosstalk is presented. SNR was calculated for different reflectivity and spacing lengths showing that the maximum number of cascaded FBGs can be significantly increased by using lower FBG reflectivity. It was also observed that the spacing length distance does not have a significant impact on the maximum number of FBGs that can be interrogated. By comparing single-pulse and double-pulse configurations, the use of double pulse was shown to provide higher SNR values when the number of FBGs is around 100 FBGs. The multi-reflection crosstalk when combined with the spectral-shadowing effect was demonstrated to create secondary crosstalk components making the interpretation of spectral analysis more difficult. [ABSTRACT FROM AUTHOR]

The stress concentration appears in the form of a quadratic or higher-order strain distribution, which affects the measurement accuracy of fiber Bragg grating (FBG). The FBG spectrum presents asymmetric deterioration when the stress concentration exceeds a specific limit, making the symmetric Gaussian fitting algorithm no longer suitable for seeking its peak wavelength. In this paper, the degree of stress concentration is defined by the quadratic coefficient of the strain field. The bearing capacity of different length FBGs to the Gaussian fitting algorithm is investigated under the quadratic strain distribution. The experimental results show that the FBG with shorter length had a higher tolerance to the Gaussian peak-seeking algorithm. The Extreme fitting algorithm based on a single peak has been proposed to adapt to the asymmetric changes of FBGs spectra. The exceptional adaptability of the Extreme peak-seeking algorithm compared with Gaussian fitting has been proved, which provides a reliable reference for the accurate peak seeking of FBG reflect spectrum in a nonuniform strain field. [ABSTRACT FROM AUTHOR]

• Designed a reliable self-healing communication integrated with a sensor network for the function of fault protection. • Design a cost-effective and flexible network, and increase the coverage of transmission distance. • Designed communication integrated with a sensor network for simultaneous transmission of communication and sensing signals. • Proposed machine learning algorithm to predict the wavelength of sensors. In this paper, we proposed a novel self-healing fiber/free-space optics (FSO) communication integrated with a sensor network for simultaneous transmission of optical communication signal and sensing signal as well as a function of fault protection by using a coarse wavelength-division multiplexer (CWDM). The combination of CWDM and fiber integrated with FSO transmission channel is incorporated to design a cost-effective, flexible network and increase the coverage of transmission distance. The experiment result proves that the fiber integrated with an FSO transmission channel achieves a clear constellation diagram and best error vector magnitude (EVM) value. On the other hand, the scanning-laser interrogation system is used for data acquisition in the strain-based fiber Bragg grating (FBG) sensor system. Moreover, the radial basis function neural network (RBFNN) algorithm is used to estimate the central wavelength of FBGs. [ABSTRACT FROM AUTHOR]

• Both directions outside the main vibration direction have excellent anti-interference ability. • Extending the base to reduce the effective length of the fiber plays a role in improving the sensitivity. • In the beam type FBG accelerometer, the accelerometer in this paper has the characteristics of small volume and easy processing. A Fiber Bragg Grating (FBG) accelerometer with cantilever beam is presented and experimentally demonstrated. The effective length of the optical fiber is reduced by changing the base of the accelerometer, so as to increase the sensitivity. The resonant frequency and the sensitivity of the accelerometer are deduced theoretically. The structural parameters were optimized by numerical simulations and the characteristics of the accelerometer were simulated by finite element simulations. The experimental results show that the resonant frequency of the accelerometer is 119 Hz, the sensitivity (the main direction z) is 134.29 pm/g within 2–60 Hz flat region. It also designed with good immunity to interference, the cross sensitivity in the x and the y directions are 3.86 % and 2.97 %, respectively. In addition, the accelerometer has the characteristics of small volume and easy processing. In general, this accelerometer has a research significance in the field of oil and gas exploration. [ABSTRACT FROM AUTHOR]

• This paper proposes a FBG acceleration sensor based on the double-mass block structure. • The resonance frequency of the sensor is 1000 Hz, and the sensitivity of the sensor is 26.7 pm/g at 250 Hz. • The sensor has a simple structure, low cost and easy to fabricate. A double-mass block acceleration sensor based on Fiber Bragg Grating (FBG) is proposed to meet the needs of acceleration sensors in vibration monitoring. The proposed sensor is composed of FBG, beam and double mass block, and the FBG is fixed in the middle of the upper surface of the dual mass block by epoxy glue. Through theoretical analysis and experimental verification, the resonant frequency, sensitivity and lateral anti-interference ability of the sensor have been investigated. The experimental results show that the natural frequency of the sensor is 1000 Hz, the sensitivity is 26.7 pm/g when the frequency is 250 Hz, and the lateral anti-interference degree is less than 6%. And the sensor structure in this paper is simple and easy to fabricate. Therefore, it has a great application prospect in the vibration monitoring of bridges, roads and buildings. [ABSTRACT FROM AUTHOR]

• To survive impact the contact radius between the fiber and impactor must be increased. • Silicone was chosen due to its high strain to failure and elastic properties. • In situ silicone sensors have shown to minimally effect back face deformation depth. • Silicone sensing mats are able to protect optical fibers during impact. The authors previously demonstrated the use of FBG sensors in Kevlar mats behind body armor to measure the transient back face deformation (BFD) during ballistic testing. This paper presents a novel sensor system based on a Fiber Bragg Grating embedded in silicone mats to improve the survivability of the body armor in-situ strain sensing layers. Due to the large amount of deformation, a relative slip between the optical fibers and the supporting structure is needed to maintain the performance of the sensors and determine the relationship between the measured strain and deformation shape. Two silicone materials were tested, Smooth-Sil 950 and Sorta-Clear 40, in both 1 mm and 2 mm thicknesses to evaluate their survivability and impact on BFD. To enhance slipping between the fibers and surrounding silicone a thin layer of petroleum jelly was placed on the fibers prior to being cast in the silicone mats. The 1 mm Sorta-Clear 40 mats performed best in silicone survivability, FBG survivability and minimal impact on the BFD. The new system improves on key deficiencies that were found from inserting the fibers directly into the Kevlar with minimal to no impact on the back face deformation. [ABSTRACT FROM AUTHOR]

• The VMD algorithm is improved by optimizing the number of decompositions K; • A high-sensitivity flexible FBG packaging structure is designed; • Constructed a mapping algorithm with inversion of respiratory wave and ECG characteristics; • The accuracy rate of the mapping algorithm is as high as 99.28 %, and the error is only 0.2 bpm. We propose a pulse and respiration monitoring system based on fiber optic sensing, to achieve accurate monitoring of human pulse and respiration rate with wearable and high comfort. We designed a flexible packaging structure -- to embed the fiber Bragg grating sensors (FBGs) fiber optic sensor into a nylon bandage, which constitutes a wearable real-time monitoring system that can acquire pulse and respiration rates simultaneously. The system uses the Variational Mode Decomposition - Feature Point Recognition (VMD-FPR) signal processing algorithm to demodulate and analyze the FBG echo signal to invert the pulse and respiration rate. The experiments were conducted on human heart rate and respiration rate in different states. The experimental results showed that the error range of respiration rate was within ±0.2 bpm, and the accuracy of the respiration experiment feature point recognition algorithm was 93.29 %. The error range of heart rate was within ±2 bpm, and the accuracy of the pulse experiment main wave feature point recognition algorithm was 99.28 %. Thus, the system can accurately collect and analyze the feature points in the respiratory and pulse wave signals. The information obtained from the experiments is used to build an information base, which provides data support for establishing the analysis model of pulse characteristics and respiratory characteristics of the detectors, and to achieve the function of real-time monitoring of human physiological parameters. [ABSTRACT FROM AUTHOR]

• Having established a simulation model, the spectrum broadening characteristics of the narrow-linewidth FBG-based amplifier were analyzed based on multiple factors. • A plum-shaped coiling design for the gain fiber was used to filter out high-order modes thus obtaining a beam quality m2 of 1.32 at the maximum output power of 3.3 kW. • A Raman suppression ratio of 26.3 dB was obtained at the maximum output power of 3.3 kW. • To the best of our knowledge, this is the highest record of narrow-linewidth fiber laser based on one stage MOPA configuration with normal commercial narrow-bandwidth fiber gratings and non-wavelength-locked laser diodes. This paper demonstrates a narrow-linewidth fiber laser with near-diffraction-limited beam quality and high Raman suppression ratio by utilizing Master Oscillator Power-Amplifier structure based on narrow-bandwidth fiber Bragg gratings. The output power reaches 3.31 kW with a slope efficiency of 85.0 %. At the maximum output power, the 3 dB, 10 dB, and 20 dB linewidths are 0.40 nm, 1.20 nm, and 3.09 nm, respectively. The beam quality M2 factor is 1.32 and the Raman suppression ratio reaches 26.3 dB. A pair of narrow-bandwidth fiber gratings were used in the oscillator to compress the linewidth of the seed laser. We used backward-pumping configuration and plum-coiled gain fiber in the amplification stage to suppress the Raman effect and select transverse mode by fiber-bending. Finally, a compact all-fiber laser was obtained with narrow-linewidth, high beam quality, and effective suppression of the Raman effect. To the best of our knowledge, this is the highest output power for a sub-nanometer-linewidth laser using narrow-bandwidth fiber gratings and none-wavelength-locked laser diodes, which achieves near-diffraction-limit beam quality. This provides a solution for high-power narrow-linewidth laser sources with high beam quality. [ABSTRACT FROM AUTHOR]

• The sensor can be used to monitor crack growth at structural joints. • Equal-strength beam is used to realize the desensitization mechanism of the sensor. • The sensor shows good linearity and repeatability in calibration tests. • The sensor is used in model experiments and shows good consistency with simulation. Monitoring of cracks on offshore platforms is necessary to ensure the safe operation and prolong their service life. The crack extension width is of great significance for the calculation of stress intensity factor (SIF). This paper developed a measuring device for crack extension width based on Fiber Bragg Grating (FBG) sensing technique. The designing principle was detailed introduced upon which a sensor with measuring range of 0–5 mm and sensitivity of 0.4 nm/mm was encapsulated according to engineering requirements. The calibration results indicated that the sensor has good linearity and repeatability within the design range. Based on local model experiment with prefabricated crack, results demonstrated that the sensor has high precision and feasibility for different crack damage conditions as well as good stability towards cyclic loading tests, showing potential prospective for offshore platform crack monitoring. [ABSTRACT FROM AUTHOR]

• Optical fiber sensors for impact amplitude and localization. • Analysis of spectral features of FBG embedded in diaphragm. • Impact localization estimation in the x and y planes with errors below 5%. • Diaphragm-embedded FBG sensor for force assessment with errors below 0.47 N. This paper presents the development, numerical and experimental analyses of a fiber Bragg grating (FBG) based sensor for force amplitude and localization (in 2D plane) detection. The sensor system is based on a diaphragm-embedded FBG, where the spectral features, namely wavelength shift, optical power and full width at half maximum (FWHM) are analyzed as a function of the force application parameters (amplitude and position). The diaphragm results in a higher spatial range for the sensor as its larger dimensions (when compared with the optical fiber) enable the strain transmission to the optical fiber even when the force is not directly applied to the fiber. In order to verify the proposed approach, finite element method is applied on the strain analysis in the optical fiber when the diaphragm is subjected to forces at different positions. Then, the FBG reflected spectra are simulated considering the strain distribution obtained at each condition, where there are variations not only in the Bragg wavelength, but also in the FWHM and optical power as a function of the force amplitude and position. The experimental analysis confirms the results numerically obtained and shows the possibility of simultaneously measuring force amplitude and position (x and y components) using a single FBG. Results show the feasibility of the method, where root mean squared errors of 0.47 N (in 10 N range), 0.48 mm (in 10 mm range) and 0.33 mm (in 6 mm range) are found for the force, horizontal (x-) and vertical (y-) components estimations, respectively. Thus, the proposed approach shows relative errors below 5% and is an interesting approach for sub-millimeter resolution force and impact detection, which can also be used in multiplexed systems with the advantage of higher spectral efficiency, since a single FBG can provide multiple measurements that are commonly made employing a higher number of sensors. [ABSTRACT FROM AUTHOR]

• Systematic study on the effects of external environmental vibrations on fiber gratings for different sized capillary metal tube packaging methods. • A research method is applied that selects the optimal packaging method under different vibration environments, to reduce the impact of vibrations on the proposed fiber gratings. • This work is based on theoretical research, simulation and numerous experimental verifications. Given the current limitations of traditional electrical sensors in practical engineering applications, alongside the difficulties of temperature monitoring in key components of industrial equipment, we propose a novel passive sensor based on fiber Bragg grating (FBG) that is encapsulated in a capillary metal tube. In addition, to provide a solution to the cross-sensitivity problem of the FBG sensor on measuring vibrations and temperature, this paper analyzed the influence of environmental vibrations on the FBG temperature sensor. This was achieved, in detail, through vibration experiments on the FBG sensor. The results showed that account should be performed for the interference of the FBG temperature sensor with factors such as environmental vibrations, acceleration, and the first-order natural frequency of the metal tube that was used in the packaging. Finally, an optimal analysis method for a metal tube armored fiber grating sensor is proposed. [ABSTRACT FROM AUTHOR]

• The parallel theoretical model is proposed. • The theoretical design idea of expanding the frequency band and maintaining higher sensitivity based on adjusting the fiber length is clearly proposed. • The thought of simplifying boundary conditions is proposed. • The figure of merit is higher. A double-diaphragm acceleration geophone based on Fiber Bragg Grating (FBG) is proposed and demonstrated experimentally in this paper. The proposed geophone is mainly composed of a FBG, double diaphragms, and an inertial mass. Through theoretical analysis and experimental verification, the FBG acceleration geophone has good comprehensive response characteristics. The experimental results obtained indicate that the flat region sensitivity range is from 651.0 to 850.5 pm/G, the corresponding fluctuation is less than 2.20 dB within 1–70 Hz, the cross sensitivity is less than −16.4 dB, and the quality factor is better than 89.1 nm·Hz/G. Compared with the traditional geophones, the proposed geophone has a higher quality factor. Therefore, this type FBG geophone is very good candidate for low-frequency oil and gas exploration. [ABSTRACT FROM AUTHOR]

• A slide sensing unit based on cantilever beams with nine FBGs is developed. • A stress-strain model detects the one-dimensional slide. • The two-dimensional slide is perceived by modeling for fluctuation of contact force. This paper presents a slide sensing prototype system based on cantilever beams with nine optical fiber Bragg grating sensors in a differential arrangement. We have developed models to assess the two-dimensional sliding. If the object slides only along X axis or Y axis, it is an X-slide or a Y-slide. If there are slides on X-axis and Y-axis at the same time, we denote it as an X-Y-slide. We have employed a stress–strain model to judge the sliding direction and measure the sliding sensitivity. And the fluctuations of contact force have been modeled to perceive the X-Y-slide. The maximum relative error of the sliding sensitivity of X-slide between the experimental and the theoretical value is 3.71%. The experimental absolute error of Y-slide is 1.3 pm. For X-Y-slide, the maximum experimental absolute error of Y direction is 0.5 pm and the maximum absolute error of X direction is 1.93 pm. The proposed models for assessing sliding can work well and the sensing unit is able to perceive a two-dimensional sliding. The research results can be used in further studies to develop FBG sensors for the total detection of slide. [ABSTRACT FROM AUTHOR]