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The mining face under the close coal seam group is affected by the superposition of the concentrated stress of the overlying residual diagonally intersecting coal pillar and the mining stress, which can easily cause the instability and damage of the section coal pillars during the process of mining back to the downward face. Additionally, the traditional methods of monitoring such as numerical simulation, drilling peeping, and acoustic emission fail to realize the real-time and accurate deformation monitoring of the internal deformation of the section coal pillars. The introduction of the drill-hole-implanted fiber-optic grating monitoring method can realize real-time deformation monitoring for the whole area inside the coal pillar, which solves the short board problem of coal pillar deformation monitoring. However, fiber-optic monitoring is easily disturbed by the external environment, which is especially sensitive to the background noise of the complex underground mining environment. Therefore, taking the live chicken and rabbit well of Shaanxi Daliuta Coal Mine as the engineering background, the ensemble empirical modal decomposition (EEMD) is introduced for primary noise reduction and signal reconstruction by the threshold determination (DE) algorithm, and then the singular matrix decomposition (SVD) is introduced for secondary noise reduction. Finally, a machine learning algorithm is combined with the noise reduction algorithm for the prediction of the fiber grating strain signals of coal pillar in a zone, and DBO-LSTM-BP is constructed as the prediction model. The experimental results demonstrate that compared with the other two noise reduction prediction models, the SNR of the EEMD-DE-SVD-DBO-LSTM-BP model is improved by 0.8–2.3 dB on average, and the prediction accuracy is in the range of 88–99%, which realizes the over-advanced prediction of the deformation state of the coal column in the section. [ABSTRACT FROM AUTHOR]

The mining face under the close coal seam group is affected by the superposition of the concentrated stress of the overlying residual diagonally intersecting coal pillar and the mining stress, which can easily cause the instability and damage of the section coal pillars during the process of mining back to the downward face. Additionally, the traditional methods of monitoring such as numerical simulation, drilling peeping, and acoustic emission fail to realize the real-time and accurate deformation monitoring of the internal deformation of the section coal pillars. The introduction of the drill-hole-implanted fiber-optic grating monitoring method can realize real-time deformation monitoring for the whole area inside the coal pillar, which solves the short board problem of coal pillar deformation monitoring. However, fiber-optic monitoring is easily disturbed by the external environment, which is especially sensitive to the background noise of the complex underground mining environment. Therefore, taking the live chicken and rabbit well of Shaanxi Daliuta Coal Mine as the engineering background, the ensemble empirical modal decomposition (EEMD) is introduced for primary noise reduction and signal reconstruction by the threshold determination (DE) algorithm, and then the singular matrix decomposition (SVD) is introduced for secondary noise reduction. Finally, a machine learning algorithm is combined with the noise reduction algorithm for the prediction of the fiber grating strain signals of coal pillar in a zone, and DBO-LSTM-BP is constructed as the prediction model. The experimental results demonstrate that compared with the other two noise reduction prediction models, the SNR of the EEMD-DE-SVD-DBO-LSTM-BP model is improved by 0.8–2.3 dB on average, and the prediction accuracy is in the range of 88–99%, which realizes the over-advanced prediction of the deformation state of the coal column in the section.

Drill hole gas extraction is an important method to solve the problem of gas overrun. However, the mine gas extraction drilling hole is prone to collapse and deformation due to the influence of ground stress and drilling disturbance, which seriously affects the effect of gas treatment. Aiming at the problem that it is difficult to monitor the location of collapsed hole in the gas extraction drillhole of coal seam, a monitoring technology for collapsed hole based on fiber grating intelligent sensing technology was proposed. Firstly, the principle of monitoring drilling hole collapse by fiber grating was explained from the theory of stress monitoring by fiber grating sensors. Based on this, an experimental platform for the hole collapse characterization based on fiber grating was built. The hole collapse monitoring experiments of three types of grating arrangements (0°, 90° and 180°) under different amount of collapsed coal were carried out, the changes of grating measurement wavelength offset and extraction flow rate with the amount of collapsed coal were studied, and the collapse level of drillhole was classified. The experimental results shown that, the accuracy of collapsed hole monitoring was the highest when the grating measurement point was located below the substrate material, the relationship between the central wavelength offset Δλ and the amount of collapsed coal m was \begin{document}$m = 3.017\Delta \lambda $\end{document}, and the relationship between the extraction flow rate Q and the wavelength offset Δλ was \begin{document}$ Q = 8.72E - 7x^3 + 2.30E - 4x^2 - 0.18x + 30 $\end{document}. The decay rate of extraction flow was used as the criterion of collapsing hole to divide the three levels of collapsed hole: Class I collapsed hole with the extraction flow decayed to 90%, Class II collapsed hole with the extraction flow decayed from 90% to 50%, and Class III collapsed hole with the extraction flow decayed to less than 50%. Finally, the effectiveness of fiber grating collapsed hole monitoring technology was verified through a field test in 2802 working face of Zhangcun coal mine of Lu'an Group, and it was found that the distribution of collapsed-hole locations after 30 days of drilling was similar to that of the initial period after the drilling was completed. In the field test, five drill holes were successfully located and repaired based on the proposed collapsed hole monitoring technique, and the pure volume of gas extraction from the repaired drill holes was improved by 37.79% compared to that before the repair.

Aiming at the problems of lateral force interference and non-uniform strain of robot fingers in the process of pressure tactile sensing, a flexible tactile sensor with a square hole structure based on fiber Bragg grating (FBG) is proposed in this paper. Firstly, the optimal embedding depth of the FBG in the sensor matrix model was determined by finite element simulation. Secondly, according to the size of the finger knuckle and the simulation analysis based on the pressure tactile sensor element for the robot finger, the square hole structure was designed, and the overall dimensions of the sensing element and size of the square hole were determined. Thirdly, the FBG was embedded in the polydimethylsiloxane (PDMS) elastic matrix to make a sensor model, and the tactile sensor was fabricated. Finally, the FBG pressure tactile sensing system platform was built by using optical fiber sensing technology, and the experiment of the FBG tactile sensor was completed through the sensing system platform. Experimental results show that the tactile sensor designed in this paper has good repeatability and creep resistance. The sensitivity is 8.85 pm/N, and the resolution is 0.2 N. The loading sensitivity based on the robot finger is 27.3 pm/N, the goodness of fit is 0.996, and the average value of interference in the sensing process is 7.63%, which is lower than the solid structure sensor. These results verify that the sensor can effectively reduce the lateral force interference and solve the problem of non-uniform strain and has high fit with fingers, which has a certain application value for the research of robot pressure tactile intelligent perception. [ABSTRACT FROM AUTHOR]

Copyright of Machine Tool & Hydraulics is the property of Guangzhou Mechanical Engineering Research Institute (GMERI) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

It is difficult to monitor the collapse position of a gas extraction borehole in a coal seam. In order to solve this problem, a fiber-optic grating collapse monitoring technology was proposed. Five kinds of grating arrangements (0°, 45°, 90°, 135°, 180°) were examined to simulate the hole collapse. The relationship model between the center wavelength offset and the amount of collapsed coal and extraction flow was constructed to obtain the distribution curve of the hole collapse position and quality along the length of the hole, and flow decay rates of 80% and 50% were used as the critical values to classify the three levels of hole collapse. The results show that the hole collapse monitoring accuracy is the highest with the grating measurement points located below the substrate material arrangement. Finally, the effectiveness of fiber-optic grating monitoring technology was verified in the 2202 working face of the Changcun coal mine of the Lu'an Group, and the pure amount of gas extraction from the repaired borehole after monitoring was increased by 62.6% compared with that before the repair. [ABSTRACT FROM AUTHOR]

Copyright of Piezoelectrics & Acoustooptics is the property of Piezoelectric & Acoustooptic and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Variations of seawater salinity often cause ocean internal waves, water masses and stratification, which affect the stability of the ocean environment. Therefore, the study of seawater salinity is significant for the prediction of changes in the ocean environment. However, existing methods for measuring seawater salinity generally have the disadvantages of low sensitivity and low accuracy. In this work, we proposed a seawater salinity sensor based on long period fiber grating (LPFG) in the dispersion turning point (DTP), which has demonstrated the possibility to fabricate LPFG with a shorter grating period by CO2 laser in a thin single mode fiber (SMF) of 80 μm cladding diameter without etching. For obtaining higher sensitivity that could meet the measurement requirement in practice, the proposed sensor was optimized by combining etching cladding and DTP. After the LPFG working near DTP was fabricated by a CO2 laser, the cladding diameter was reduced to 57.14 μm for making cladding mode LP1,7 work near DTP by hydrofluoric acid (HF) solutions. The experimental results have demonstrated that a sensitivity of 0.571 nm/‰ can be achieved when the salinity increases from 5.001‰ to 39.996‰, and the sensor shows good repeatability and stability. Based on its excellent performance, the optimized LPFG is a prospective sensor to monitor seawater salinity in real time. Meanwhile, a low-cost way was provided to make LPFG work near DTP instead of ultraviolet exposure and femtosecond laser writing. [ABSTRACT FROM AUTHOR]

FBG based refractive index sensors are widely used in sensing applications such as structural health monitoring, oil exploration, refractive index measurement of chemical solutions, etc., owing to benefits of FBG such as small size, real time monitoring, high resolution, resistant to high temperature and various chemicals, etc. In this paper, simple and high sensitive method employing a double D-shaped FBG (DDSF) for refractive index (RI) measurements of glucose solution is proposed and investigated. The operating principle is based on the exploitation of the evanescent wave interaction of the propagating light with the surrounding external environment, resulting in shift of Bragg wavelength. Wavelength shift is measured, when the refractive index of external medium changes from 1.33 to 1.43 approximately for glucose solution with varying concentrations, exhibiting a good linear relationship between them. By changing the depth of the D-shaped groove, the sensor's sensitivity can optimize. A high sensitivity of 47.37 nm/RIU is achieved, which indicates that proposed sensor has provided high sensitivity and linearity. [ABSTRACT FROM AUTHOR]

Owing to the assembly state changes during aircraft assembly processes, assembly force-deformation problem occurs. To obtain the structure shape in the product assembly process efficiently and accurately, a three-dimensional (3D) mapping technology for the structural deformation during the aircraft assembly process is proposed combined with a fiber Bragg grating (FBG) optical fiber sensor and binocular vision measurement system. First, this study established a curvature transformation model using optical fiber monitoring data, obtained the 3D spatial deformation of the product, and completed the unification of the optical fiber wavelength change and spatial 3D point coordinate heterogeneous data. Second, a mesh deformation optimization algorithm based on point-cloud optimization was established. Subsequently, the deformation effects of four mesh deformation models were compared to verify the feasibility and accuracy of HEC-Laplace, and the 3D mapping of the product structure shape in the assembly process was realized. Finally, a cantilever wing model was used to verify the deformation of different loading modes. The results show that the product structure changes can be accurately obtained through the proposed technology, thereby improving the accuracy control and overall assembly quality in the aircraft assembly process and providing a theoretical basis for intelligent aircraft assembly. [ABSTRACT FROM AUTHOR]

In this study, a structural health monitoring system for aircraft wings based on a fiber-optic grating sensing network was designed, and a three-dimensional position correction algorithm based on fiber Bragg grating (FBG) array test data was developed. Solving the deformation parameters using mathematical models of strain-position offset introduces errors, leading to inaccurate measurements and affecting the preciseness of three-dimensional-reconstructed surfaces. Two fitting algorithms (bitone and spline with linear interpolations) are proposed to improve this method in analyzing errors at the measured points of the wing structure. The relative error of each measurement point of the reconstructed wing structure using the bitone and spline interpolation methods was lower than 4.96%. Furthermore, the absolute error was within 0.014 mm, the average relative error was lower than 3.47%, and the root-mean-square error was lower than 0.01 mm. Compared to linear interpolation, the average relative error decreased by up to 5.84%. The experimental results show that the measurement accuracy of bitone and spline interpolation was significantly higher than that of linear interpolation. FBG array measurements using bitone and spline interpolations for the local structural health monitoring system of aircraft improve the fitting reconstruction effect for the wing structure and stabilize the compensation effect.

Optical fiber sensors working at the dispersion turning point (DTP) have served as promising candidates for various sensing applications due to their ultrahigh sensitivity. In this review, recently developed ultrasensitive fiber sensors at the DTP, including fiber couplers, fiber gratings, and interferometers, are comprehensively analyzed. These three schemes are outlined in terms of operation principles, device structures, and sensing applications. We focus on sensitivity enhancement and optical transducers, we evaluate each sensing scheme based on the DTP principle, and we discuss relevant challenges, aiming to provide some clues for future research. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering Geology / Gongcheng Dizhi Xuebao is the property of Journal of Engineering Geology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Stress in rock masses is an important parameter in the design and construction of underground engineering, such as the design and maintenance of mine roadways and the design of mining working faces. It is also a fundamental force causing the deformation and failure of geotechnical engineering excavation. At present, the abutment-pressure monitoring technology of the surrounding rocks of the coal mine roadway in China is not intelligent and systematic and lacks some high-precision sensing instruments and multi-functional monitoring systems. The mechanical model of the rocks surrounding the borehole was constructed by theoretical analysis of problems in the stress monitoring technology for underground rock masses in coal mines. Additionally, the interaction between the surrounding rocks and the borehole stress meter was analyzed. The borehole stress meters for tubular-structure fiber Bragg grating (TS-FBG) and cystic-structure fiber Bragg grating (CS-FBG) were designed by combining the sensing principle and sensing characteristics of fiber Bragg grating, and the performance of the two kinds of fiber Bragg grating borehole stress meters was compared by laboratory test. The track roadway of the 14,301 tested working faces in the Shaqu Coal Mine was taken as an example, and the stress of the rocks surrounding the 14,301-track roadway was monitored in real time by CS-FBG borehole stress meter during the mining of the working face. The following conclusions are drawn from the field application. The rig-site utilization results revealed obvious stress growth and stress peak zones in the mining-stress change curves of each measuring point on the two sides of the 14,301-track roadway in the process of mining the tested working face. Additionally, there were four stages: rapid rise, uniform growth, rapid rise to the peak, and rapid decline. Maximum stress monitored by the second station was 18.5 MPa, and the influence range of stress was over 140 m. Maximum stress monitored by the first measuring station was 19 MPa, the influence range of stress was about 80 m, and the peak stress position was about 20 m in front of the coal wall. Rig-site utilization proved the design of the CS-FBG borehole stress meter to be reasonable. Performance was stable and reliable, and the successful operation of field monitoring achieved the expected effect.

The key to evaluating the health status of cable-stayed bridges lies in the accuracy of cable force measurement. When measuring the cable force using the conventional frequency method, the clearance between the bracing cable and the protective tube is typically disregarded. Moreover, due to their large size, existing vibration sensors are difficult to install into protective tubes for steel strand-type bracing cables to measure the cable force. To address the above difficulties, a type of thin rod vibration sensor only 5 mm in diameter was designed based on the high sensitivity of Fiber Bragg grating (FBG), and high-throughput data processing software for engineering calculation (EC) was self-developed. Then, the recognition principle of the thin rod vibration sensor was theoretically analyzed and a step-by-step tension test was carried out. The results demonstrated that the relative error of the cable force measured by the thin rod vibration sensor within 12.865 Hz was less than 5% and the sensitivity reached 28.7 pm/Hz, indicating its high measurement precision. Upon subsequent application of the thin rod vibration sensor to a monitoring test in the field, the relative error of the fundamental frequency between artificial and natural excitations was less than 4%. In addition, the error relative to both the theoretical frequency and the third-party sampling frequency was less than 5%, further verifying the accuracy and applicability for monitoring the cable force of bridges under natural excitation. Compared with the traditional cantilever FBG sensor, the improved sensor with supporting data processing software has the advantages of small cross-section, high reliability, and good sensitivity. The research results can provide a reference for the subsequent accurate measurement of cable force and the development of a supporting sensor data processing system. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin University of Science & Technology is the property of Journal of Harbin University of Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

In order to minimize the electromagnetic interference in the detection system in robot finger pressure tactile measurement, a fiber Bragg grating pressure tactile sensor array with the advantages of anti electromagnetic interference, small volume, light weight and easy packaging is used in many studies. Aiming at the problem of measuring point arrangement of tactile sensor array for robot finger pressing, a measuring points optimization method based on the improved particle swarm optimization algorithm was proposed. Firstly, the distribution of strain field in the process of robot finger grasping was obtained by finite element simulation. Secondly, the sensor array measuring points were selected based on the results of strain field distribution. Thirdly, the improved particle swarm optimization algorithm was used to optimize the design of measuring points. Finally, the optimal combination of measuring points was obtained and the experiment of robot finger grasping perception was carried out. The simulation results show that the measuring points combination of single finger and multi-finger can reduce the percentage error of force state recognition to less than 10%, and improve the accuracy of the sensor. The performance of the sensor array has been verified by the robot finger grasping sensing experiment.

Aiming at the problems of lateral force interference and non-uniform strain of robot fingers in the process of pressure tactile sensing, a flexible tactile sensor with a square hole structure based on fiber Bragg grating (FBG) is proposed in this paper. Firstly, the optimal embedding depth of the FBG in the sensor matrix model was determined by finite element simulation. Secondly, according to the size of the finger knuckle and the simulation analysis based on the pressure tactile sensor element for the robot finger, the square hole structure was designed, and the overall dimensions of the sensing element and size of the square hole were determined. Thirdly, the FBG was embedded in the polydimethylsiloxane (PDMS) elastic matrix to make a sensor model, and the tactile sensor was fabricated. Finally, the FBG pressure tactile sensing system platform was built by using optical fiber sensing technology, and the experiment of the FBG tactile sensor was completed through the sensing system platform. Experimental results show that the tactile sensor designed in this paper has good repeatability and creep resistance. The sensitivity is 8.85 pm/N, and the resolution is 0.2 N. The loading sensitivity based on the robot finger is 27.3 pm/N, the goodness of fit is 0.996, and the average value of interference in the sensing process is 7.63%, which is lower than the solid structure sensor. These results verify that the sensor can effectively reduce the lateral force interference and solve the problem of non-uniform strain and has high fit with fingers, which has a certain application value for the research of robot pressure tactile intelligent perception.

Variations of seawater salinity often cause ocean internal waves, water masses and stratification, which affect the stability of the ocean environment. Therefore, the study of seawater salinity is significant for the prediction of changes in the ocean environment. However, existing methods for measuring seawater salinity generally have the disadvantages of low sensitivity and low accuracy. In this work, we proposed a seawater salinity sensor based on long period fiber grating (LPFG) in the dispersion turning point (DTP), which has demonstrated the possibility to fabricate LPFG with a shorter grating period by CO2 laser in a thin single mode fiber (SMF) of 80 μm cladding diameter without etching. For obtaining higher sensitivity that could meet the measurement requirement in practice, the proposed sensor was optimized by combining etching cladding and DTP. After the LPFG working near DTP was fabricated by a CO2 laser, the cladding diameter was reduced to 57.14 μm for making cladding mode LP1,7 work near DTP by hydrofluoric acid (HF) solutions. The experimental results have demonstrated that a sensitivity of 0.571 nm/‰ can be achieved when the salinity increases from 5.001‰ to 39.996‰, and the sensor shows good repeatability and stability. Based on its excellent performance, the optimized LPFG is a prospective sensor to monitor seawater salinity in real time. Meanwhile, a low-cost way was provided to make LPFG work near DTP instead of ultraviolet exposure and femtosecond laser writing.

Owing to the assembly state changes during aircraft assembly processes, assembly force-deformation problem occurs. To obtain the structure shape in the product assembly process efficiently and accurately, a three-dimensional (3D) mapping technology for the structural deformation during the aircraft assembly process is proposed combined with a fiber Bragg grating (FBG) optical fiber sensor and binocular vision measurement system. First, this study established a curvature transformation model using optical fiber monitoring data, obtained the 3D spatial deformation of the product, and completed the unification of the optical fiber wavelength change and spatial 3D point coordinate heterogeneous data. Second, a mesh deformation optimization algorithm based on point-cloud optimization was established. Subsequently, the deformation effects of four mesh deformation models were compared to verify the feasibility and accuracy of HEC-Laplace, and the 3D mapping of the product structure shape in the assembly process was realized. Finally, a cantilever wing model was used to verify the deformation of different loading modes. The results show that the product structure changes can be accurately obtained through the proposed technology, thereby improving the accuracy control and overall assembly quality in the aircraft assembly process and providing a theoretical basis for intelligent aircraft assembly.

Graphene, a novel form of the hexagonal honeycomb two-dimensional carbon-based structural material with a zero-band gap and ultra-high specific surface area, has unique optoelectronic capabilities, promising a suitable basis for its application in the field of optical fiber sensing. Graphene optical fiber sensing has also been a hotspot in cross-research in biology, materials, medicine, and micro-nano devices in recent years, owing to prospective benefits, such as high sensitivity, small size, and strong anti-electromagnetic interference capability and so on. Here, the progress of optical fiber biochemical sensors based on graphene is reviewed. The fabrication of graphene materials and the sensing mechanism of the graphene-based optical fiber sensor are described. The typical research works of graphene-based optical fiber biochemical sensor, such as long-period fiber grating, Bragg fiber grating, no-core fiber and photonic crystal fiber are introduced, respectively. Finally, prospects for graphene-based optical fiber biochemical sensing technology will also be covered, which will provide an important reference for the development of graphene-based optical fiber biochemical sensors. [ABSTRACT FROM AUTHOR]

According to the need of cable force monitoring of space cable structure, a new fiber Bragg grating monitoring method is proposed by analyzing the mechanism, advantages and disadvantages and application scope of existing cable force monitoring methods. The results of practical engineering show that: the mechanism of this method is clear and easy to implement; the advantages of clamp cable, tension meter, and Jack monitoring method are fully used to avoid their disadvantages; in the mutual check stage of Jack and clamp cable tension meter, the monitoring data error of the two testing methods is less than 5%, which indicates that the clamp strain gauge is accurate and effective; this method has little influence on construction, and can make full use of the engineering project The clamp cable tension meter is arranged in the process of cable tension in stages, which can realize the implementation without affecting the original construction process. It provides an experience reference for similar cable tension process and cable force control in service stage in the future. [ABSTRACT FROM AUTHOR]

Abstract In this paper, a fiber grating demodulation system based on two transmission volume Bragg gratings (VBGs) was proposed. In order to resolve the problem that the spectral resolution of the present fiber grating demodulation system is not high enough, the two transmission VBGs were applied to improve the spectral resolution and reduce the volume of the spectrometer. The diffraction characteristics of the transmission VBGs were analyzed, and the optical path of the two transmission VBGs demodulation system was designed based on the diffraction characteristics. The grating constant, lens parameters, and aberration correction of the system were analyzed and calculated. The calculation showed that the theoretical wavelength range of the demodulation system was from 1525 nm to 1565 nm and the theoretical optical resolution was 60 pm when the grating constant was 0.9168, the angle between two transmission VBGs was 89°, the focal length of the collimator was 60 mm, and the focal length of the imaging lens was 131.5 mm. The aberration of the system was well corrected by using a lens as the collimator and a reflector as the imaging lens. The system principle prototype was assembled and calibrated, and its performances were experimentally investigated. The results showed that the spectrometer worked stably, with a wavelength range from 1525 nm to 1565 nm, an optical wavelength resolution of 65.3 pm, and a high demodulation speed of 10 kHz.

Optical fiber sensors working at the dispersion turning point (DTP) have served as promising candidates for various sensing applications due to their ultrahigh sensitivity. In this review, recently developed ultrasensitive fiber sensors at the DTP, including fiber couplers, fiber gratings, and interferometers, are comprehensively analyzed. These three schemes are outlined in terms of operation principles, device structures, and sensing applications. We focus on sensitivity enhancement and optical transducers, we evaluate each sensing scheme based on the DTP principle, and we discuss relevant challenges, aiming to provide some clues for future research.

In this paper, a fiber grating demodulation system based on two transmission volume Bragg gratings (VBGs) was proposed. In order to resolve the problem that the spectral resolution of the present fiber grating demodulation system is not high enough, the two transmission VBGs were applied to improve the spectral resolution and reduce the volume of the spectrometer. The diffraction characteristics of the transmission VBGs were analyzed, and the optical path of the two transmission VBGs demodulation system was designed based on the diffraction characteristics. The grating constant, lens parameters, and aberration correction of the system were analyzed and calculated. The calculation showed that the theoretical wavelength range of the demodulation system was from 1525 nm to 1565 nm and the theoretical optical resolution was 60 pm when the grating constant was 0.9168, the angle between two transmission VBGs was 89°, the focal length of the collimator was 60 mm, and the focal length of the imaging lens was 131.5 mm. The aberration of the system was well corrected by using a lens as the collimator and a reflector as the imaging lens. The system principle prototype was assembled and calibrated, and its performances were experimentally investigated. The results showed that the spectrometer worked stably, with a wavelength range from 1525 nm to 1565 nm, an optical wavelength resolution of 65.3 pm, and a high demodulation speed of 10 kHz. [ABSTRACT FROM AUTHOR]

Optical orbital angular momentum (OAM) has become a hot research topic because of its unique properties due to its spiral distribution of phases. The production and transmission of OAM has also become a necessary condition for effective use of OAM. As an optical waveguide with good propagation properties, optical fibers are used in optical systems supporting OAM. This paper introduces the OAM generation and transmission system based on fiber, summarizes the current photonic crystal fiber, ring core fiber, fiber grating and other all-fiber systems that can support OAM modes, and explains some experimental principles. Finally, an outlook on OAM generation or transmission devices for all-fiber systems is presented, providing a useful reference for future related research.

The key to evaluating the health status of cable-stayed bridges lies in the accuracy of cable force measurement. When measuring the cable force using the conventional frequency method, the clearance between the bracing cable and the protective tube is typically disregarded. Moreover, due to their large size, existing vibration sensors are difficult to install into protective tubes for steel strand-type bracing cables to measure the cable force. To address the above difficulties, a type of thin rod vibration sensor only 5 mm in diameter was designed based on the high sensitivity of Fiber Bragg grating (FBG), and high-throughput data processing software for engineering calculation (EC) was self-developed. Then, the recognition principle of the thin rod vibration sensor was theoretically analyzed and a step-by-step tension test was carried out. The results demonstrated that the relative error of the cable force measured by the thin rod vibration sensor within 12.865 Hz was less than 5% and the sensitivity reached 28.7 pm/Hz, indicating its high measurement precision. Upon subsequent application of the thin rod vibration sensor to a monitoring test in the field, the relative error of the fundamental frequency between artificial and natural excitations was less than 4%. In addition, the error relative to both the theoretical frequency and the third-party sampling frequency was less than 5%, further verifying the accuracy and applicability for monitoring the cable force of bridges under natural excitation. Compared with the traditional cantilever FBG sensor, the improved sensor with supporting data processing software has the advantages of small cross-section, high reliability, and good sensitivity. The research results can provide a reference for the subsequent accurate measurement of cable force and the development of a supporting sensor data processing system.

Acceleration detection is an important technical means in the fields of seismic monitoring, structural health monitoring, and resource exploration. Aiming at the problem of low sensitivity of fiber optic acceleration sensor, a low-frequency dual fiber grating acceleration sensor based on flexible hinge structure is proposed. Theoretical analysis of its sensitivity and resonant frequency is carried out. MATLAB software is used to analyze and optimize structural parameters such as hinge thickness, hinge radius, mass size, etc., and ANSYS software is used to carry out structural static stress and modal simulation, and a test system is constructed to carry out the physical performance of the sensor. Performance test experiment. The results show that the resonant frequency of the sensor is 117 Hz, and it has a flat sensitivity response in the low frequency range of 5 to 90 Hz. The sensitivity is about 278 pm/g. The drift of the fiber center wavelength has a good linear correlation with the acceleration. The degree of interference is less than 5%. Provide reference for the engineering application of fiber grating acceleration sensor. [ABSTRACT FROM AUTHOR]

This study presents the development of a monitoring probe for characterizing aromas during the coffee fermentation process. Using an optical device created through a Twin Long Period Fiber Grating interferometer in conjunction with auxiliary measurements of carbon dioxide and temperature, the probe was designed to capture relevant data throughout the fermentation process. A set of analyses were conducted, examining carbon dioxide levels, temperature, and spectral analysis, to effectively identify and classify aromas. The results demonstrate that the developed device proved to be a promising tool for monitoring and characterizing the coffee fermentation process. Furthermore, this work opens perspectives for the application of sensors and monitoring techniques in industrial processes, with the potential to enhance quality and efficiency in agro-industry. [Display omitted] • Advanced assessment of coffee fermentation using Fiber-Based Twin Long-Period Fiber Grating Interferometry. • Correlation of CO2 and temperature profiles with distinct coffee aroma transitions. • Significant shifts in attenuation wavelength valleys coincide with changing coffee aromas. • Effective data simplification through Principal Component Analysis (PCA), retaining 57.37% variability with two key components. • Promising integration of auxiliary data enhances real-time monitoring and identifies optimal coffee fermentation stages. [ABSTRACT FROM AUTHOR]

Via using an L-band optimized in-fiber polarizing grating device, a GHz L-band femtosecond passively harmonic mode-locked Er-doped fiber laser based on nonlinear polarization rotation is first demonstrated. In total, 4.22 GHz pulses with the duration of 810 fs and super-mode suppression ratio (SMSR) of 32 dB are obtained under the pump power of 712 mW corresponding to 215th harmonic order. The central wavelength of 4.22 GHz pulses is 1581.7 nm with 10.1 nm 3 dB bandwidth. Furthermore, under this fixed pump power, higher harmonic orders can also be attained by rotating the polarization controllers properly. The highest repetition rate we obtained is 7.41 GHz with the SMSR of 20.7 dB.

We demonstrate fiber few-mode interferometers based on a self-assembly surface corrugated grating using charged nano-particles. Initially, an abrupt taper (AT) was first created using a micro flame. The AT was then further outwardly stretched to make an elongated uniformed taper until the tapered diameter achieved a micron scale. The high order core modes (HOCMs) were excited at the AT and the optical path difference (OPD) among the modes was enlarged through the uniformed taper to achieve the few-mode interference effects seen. However, to significantly enhance the interference effects with higher extinction ratios (ER) over such a short length of interferometer, an external assisted grating was made using charged nanoparticles to form surface corrugated grating with a period, Λ, of approximately 14 μm. This intermediate period of the fiber grating was helpful in scattering and attenuating some unwanted high-order modes to change the optical characteristics of the few-mode interferometer (FMI). This FMI with a self-assembly fiber grating (SAFG) was further used to make fiber temperature sensors, with a maximum resonant wavelength shift of 4.6 nm, over a temperature range from 20–60 °C. The temperature sensitivity achieved was 112.6 pm/°C and the coefficient of determination, R2, was as high as 0.99, which revealed the high linearity of the results.

Graphene, a novel form of the hexagonal honeycomb two-dimensional carbon-based structural material with a zero-band gap and ultra-high specific surface area, has unique optoelectronic capabilities, promising a suitable basis for its application in the field of optical fiber sensing. Graphene optical fiber sensing has also been a hotspot in cross-research in biology, materials, medicine, and micro-nano devices in recent years, owing to prospective benefits, such as high sensitivity, small size, and strong anti-electromagnetic interference capability and so on. Here, the progress of optical fiber biochemical sensors based on graphene is reviewed. The fabrication of graphene materials and the sensing mechanism of the graphene-based optical fiber sensor are described. The typical research works of graphene-based optical fiber biochemical sensor, such as long-period fiber grating, Bragg fiber grating, no-core fiber and photonic crystal fiber are introduced, respectively. Finally, prospects for graphene-based optical fiber biochemical sensing technology will also be covered, which will provide an important reference for the development of graphene-based optical fiber biochemical sensors.

Fiber grating (FBG) is an important optical device of fiber, which is widely used in optical fiber communication and sensing. At the present stage, the fiber grating is almost prepared in the static state of the fiber, and then the grating is welded into the fiber grating array or network. With the continuous improvement of the application effect of fiber grating, has become one of the most promising, representative and fastest developing fiber passive devices. In this paper, fiber grating is classified according to the refractive index distribution of grating axis. The central wavelength of Bragg fiber grating is modulated by using external parameters (temperature or stress strain). The wave equation of insulating medium is obtained by the reflection characteristic and coupling mode theory of fiber grating. Using sparse matrix model of nerve action potential signal with wavelet decomposition layers, nerve action potential signal reconstruction of the relative error between the value and the original value contrast found that reconstructed and original signals are very close. Good results have been obtained for the sampling reconstruction of the filtered high signal-to-noise ratio neural action potential signal. Researchers have conducted extensive and in-depth research on fiber grating sensing technology, and achieved gratifying results. But with the of the engineering application of technical requirements, they need real-time monitoring. Due to the cross sensitivities of fiber grating, it became the bottleneck of multiple parameter measurement.

In order to collect the real-time strain distribution of complex surface shapes, and provide data support for the health assessment of complex surface structures, the strain field detection system for complex surface was designed with fiber-optic sensor network. The system consisted of a fiber laser, a coupler, a demodulator, and a fiber sensing array. The method was compared with the optical scanning detection data, and the theoretical analysis and simulation calculation of the strain distribution were carried out under many different conditions. The strain distribution of the device under test with a variety of different force conditions was simulated and analyzed. The results show that the strain distribution is related to the applied position, size and surface structure. A 5.0mm aluminum plate was tested and compared with simulation data in the experiment. Four groups of fiber grating sensors were placed on the surface to be measured in an orthogonal structure arrangement. The test results show that the maximum wavelength offsets are 1.324nm, 2.547nm, and 1.643nm, and the corresponding shift offsets are 0.244mm, 0.523mm, and 0.347mm, respectively. Compared with the calibration data of laser scanning method, the offset of this method is relatively less than 10%. The test data can reflect the trend of surface shape change, and it meets the design requirements. [ABSTRACT FROM AUTHOR]

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A four-channel wavelength-selectable in-series DFB laser array with good single-longitudinal-mode (SLM) property is proposed and theoretically analyzed. To realize good SLM property, a linearly chirped Bragg grating with multiple π phase shifts are implemented in the laser cavity. In addition, two reflector sections with chirped Bragg grating are specially designed for reflection compensation at both ends. In our simulation, the split-step time-domain model with coupled-wave equations and carrier rate equations are used. Based on the simulation results, all the channels work with low threshold currents and the SLM properties are considerably enhanced compared with the conventional in-series laser array. Notably a high precision control of grating phase is required in the practical fabrication due to the ultra-small variation of grating period. Therefore, the reconstruction-equivalent-chirp technique is discussed to provide the fabrication of high-precision grating. In addition, the complex heterogeneous integration or butt-joint technique is not required during the whole fabrication process, which simplifies the fabrication and reduces the cost. The proposed wavelength-selectable in-series laser array is deemed to be applied in the dense wavelength division multiplexing technology, especially when fast wavelength switching is needed, such as in the application of NG-PON2. [ABSTRACT FROM AUTHOR]

We demonstrated a wavelength-tunable, ultra-wideband, biconical, long-period fiber grating (BLPFG) mode converter in a two-mode fiber based on fusion taper technology and CO2 laser writing technology. Theoretical and experimental results show that after changing the diameter of the two-mode fiber by fusing and tapering, the dispersion turning point of the fiber is adjusted and wavelength-tunable broadband mode conversion is achieved efficiently. Theoretical simulation shows that the mode conversion bandwidth can cover the O + E + S + C band. In the experiment, we fabricated adiabatic tapers with cladding diameters of 113 μm and 121 μm and wrote gratings on these tapers to achieve dual-resonance coupling, thus realizing mode conversion from LP01 to LP11, with a 15 dB bandwidth of 148.8 nm from 1229.0 nm to 1377.8 nm and of 168.5 nm from 1319.7 nm to 1488.2 nm, respectively. As far as we know, this is the first time that fusion taper technology has been used to adjust the window of the dual-resonant coupling of an optical fiber. This work broadens the scope of application of the dual-resonance effect and proposes a general method for widening the bandwidth of a fiber grating with tunable wavelength.

Based on electrical arc discharges mechanisms, we inscribed spatially arrayed long period gratings (LPGs) into a heterogeneous seven-core fiber. The LPG fabrication platform was built upon the commercial fiber fusion splicer with a self-developed real-time control software. With the help of homemade fan-in/fan-out multiplexer, the high-quality grating spectra in seven cores are measured. The largest resonant dip of -42 dB and minimum insertion loss of 0.5 dB are achieved in the central core. To demonstrate potentials of the spatially diversified gratings, we measured the strain and temperature simultaneously with the spatial response matrix to eliminate the cross sensitivity. Accurate sensing results have been achieved with relative errors less than 4%.

We report on the multi-state soliton generation in a chirped fiber Bragg grating (CFBG) based bidirectional mode-locked fiber laser. In particular, a $\theta $ -shape associate cavity is designed to introduce reverse dispersion distributions in counter-propagation directions through incorporating the bidirectional reflection of the CFBG into the lasing oscillator. With a relatively compact cavity configuration, the clockwise (CW) and counterclockwise (CCW) directions are managed into large anomalous and normal dispersion regimes, respectively. Consequently, conventional solitons (CSs) and dissipative solitons (DSs) can be emitted from the counter-propagation directions. To the best of our knowledge, it is the first report of CS/DS generation in a CFBG based bidirectional fiber laser. This investigation can contribute to research community of multiplexed ultrafast fiber lasers. [ABSTRACT FROM AUTHOR]

We demonstrate a DNA-based optical fiber device that uses an in-fiber grating, a light absorbing coating with surface anchored DNA, and a built-in optical thermometer. This device is used for precisely thermal cycling surface DNA spots bound by a simple UV cross-linking technique. Near-infrared light of wavelengths near 1550 nm and guided power near 300 mW is coupled out of the fiber core by a tilted fiber Bragg grating inscribed in the fiber and absorbed by the coating to increase its temperature to more than 95 °C. A co-propagating broadband light signal (also in the near-infrared region) is used to measure the reflection spectrum of the grating and thus the temperature from the wavelength shifts of the reflection peaks. The device is capable of sensitive DNA melt analysis and can be used for DNA amplification. [ABSTRACT FROM AUTHOR]

Via using an L-band optimized in-fiber polarizing grating device, a GHz L-band femtosecond passively harmonic mode-locked Er-doped fiber laser based on nonlinear polarization rotation is first demonstrated. In total, 4.22 GHz pulses with the duration of 810 fs and super-mode suppression ratio (SMSR) of 32 dB are obtained under the pump power of 712 mW corresponding to 215th harmonic order. The central wavelength of 4.22 GHz pulses is 1581.7 nm with 10.1 nm 3 dB bandwidth. Furthermore, under this fixed pump power, higher harmonic orders can also be attained by rotating the polarization controllers properly. The highest repetition rate we obtained is 7.41 GHz with the SMSR of 20.7 dB. [ABSTRACT FROM AUTHOR]

We propose the first black phosphorus (BP) – fiber optic biosensor for ultrasensitive diagnosis of human neuron-specific enolase (NSE) cancer biomarkers. A novel optical-nano configuration has been exploited by integrating BP nanosheets with a largely tilted fiber grating (BP-TFG), where the BP is bio-functionalized by the poly-L-lysine acting as a critical cross-linker to facilitate bio-nano-photonic interface with extremely enhanced light-matter interaction. BP nanosheets are synthesized by a liquid ultrasonication-based exfoliation and deposited on fiber device by an in-situ layer-by-layer method. The BP-induced optical modulation effects in terms of thickness-tunable feature, polarization-dependence and enhanced light-matter interaction are experimentally investigated. The anti-NSE immobilized BP-TFG biosensor has been implemented to detect NSE biomarkers demonstrating ultrahigh sensitivity with limit of detection down to 1.0 pg/mL, which is 4 orders magnitude lower than NSE cut-off value of small cell lung cancer. The enhanced sensitivity of BP-TFG is 100-fold higher than graphene oxide or AuNPs based biosensors. We believe that BP-fiber optic configuration opens a new bio-nano-photonic platform for the applications in healthcare, biomedical, food safety and environmental monitoring. • First black phosphorus (BP) – fiber optic biosensor. • Novel photonic-nano-bio configuration. • Successful bio-functionalization of BP. • LOD of 1.0 pg/mL, 4 orders magnitude lower than NSE cut-off value of small cell lung cancer. • Ultrahigh sensitivity, 100-fold higher than GO or AuNPs based biosensors. [ABSTRACT FROM AUTHOR]

Based on the coupled mode theory of the fiber grating, a novel cascaded grating sensor with film-modified long period fiber grating (LPFG) and fiber Bragg grating (FBG) is designed. The sensing principles are investigated, and the temperature and humidity sensing experiments are carried out. The experimental results show that LPFG and FBG have different response sensitivities to the temperature and humidity, and the temperature sensitivities are 34.99 and 8.65 pm·℃−1, respectively. The humidity sensitivity of LPFG is 94.34 pm·%RH-1 while FBG is insensitive to the humidity. Therefore, the simultaneous monitoring of environmental temperature and humidity is realized by using the sensitivity matrix. The minimum resolutions of the sensor for temperature and humidity are ±2.312 °C and ±1.069%RH, respectively. Given the simple structure, high sensitivity, stable performance and anti-electromagnetic interference, the sensor promises an excellent application prospect. [ABSTRACT FROM AUTHOR]

Nonlinear ultrasonic technique that evaluates the growth of the barely visible fatigue crack in metal is difficult to be achieved by conventional optical fiber sensor because of the critical requirements for sensor sensitivity and bandwidth. In this paper, a high-performance optical fiber sensor using phase-shifted fiber Bragg grating and balanced demodulation system was applied to detect the fundamental and second harmonic Lamb waves propagating in an aluminum plate. After the nonlinearity was calculated from the modes of the Lamb waveform detected by the optical fiber sensor, its change due to the second harmonic generation shows an exponential trend in a fatigue test, which can be used to evaluate the crack growth with sufficient reliability. The experimental results from the optical fiber sensor were validated by comparing with results from the PZT sensor. This paper facilitates the development of optics-based nonlinear ultrasonic structural health monitoring for fatigue crack evaluation. [ABSTRACT FROM AUTHOR]

Because of the characteristics of heavy-duty machine tools such as large self-weight and heavy load, the working precision and service life of their lathe beds, columns, and other large structural parts are all directly influenced by the foundation. In view of the considerable influence of joint surfaces on system characteristics, this study involved obtaining joint surface parameter values from a microscopic perspective, deriving a static joint surface parameter model from Reynolds equation, adopting fractal theory to develop a bolted joint surface parameter model, and thus completing the embedding of joint surface parameters under uneven loads; a simulation model for a heavy-duty machine tool-foundation system was also devised considering the influence of joint surfaces. To identify the structural micro-deformation status of heavy-duty numerical control machine tool-foundation systems, the authors constructed a fiber grating technology-based experimental platform for detecting the deformation of structural parts, verified the correctness of the above simulation model via experiments, and proposed a method for detecting the deformation of heavy-duty machine tool-foundation systems using fiber grating technology. Based on the above simulation model, the influence of reinforced layer position, foundation outline specifications and soil properties around the foundation on the bearing deformation of heavy-duty machine tool-foundation systems were studied, and some guidelines on the construction of concrete foundations were formulated. This model and the related detection method laid a theoretical foundation for guiding the design and optimization of heavy-duty machine tool structure and foundation. [ABSTRACT FROM AUTHOR]