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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]

• 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]

• 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]

• 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]

• 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]

• 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]

• 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]

• Three optic methods monitoring were applied in the model test. • A new method to forecasting goaf pressure was proposed. • The stress evolution model of coal seam mining space is established. • This research is a combination of mining engineering and fiber optic sensing, provide the theoretical basis for fiber optic sensing technology to be better applied in mining engineering. The spatial distribution of rock mass stress in goaf seriously affects the stable operation of underground reservoir. Measurement of stress distribution in the goaf of coal mines is difficult due to the complex internal environment and other restrictive conditions of goaf. This paper presents a new method of monitoring the deformation of key stratum in overburden by optical fiber sensing, and then characterizing the pressure of goaf. The internal mechanism of key stratum breakage and the variation of goaf stress is investigated by physical model with the geometric dimension of 3 (length) × 1.3 (height) × 0.4 (width) m), in which the Brillouin Optical Time Domain Analysis (BOTDA) fiber-optic sensing technology and Fiber Bragg Grating (FBG) technology are used to monitor the stability of key stratum and the non-contact Digital Image Correlation (DIC) technique is employed to provide real-time deformation measurements. Combining the fracture mechanics model of key strata with the parabolic function model of subsidence trajectory, the stress evolution model of coal seam mining space is established for the first time, the goaf pressure change and the corresponding relation between the key layer breakage was quantitatively characterized. The research results show that: The fracture location of the key strata is consistent with the peak stress location of the goaf. The breaking limit strain threshold value of the key strata based on optical fiber sensing detection is 4000 με. Through the inversion of optical fiber monitoring results, the fracture degree and range of overburden rock in goaf are obtained, thus reflecting the position and change of the peak pressure in goaf. The consistency between key layer deformation and goaf pressure variation is verified. [ABSTRACT FROM AUTHOR]