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• 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]

• 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]

• Combination of advantages of FBG and cascading FPI. • Cascade FPI based on optical fiber splicing based on Vernier effect. • High-temperature sensitive fiber optic temperature sensor. • Measured simultaneously to temperature changes and absolute values. The Vernier effect is an effective way of magnifying the sensitivity of the sensor for higher resolution sensing. The Fiber-optic cascaded Fabry-Perot Interferometric (CFPI) temperature sensor has been theoretically studied and experimentally investigated in this paper base on Vernier effect. It has been manufactured by a segment of pure silica hollow-core fiber (HCF) fused between two sections single-mode fibers (SMF), where the optical path length of the HCF is similar to that of the terminal SMF. The combination of a fiber Bragg grating (FBG) and a CFPI structure could be used not only obtain the amount of temperature variation but also know the specific value of the temperature. The temperature sensitivity of the sensor was 706.9 pm/°C, which is 76 times higher than that of single one fiber-optic cavity FP temperature sensor. The sensor has a wide range of potential applications such as high-precision temperature sensing and high-temperature monitoring. [ABSTRACT FROM AUTHOR]

• The sensing models of the humidity in different temperatures are established. • A temperature compensation method is proposed to replace the reference grating. • The performance parameters of this humidity sensor are tested experimentally and discussed. • The cross-sensitivity problem of FBG humidity sensors are solved. Humidity is an important parameter in many industry fields and accurate humidity measurement is of great significance to improve production efficiency and ensure product quality. This paper presents a sensing model and scheme for utilizing thermoplastic polyimide (PI) coated fiber Bragg grating (FBG). The humidity sensitive PI layer was firstly prepared and coated on the FBG. The humidity calibration experiment was designed to verify the sensor performance based the saturated salt solution method. The experimental results show this FBG humidity sensor can work steady in the relative humidity (RH) condition ranging from 11%RH to 83% RH, with the sensitivity of the sensor less than 2 pm/% RH and measurement uncertainty of ±1% RH in the temperature range of 30 °C to 70 °C. In order to solve the problem that the PI-coated FBG is both sensitive to the humidity and temperature (cross sensitivity problem), a novel method of temperature compensation has been proposed. The method uses the character that the coefficients of expansion of the PI-coated layer for humidity and temperature are linearly superimposed and obtains the temperature compensation coefficient by linear interpolation of the function relationship between the measured temperature and the FBG central wavelength shift. The analysis of experimental data shows that within the temperature variation range of 20 °C, this temperature compensation method can effectively guarantee the average wavelength errors within 0.03 nm without reference grating. In this way, it can extend the FBG humidity measurement under environmental temperature up to 90 °C making it possible for application in the measurement the humidity in the high temperature environment. [ABSTRACT FROM AUTHOR]

• An effective method for the determination of temperature is proposed. • A fiber Bragg grating temperature sensor based on vortex light is designed. • The sensor achieves temperature measurement of high resolution and wide range. • The resolution can be adjusted by the length of the grating. The vortex beam owns helical phase factor, orbital angular momentum, and hollow structure of intensity distribution. It is widely applied in information coding, optical manipulation and optical sensing. This paper attempts to design a fiber Bragg temperature sensor based on vortex light, including an orbital angular momentum (OAM) beam transmitted by fiber Bragg grating (FBG), an optical fiber path used to eliminate errors, and a Gaussian beam, which is used to interfere with FBG transmission light. The interference pattern of a Gaussian beam and an OAM beam rotates with the changes of phase difference between the two beams. When the temperature changes, FBG is affected by the thermal optical effect and thermal expansion effect, and the central wavelength of the reflected spectrum will shift. Based on the relationship between wavelength shift and temperature change, temperature measurement in a large range can be realized. Additionally, the Filter is placed behind the FBG, and the transmission light (λ = 1.466 μ m) with less reflection is selected to interfere with the Gaussian light, with the change of phase of transmission light, the rotation of the interference pattern will change accordingly. The temperature change corresponding to the 2π rotation of the interference pattern can be obtained, thus temperature measurement with high resolution can be realized. The simulation results show that when the temperature range is 27 °C–427 °C, the sensitivity of this sensor is 14.42 pm/°C. On the basis of the rotation of the interference pattern, when the rotation angle of the temperature-induced interference pattern change within 2π the temperature sensitivity is 1.529 rad/°C, based on the angular resolution of CCD, the theoretical resolution of temperature measurement is 6.3e−7 °C. Similar to the cursor effect, the main ruler measures a wide range of temperature changes according to wavelength drift, and the cursor is a high-resolution measurement of temperature according to the rotation angle of the interference pattern. [ABSTRACT FROM AUTHOR]

• A high sensitive seawater temperature sensor was demonstrated. • It was fabricated by splicing a piece of no-core fiber between single mode fibers. • A linear temperature response with sensitivity about −4.677 nm/°C was obtained. In this paper, a high sensitive seawater temperature sensor fabricated by splicing a piece of no-core fiber (NCF) with coating between single mode fibers (SMF) was demonstrated. Part of the incident light in SMF transmitted to the coating of NCF and reflected to the cladding at the boundary of coating-air, which excited multimode interference (MMI) and clear interference curve can be seen in the transmission spectrum. The characteristic wavelength exhibits a linear temperature response with maximum sensitivity about −4.677 nm/°C in the range of −5 to 45 °C. In addition, different order interference dips with similar sensitivity disappear and generate when temperature changes, and the order can be determined by cascading a fiber Bragg grating (FBG). Hence, such a coated-NCF sensor can be used for wide range and high precision seawater temperature measurement. [ABSTRACT FROM AUTHOR]

• Optical and Chemical Characteristics of CYTOP. • Review of the Bragg grating components in CYTOP. • Sensing Characteristics of CYTOP sensors. • Latest CYTOP applications. Polymer optical fibre (POF) sensors have been the focus a many research studies for approximately twenty years, due to their unique mechanical characteristics that offer the potential for new and exciting applications; they are however, plagued with high loss. As we discuss in this paper efficient polymer sensing is possible using perfluorinated polymer fibres. We present a brief review of the latest achievements regarding the Bragg grating inscription in multimode gradient index CYTOP POFs, presenting their characterisation for different measurands and a number of applications that have been examined during the last five years. [ABSTRACT FROM AUTHOR]

• A Fiber Bragg Grating sensor embedded in 3D printed casing is proposed. • The casings are conceptualized based on numerical analysis. • Two printers, with different technologies, are adopted. • Two applications are analyzed numerically and experimentally. • Calibration factors are defined and discussed for the embedded sensors. This paper aims to numerical and experimentally study and propose Fiber Bragg Grating (FBG) sensors embedded in 3D printed casings. 3D printed casings offer design freedom, enabling strain measurement of structural systems with complex configuration, and, at low cost due to its easiness to manufacture. The casings were first numerically modeled using a commercial finite element software. After the casing was conceptualized to the user needs, they were printed and introduced in two different applications. Assessing the behavior of 3D printed casings in realistic applications has been explored only by few authors, which is the novelty introduced herein. In both applications, the casings' behavior was analyzed numerical and experimentally. The experimental results were compared with electrical strain gauges. As expected from the numerical analysis results, a calibration factor was determined to accurately predict strain using the embedded FBG sensors. A comprehensive discussion is presented on the topic of calibration factors defined numerical and experimentally. [ABSTRACT FROM AUTHOR]