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• An optical fiber strain sensor based on metal welding has been presented. • The proposed fiber sensor has the strain sensitivity of 9.11 pm/με. • The fiber sensor has the advantages of easy fabrication and low cost. • The relationship between capillary length and strain sensitivity is investigated. This paper presents a strain sensor based on Fabry-Perot (F-P) interference. The F-P strain sensor is composed of a single-mode fiber (SMF) and a silica capillary, which are encapsulated by metal welding. The relationship between the cavity length and sensitivity is analyzed theoretically, then the F-P strain sensor is calibrated and tested. The experiment results show that the strain sensitivity of the fabricated sensor reaches 4.4 pm/με when the length of the silica capillary is 12 mm and the F-P cavity length is 436 μm. In addition, the temperature sensitivity of the F-P sensor is 11.7 pm/°C. With the increase of the capillary, the strain sensitivity increases. When the capillary length is 20 mm and the cavity length is 341 μm, the sensitivity of the sensor reaches 9.11 pm/με. The sensor proposed fabricated by metal welding, has the advantages of high sensitivity, simple fabrication, simple structure, and low cost. [ABSTRACT FROM AUTHOR]

• Wearable Optical Fiber Sensor based macro-bending loss for Biomechanical Motion Evaluation. • A U-shaped fiber sensor has been integrated into elbow and knee joint phantoms and measuring its response to various elbow and knee joint movements in the range of 0°-90°. • The recorded sensing data revealed an extra sensitivity of −0.457 nm/° with linear regression coefficients of more than 98.7 % for elbow flexion angle and a 0.506 nm/° sensitivity with a high linear regression coefficient of ∼ 99.6 % for knee flexion angle. • The proposed sensor exhibits a high sensitivity which it significantly improves the optical sensing signal to more than 5.5 times in correspondence with many previous published works. The biocompatibility and other unique physical properties of optical fibers reflect their potential for use in biomechanical motion sensing. In this paper, a wearable macro-bending structure fiber sensor has been fabricated and its potential is demonstrated for observing key motions during physical therapy for patients with joint stiffness. The sensor's performance has been evaluated by integrating it into elbow and knee joint phantoms and measuring its response to various elbow and knee joint movements in the range of 0°-90°. The recorded sensing data revealed the extra sensitivity of −0.457 nm/° with linear regression coefficients (R2) of more than 98.7 % for elbow flexion angle and a 0.506 nm/° sensitivity with a high linear regression coefficient (R2) of ∼ 99.6 % for knee flexion angle. Additionally, good reproducibility and swift response facility were demonstrated during the experiment, indicating that the designed sensor possesses enormous potential in terms of different body motion monitoring applications, even subtle ones. The results show that the sensor's high sensitivity leads to a very good correlation between the sensor's output and the actual elbow joint movement. [ABSTRACT FROM AUTHOR]

• A highly sensitive MZI microstructured fiber optic temperature sensing unit based on SNMNS was designed and fabricated by combining simulation analysis. • The novel intrinsic MXene-Ti 3 C 2 T x material was prepared by water bath method, and the preparation was verified by SEM and XRD characterization, and the temperature sensing properties were experimentally verified by combining with microstructure sensing units. • The effects of the length of MMF and whether or not to coat MXene-Ti3C2Tx material on the sensing performance were investigated in depth, and it was concluded that for the designed sensor intrinsic MXene-Ti3C2Tx has a sensitizing effect. • The superior sensitivity and repeatability of the designed temperature sensor is achieved over a temperature range of 30 to 80 degreesC. A temperature sensing method with simple fabrication, real-time detection, and high sensitivity is proposed, and the feasibility of the method is experimentally verified. The sensing unit uses a single-no-multi-no-single (SNMNS) Mach-Zehnder Interferometric (MZI) structure to excite a stronger evanescent field and compares the effect of two different lengths of multimode fibers on the temperature sensing performance. To enhance the sensor sensitivity, a novel 2D material with superior photothermal, optoelectronic, and structural properties, MXene-Ti 3 C 2 T x , was innovatively coated on the optical fiber surface. The intrinsic MXene-Ti 3 C 2 T x material was prepared by the water bath method and characterized by scanning electron microscopy (SEM) and X-ray diffraction spectroscopy (XRD). The same temperature sensing experiments were carried out on uncoated and coated material fibers respectively, and the results showed that the temperature sensitivity improved from a minimum of 61.97 pm/℃ to 120.71 pm/℃ over the temperature range of 30℃∼80℃, an improvement of 94.7 % in sensitivity, with linearity all above 0.98. This paper combines for the first time the sensitive 2D material MXene-Ti 3 C 2 T x with a microstructured fiber-optic temperature-sensing unit, providing a new way of thinking in the field of temperature detection and showing great promise in the field of fiber-optic sensing. [ABSTRACT FROM AUTHOR]

• This paper review the research status of microhole fiber based nanoliter liquid sensors. • This paper illustrates the microhole fiber based nanoliter liquid sensors for several variables measurement. • The microhole fiber gives a new possibility for nanoliter liquid measurement. • The future development directions of microhole fiber-optic sensors for nanoliter liquid are discussed. This article reviews the development of nanoliter liquid sensors based on microhole fiber. To measure liquid characteristics in microscale is crucial in the field of environmental pollution monitoring, food and drug safety testing, life science research, clinical application, chemical analysis and biomedical research. Microhole optical fiber with micro air holes can provide fitting space and circulation channel for the measurement of miniature volume liquid. Microhole optical fiber sensors have the characteristics of simple structure, enclosed environment, strong light and liquid interaction, which can meet the requirements of multi-parameter, high sensitivity and pollution-free measurement of minimal specimen. Here, we outline the cumulative introduction and the previous experimental research work on microhole fiber-optic sensors for nanoliter liquid, and ends with some conclusions and prospects. We hope this review can provide rigorous and organized literature to researchers/students for the basic understanding, trends, and practical utility of microhole fiber sensors. [ABSTRACT FROM AUTHOR]

• An overburden failure volume calculation model based on the BOTDA was established. • The optical fiber mechanical models in different zone of overburden are established. • This research is a combination of mining engineering and fiber optic sensing. The determination of overburden failure volume is a great challenge because of the complexity and concealment of mining fractures. Taking the 61,601 working face of Longwanggou Coal Mine as the engineering background, this paper established a two-dimensional model of 3000 mm × 200 mm × 1350 mm (length × width × height) in the laboratory. By adopting the Brillouin optical time-domain analysis (BOTDA) technology, considering the fiber deformation and the coordination of rock strata, Of deformation and damage state of overburden strata above the mined-out area, discusses the strata deformation partitioning features and characteristics of fiber optic monitoring strain curve partition interaction relations, is derived based on the above theoretical calculation formula of the rock damage volume, comparing the calculated results with measured results, verify the accuracy of volume calculation formula of mined-out area. The results show a good correspondence between the strain zoning characteristics monitored by optical fiber and the deformation zoning characteristics of overburden. Based on considering the discordant deformation between the overlying rock mass and the optical fiber sensor, the range and degree of overlying mining rock can be obtained through the inversion of the horizontal optical fiber monitoring results to reflect the position and degree of rock strata movement and deformation with the optical fiber. By analyzing the development process of the vertical fiber optic strain curve, it can be found that the rock strata with different propelling distances and different heights bear different tensile stresses, which have remarkable range characteristics. By comparing the results of the calculation model proposed in this paper with the monitored values of the model, it is found that the void volume size of goaf is the same, and the relative error is less than 9%. The results show that the model is reliable in calculating the failure volume of overburden rock in goaf. [ABSTRACT FROM AUTHOR]

Single-mode-no-core-single-mode (SNS) optical fiber structures have valuable potential to encapsulate as high-precision temperature sensors, due to their great sensitivity. This paper is to improve the temperature sensitivity of SNS fibers by studying the parameters of fiber length, diameter, and refractive index of the seal liquid. Transmission spectra of the fibers are given in both aspects of experiment and numerical simulation. We find that using a slim no-core fiber section sealed with liquid of high refractive index can effectively improve the sensitivity, while the choice of fiber length mainly affects the width of transmission peak. The results may contribute to the development of high-sensitivity optic temperature sensor. [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]

• A controllable cavity length, high sensitivity, simple to fabricate and cost-effective axial strain sensor based on optical fiber Fabry-Perot microcavity is presented. • To further improve the sensitivity of axial strain, a sensing structure based on parallel Vernier effect is proposed. • These two FPIs are paralleled by a coupler, and the sensitivity of axial strain can be amplified by 22 times. • The axial strain sensitivity of the proposed sensor can reach 33.5 pm/με owing to the parallel Vernier effect with excellent linear property (R-square is 0.9981). In this paper, a Fabry-Perot interferometer (FPI) based on the fusion structure of hollow-core fiber (HCF) and single-mode fiber (SMF) is reported. Its length can reach a level of 200 μm, which has the advantage of axial strain sensitivity, low temperature crosstalk, and a wide free spectrum range (FSR). The experimental results show that the axial strain and temperature sensitivity of FPI with the length of 200 μm is 1.50 pm/με (R-square is 0.9999) and 1.5 pm/°C during the range of 20 °C–100 °C. A sensing structure based on a parallel Vernier effect is proposed to improve axial strain sensitivity and eliminate temperature crosstalk. Two FPIs with similar lengths are fabricated in this experiment. A 2*2 coupler parallels these two FPIs, the sensitivity of axial strain can be amplified by 22 times, and the optical path difference caused by temperature change can be eliminated. The axial strain sensitivity of the proposed sensor can reach 33.5 pm/με owing to the parallel Vernier effect with excellent linear property (R-square is 0.9981) and negligible crosstalk of temperature. In addition, the parallel sensing structure has the advantages of repeatability, reproducibility, flexible collocation, low cost, easy preparation. [ABSTRACT FROM AUTHOR]

• A Pb2+ sensor with BP nanosheets integrated tapered optical fiber is manufactured. • The exciting cladding mode interferes with the fundamental mode in the core, realizing rapid response to external changes. • The adsorption complexes formed by the electrostatic adsorption of BP nanosheets and Pb2+ will change the effective refractive index of the fiber surface, and cause the interference spectrum to shift. • The on-line optical fiber sensor not only exhibits high sensitivity for the detection of trace Pb2+ but also has a very low detection limit. In this paper, a high-sensitivity Pb2+ on-line sensor based on black phosphorus (BP) integrated tapered fiber is proposed. The BP nanosheets which are deposited on the surface of the tapered fiber by light deposition technology are obtained by one-pot solvothermal reaction and ultrasonic peeling in the laboratory. Due to the electrostatic adsorption between BP and Pb2+, the effective refractive index (RI) of the tapered fiber surface is changed by the adsorption complexes, which eventually causes the interference wavelength to shift. The experimental results show that the sensor has a sensitivity of 0.03714 nm/ppb when the range of Pb2+ solution concentration is 0.1–105 ppb, and its limit of detection (LOD) is 0.0206 ppb. The response time of 1 ppb and 106 ppb Pb2+ solution is 44 s and 49 s, respectively. Meanwhile, the sensor has good stability after repeated measurement of the same concentration solution. In general, the sensor has the characteristics of simple preparation, high sensitivity, low detection limit, integration, and low-cost, and has potential application prospects to detecting trace Pb2+ in water quality monitoring. [ABSTRACT FROM AUTHOR]

• An ultra compact SPR microtip sensor on the end facet of optical fiber is proposed and realized. • The fabrication process is simple and reproducible. • The SPR dip can reach high sensitivity of 1.985 nm/°C. • By simply replacing the sensitive material, the structure can be easily extended to the measurement of other parameters. In this paper, we report an ultra-compact reflective SPR microtip temperature probe, which can achieve 2-D spatial resolution of several microns. The pencil-shaped all-fiber microtip proposed by us is a SO 2 cone-cylinder with a length of 40 μm and a gradient diameter of 2 μm-10 μm, which can be used to develop multi-type tiny functionalized sensors. Due to the strong evanescent field of the ultra-fine microtip, the surface plasmon resonance (SPR) can be effectively stimulated by depositing nano-silver (Ag) film on its surface. Owing to the high refractive index (RI) sensitivity of SPR effect and the high thermo-optic coefficient (TOC) of polydimethylsiloxane (PDMS), the temperature sensitivity of microtip SPR probe coated with PDMS micron-film can reach to 1.985 nm/°C. Teflon 1600 nano-film on the surface of the SPR thermometer can effectively enhance the non-stickiness, moisture-proof and wear resistance. The test results show that the probe is hardly disturbed by humidity. This new sensing platform can be demonstrated by assembling high-performance sensing materials and ultra-compact fiber-optic structures. [ABSTRACT FROM AUTHOR]