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• GaS nanosheets used for the first time as a sensitizer material in optical fiber SPR sensors is proposed. • GaS modification of optical fiber is achieved through layer-by-layer self-assembly. • The experimental results show that the SPR sensor based on GaS nanosheets offers high sensitivity and FOM value. Optical fiber Surface plasmon resonance (SPR) sensors are applied in the field of biological monitoring widely. This paper presents a novel method to improve the sensitivity of optical fiber SPR sensor by using gallium sulfide (GaS) nanosheets. Compared with traditional two-dimensional materials, gallium sulfide has a higher refractive index and higher carrier mobility, which can effectively enhance the surface plasma wave (SPW), thereby improving the sensitivity. By testing sensors coated with gallium sulfide of different thicknesses, the team finally found that when the sensor structure is set to MSM, the thickness of gold film and gallium sulfide nanosheets dispersion is 50 nm and 40 nm, the sensitivity and the FOM value of SPR fiber sensor are 5858.78 nm/RIU and 36.62 RIU−1. The sensitivity to refractive index is 2.9 times that of the traditional gold film sensor, and the sensor has good stability and reliability, and can be reused. This sensor can be applied to monitor low concentrations of biomolecules and weak signals. The use of this material has great implications for the production of new SPR-based chemical and biosensing devices. [ABSTRACT FROM AUTHOR]

• A high-sensitivity, quasi-temperature-insensitive magnetic field sensor is experimentally demonstrated. • Temperature affect induced by thermal expansion is eliminated by the symmetry of optical structure. • Experimental results show that the maximum sensitivity could be 4.36 nm/mT. This paper proposed and experimentally demonstrated a high resolution and quasi-temperature-insensitive magnetic field sensor based on hybrid interferometer. The sensor is created by cascading a Mach Zehnder interferometer (MZI) and a Sagnac loop interferometer (SI). Due to the symmetry of the MZI's two arms, the length change induced by thermal expansion is approximate identical, meaning the temperature perturbation would be eliminated. Moreover, the free spectrum ranges of MZI and SI are extremely close, thus the sensitivity is amplified by the vernier effect. Experiment results indicate that the magnetic field sensitivity of the MZI is 118.8 pm/mT, while the sensitivity of the sensor is 4.36 nm/mT, which is about 36.9 times higher than it in the MZI. [ABSTRACT FROM AUTHOR]

• In this research, we propose for the first time to use UV glue as a DML to excite LRSPR. Compared with the existing methods, the material is easy to obtain and the method is simple. • Through simulation and experimental analysis, the effects of the thickness of CNTs and UV glue on the performance of the sensor were optimized, and the best sensor parameters were obtained. • The LRSPR sensor modified by CNTs and gold nanorods produced in this paper has a sensitivity of 5056.35 nm/RIU and a FOM of 23.52 RIU−1, which are 150.64% and 24.38% higher than conventional SPR sensors. In order to solve the problem that the sensitivity of the optical fiber surface plasmon resonance (SPR) sensor is limited and the full width at half maxima (FWHM) is greatly increased after modification of two-dimensional materials such as carbon nanotubes (CNTs), this paper proposes a Long-Range Surface Plasmon Resonance (LRSPR) Sensors co-modified by CNTs and gold nanorods. Gold nanorods were used to further enhance the sensitivity, and a simple and convenient method was used to excite the LRSPR by using low refractive index UV glue as a dielectric matching layer (DML). Compared with SPR sensor, the FWHM is reduced. The LRSPR sensor produced in this paper has a sensitivity of 5056.35 nm/RIU and a figure of merit (FOM) of 23.52 RIU−1, which are 150.64% and 24.38% higher than conventional SPR sensors, providing a method to achieve high sensitivity and FOM of the sensor. At the same time, the large specific surface area of CNTs and gold nanorods can also improve the biocompatibility of the sensor surface, which makes it have the potential of biosensing. In this paper, the use of UV glue as a DML to excite LRSPR is proposed for the first time. [ABSTRACT FROM AUTHOR]

• The introduction of InSe nanosheets significantly improves the sensitivity and stability of optical fiber SPR sensors. • The introduction of InSe nanosheets improves the FOM of the sensor. • The sensor film was coated by layer-by-layer self-assembly method. • The superiority of InSe in optical fiber SPR sensor is revealed and compared with traditional sensitization methods. • This paper provides a new direction for the further development and application of optical fiber SPR sensors in related fields. Surface plasmon resonance (SPR) optical fiber sensors exhibit advantages such as small size, immunity to electromagnetic interference, and corrosion resistance, and are currently widely applied in the field of biodetection. However, their sensitivity and reusability are still limited in detecting biomolecules, trace concentrations, and weak signals. This paper proposes an approach by utilizing indium selenide (InSe) nanoflakes to enhance the surface electric field strength of the metal layer. InSe is a two-dimensional material that exhibits higher charge carrier mobility and refractive index compared to traditional two-dimensional materials, thereby promoting electron transfer and inducing a larger SPR effect. By testing the sensors composed of InSe nanosheets with different thicknesses, it is found that the sensitivity of InSe nanosheets with a thickness of 40 nm is up to 4655.76 nm/RIU, which is much higher than the sensitivity of traditional Au film sensors and 52 % higher than that of MoS 2 -Au film SPR sensors with the same structure. Moreover, the sensor has a higher FOM value of 33.25 RIU−1 and shows excellent characteristics in repeatability and stability experiments. In this paper, a new method is proposed to enhance the sensitivity of SPR sensors and realize the monitoring of lower biomolecule concentration and weak signal. In addition, the great potential of indium selenide nanosheets in optical applications is emphasized. [ABSTRACT FROM AUTHOR]

• 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]

• 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]

• An intelligent wearable plastic optical fiber (POF) integrated sensor system. • Video image sampling greatly improves system integration and sampling frequency. • Radial basis function neural network (RBF-NN) accurately recognizes human motion patterns. In recent years, the development of flexible bend sensors and their detection devices has attracted great interest. In this paper, an intelligent wearable plastic optical fiber (POF) integrated sensing system for human motion detection is presented. The system consists of POF bend sensors and a wearable fiber bend detection meter (WFBDM). By capturing the POF array image from the video stream, it realizes a sampling frequency of 91.7 times/s. It also greatly increases the integration of multiple sensors, with an integrated sensing area of only 0.35*0.35 mm2 for a single sensor. POF bending sensors with a diameter of 0.25 mm were tested using this system with a linear range of −40° to 90°, an accuracy of up to 0.4°, and a response time of 26.2 ms. The system has been successfully applied to human joint and muscle motion monitoring, and combined with machine learning to realize the intelligent recognition of human motion state. The sensing system is small in size, large in sensing range, high in sensitivity and good in stability, and has the ability of intelligent recognition and highly integrated multi-channel bend signals sensing. [ABSTRACT FROM AUTHOR]

• A compact concentration sensor based on the modified thin-diameter polarization-maintaining fiber is demonstrated. • The relationship between the measurement sensitivity and the length and corroded time of the TDPMF is discussed. • The SC concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%, and the temperature sensitivity is 0.08179 nm/°C. • The MMF-MTDPMF-MMF structure sensor has excellent repeatability and stability in SC concentration sensing. Sodium citrate, as strong alkali and weak acid salt, play an essential role in the food and pharmaceutical industries. However, lack of detection method selectivity, limited biocompatibility and complexity of the manufacturing process are challenges that hinder the practical application of the sensors. In this paper, a improved-sensitivity sodium citrate concentration sensor with simple and compact interrogation setup is proposed. The sensor is fabricated by fusion splicing the sandwich structure of modified thin-diameter polarization-maintaining fiber, two pieces of multimode fiber, and single-mode fiber. In theoretical modeling, the properties of the structure are investigated. In the experiment, the evanescent field generated by the modified thin-diameter polarization-maintaining fiber plays a vital role in the detection of different sodium citrate concentrations. The sodium citrate concentration sensor exhibits a high linear sensitivity response up to 4.65 nm/%. The sensor provided refractive index sensitivity of 3173.2 nm/RIU. In addition, the response, repeatability, and stability are also evaluated by the experiments. Our results show that the sodium citrate concentration sensor holds great promise for serving as a soft and biocompatible probe for monitoring sodium citrate in situ. [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]

• A RI sensor was researched on eliminating the influence of ambient temperature. • A F-P interferometer based on PVC film is used to demodulate temperature shift. • The accuracy of the sensor is high and it has good repeatability. An optical fiber sensor based on a gradually hot-pressed flatted plastic optical fiber (POF) and F-P interference structure is proposed to eliminate the effect of ambient temperature drift in the refractive index (RI) detection of glucose concentration. Based on the analysis of the influence of the sensor itself and the RI change of solution on the transmitted light intensity under the effect of temperature, a self-calibration formula was established by means of the peak wavelength and the transmitted light intensity of interference peak were used as information carriers to calibrate the parameters. The results of the validation show that the temperature drift compensation method proposed in this paper is feasible. The maximum relative error between the measured RI after compensation and actual value is only 0.032%, and it has good repeatability. The precision to the RI with temperature compensation of the gradually hot-pressed flatted POF and F-P interference structure has potential applications in the biochemistry, medical diagnosis, and environmental monitoring. [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]

The all-fiber voltage sensor is difficult for application because of its complex structure, difficult signal demodulation, and sensitivity to the environment. In this paper, a wavelength modulated all-fiber voltage sensor with a simple structure is designed and proposed using straight tapered single-mode fiber and stacked piezoelectric crystal as a voltage sensing unit. The simulation analysis of its performance shows that it can realize the voltage measurement of 0 kV ∼ 10.41 kV, and the accuracy can reach 0.2%. Additionally, the tapered fiber has good strain sensitivity and temperature insensitivity, and its sensitivity is up to 0.03206 nm / μ ε with low bending loss. [ABSTRACT FROM AUTHOR]

This paper presents a respiratory frequency system composed by a polymer optical fiber sensor using a smartphone as the interrogator and a remote access to information through a Web server. The system developed a locally-based signal processing method for a dedicated designed application while ensuring remote access using the Thingspeak Web server and application. To check the sensor accuracy, tests with known respiratory rates of 30, 35 and 40 Breaths per Minute (BPM), respectively, are performed using a metronome. The sensor is manually stretched on the metronome beats frequency and the results achieved by the system were of 30.3599, 35.88 and 41.4 BPM. Furthermore, tests are performed in volunteers to check system performance in a real environment, where the volunteers are asked to breathe normally so their respiratory rate could be counted manually and compared to the values delivered by the experimental system in a given period. The error rate verified in the validation experiments are less than 2%. To verify the remote access to information, it is checked whether the respiratory rate data measured by the acquisition system is accordingly sent and visualized by Thingview: the application used in this prototype. The frequency read on the app is the same as measured by the sensor, which shows the suitability of the proposed approach for remote sensing applications with cloud integration. • Local data acquisition using a smartphone and dedicated platform. • Smartphone-integrated optical fiber sensor for real-time respiration monitoring. • Remote Access to information using free web server and application. [ABSTRACT FROM AUTHOR]

• Bending angle sensing based on tapered POF with double U-shaped configuration. • POF taper encapsulated with PDMS film for highly sensitive bending angle sensing. • The sensor shows highly sensitive response for joint bending detection. Finger joint angle measurements are important for the fields such as treatment of joint diseases, health monitoring, wearable devices, etc. As an effective detecting device, the joint angle monitoring sensor should have wide measurement angle range, and high sensitivity. In addition, the sensor should have good skin affinity, so as to enhance the degree of comfort for the user. In this paper, high-sensitivity bending angle sensing based on tapered plastic optical fiber (POF) configuration is proposed. The sensing performance is enhanced by tapering the POF and bending the POF taper to form a double U-shaped configuration then encapsulated with Polydimethylsiloxane (PDMS) film, therefore, the POF taper will be bent at three positions simultaneously, so that the bend induced loss will increase. The experimental preparation process of tapered POF and PDMS film package are presented in detail. The influence of taper waist diameter, and bending diameter on the bending loss has been investigated and optimized to enhance the angle sensitivity of the double U-shaped configuration. The results show that the sensitivity can reach 0.070 dB/° with a linearity of 0.995 in a wide bending angle range of 0-105°. The POF sensor is then applied for the joint bending detection. The experimental results show that the sensor can quickly respond to small pressures, and the variation of bending angles can be distinguished in the detection of joint bending. The proposed POF bending sensor shows the advantages of wide measurement angle range, high sensitivity, low cost and good portability, and shows important potential applications in the fields such as human joint movement detection. [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]

• A highly sensitive temperature and strain sensor based on Lyot filter with Vernier effect has been demonstrated. • Because the interference spectra of the Lyot filter and fiber Sagnac interferometer are both relatively uniform, it is easy to achieve the Vernier effect. • The FSRs of the Lyot filter and fiber Sagnac interferometer can be regulated conveniently, it is easy to achieve higher magnification based on Vernier effect. • Compared with the sensors based on Lyot structure with Vernier effect, the sensor in this work realized highest magnification and sensitivity. A highly sensitive temperature and strain sensor cascaded with a fiber Lyot filter (FLF) and a fiber Sagnac interferometer (FSI) is proposed in this paper. The temperature and strain characteristics of Lyot filter without and with Vernier effect are experimentally studied. Because the free spectral ranges (FSRs) of the two interferometers can be controlled simply by adjusting the length of polarization-maintaining fiber (PMF), the Vernier effect can be realized conveniently. High temperature and strain sensitivity measurement is obtained by this structure for the first time. The experimental results indicate that the temperature and strain sensitivity of the cascade structure can reach −61.52 nm/°C and 58.63 pm/με respectively, which is about 50 times higher than that of a single FLF whose temperature and strain sensitivity is 1.23 nm/°C and 1.21 pm/με. By carefully optimizing the structural parameter, the sensitivity can be further improved, which provides a new choice for temperature and strain detection. [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]

In this paper, a Mach-Zehnder interference (MZI) based on asymmetric taper-shaped ultraviolet glue is proposed for highly sensitive temperature sensing. Due to the small diameter and the deflected two ends of the tapered UV glue, part of the light leaked into the air during the propagation of the UV glue taper, resulting in interference. We tested the temperature sensing of several samples with different parameters. Experimental results show that with the increase of temperature, the wavelength moves linearly to a shorter wavelength. The highest temperature sensitivity can reach −15 nm/℃, and the linear correlation coefficient is 0.995. The sensor has the advantages of low cost, simple manufacture and higher temperature sensitivity is suitable for temperature measurement. [ABSTRACT FROM AUTHOR]

• An unclosed fiber optic current sensor based on the helically wound spun highly birefringent fiber half waveplate on the tunnel boring machine is proposed and experimentally demonstrated. • The unclosed fiber optic current sensor is designed to coil the spun highly birefringent fiber around the measured current clockwise and counter clockwise alternately in order to be mounted on the measured object with large diameter easily. • The helically wound spun highly birefringent fiber half waveplate is proposed to accumulate the Faraday effect. • The good performance of the unclosed fiber optic current sensor is confirmed by the laboratory experiment. In the application of the fiber optic current sensor that is mounted on tunnel boring machine (TBM) for measuring the probe current in tunnel geological prediction, the field installation and the influence of the linear birefringence caused by external perturbations are problems needing to be solved. In this paper, an unclosed fiber optic current sensor (UFOCS) based on the helically wound spun highly birefringent (SHB) fiber half waveplate is proposed and demonstrated. The UFOCS is designed to coil the fiber around the measured current clockwise and counter clockwise alternately in order to be mounted on the measured object with large diameter easily. The all-fiber half waveplate made by the helically wound SHB fiber is used to accumulate the Faraday effect. The simulation is carried out based on the polarization model and a linear response is obtained. The laboratory experiment is conducted and the results confirm the good performance of the UFOCS. [ABSTRACT FROM AUTHOR]

• Investigated POF integrated with smartphone for HR and BR monitoring. • The performance of the POF sensor with different sensitivity zone is analyzed. • The shutter speed of the smartphone is optimized. • Investigated the POF sensor under different motion states. Human physiological signal monitoring is a key factor in diagnosing health conditions, such as monitoring heart rate and respiration. To meet the increasing monitoring requirements, such as safety, economy, and equipment miniaturization, it is necessary to optimize and improve the monitoring device. Polymer optical fiber sensor has great potential in human physiological monitoring because of their inherent safety, compactness, electrical isolation, and flexibility. In this paper, we report the integration of low-cost plastic optical fibers in smartphones for the measurement of respiratory rate and heart rate in human physiological monitoring. The performance of the plastic optical fiber (POF) sensor with different sensitivity zone is analyzed. With three sensitivity zones, the respiratory rate and heart rate can be displayed in the power spectral density simultaneously. For improving the POF sensor performance, the shutter speed of the smartphone is also optimized to 30 s−1. Finally, the POF sensor is tested under different motion states indicate that the sensor can monitor the heart rate and breathing rate under different postures (running, walking, standing, squatting and lying), where the spectral energy is 2.8 ± 0.11 × 108 pt during running and is 1.4 ± 0.55 × 107 pt in the supine state. And it also can be used to assess the dynamic posture, which is important for some potential sensory applications in clinical and home settings. [ABSTRACT FROM AUTHOR]

• All fiber-optic sensor based on a Balloon-like interferometer for pH measurement was demonstrated. • The Mach-Zehnder interferometer was fabricated by bending standard single-mode fiber into a Balloon shape. • In addition to the high sensitivity, the proposed sensor exhibits a fast response time with excellent stability. In this paper, we present the fabrication of a new fiber-optic pH sensor based on the macro-bending loss properties in a droplet-shaped single-mode fiber (SMF) bending structure for the first time. To form the droplet-shaped probe, a segment of straight SMF is properly bent into a droplet shape using a short length of a thin silica capillary tube and without any splicing. By gradually varying the bending diameter of the droplet-shaped structure, interference results between the core and cladding modes, and a Mach–Zehnder interferometer (MZI) is effectively introduced within the bending section. Sodium hydroxide (NaOH) or hydrochloric acid (HCl) were added to the pH indicator utilizing buffers solution to achieve a wide pH range from 1 to 14. Experimental results show that the proposed sensor exhibits maximum sensitivity of 3.264 nm/pH and 5.336 nm/pH for acidic and alkaline solutions, respectively. In addition to the high sensitivity, the proposed sensor structure provides high sensitivity with a high linear response and good repeatability. [ABSTRACT FROM AUTHOR]

• We fabricated the V-grooves in the step refractive index multimode fiber, sensing channel I is formed by coating gold film on the core surface, due to the V-grooves, the cladding mode of the fiber is excited, sensing channel II is formed by coating gold film on the 2 cm cladding area behind the sawtooth area, the dual-channel sensor is fabricated. • The dual-channel sensor adjusted the resonance wavelength range by utilizing the angle of the V-grooves to change the SPR resonance angle. • Due to the existence of V-shaped grooves, the total reflection angle of transmitted light in the sawtooth area changes, which significantly improves the SPR sensitivity, the sensitivity is 3114.86 nm/RIU, which is higher than that of traditional transmission type multimode fiber SPR sensor. • The sawtooth structure fiber SPR sensor proposed in this paper can effectively improve the sensitivity and realize the dual-channel measurement based on wavelength division multiplexing technology which will promote the pace of simultaneous detection of multiple analytes, and regardless of the working mode, the SPR resonance wavelength and sensitivity of the sensor have a good consistency. The fiber SPR sensor with a single sensing channel can not detect multiple analytes at the same time and introduce the reference channel to eliminate nonspecific interference. Hereby we proposed a sawtooth dual-channel fiber SPR sensor based on WDM technology. By CO 2 laser, multiple V-shaped grooves are processed on the step multimode fiber to form a sawtooth structure. The first stage of the sensor was fabricated by coating gold film on the sawtooth area, and the second stage was fabricated by coating gold film on the cladding area 2 cm behind the sawtooth area. The sensitivity and dynamic range of the two channels are 3114.86 nm/RIU and 1584.23 nm/RIU, 719.93 nm-854.2 nm, and 584.24 nm-667.62 nm, respectively. The proposed fiber SPR sensor employs the sawtooth structure to adjust the SPR resonance wavelength, so as to realize dual-channel sensing, which is of significance in the fields of simultaneous detection of multiple analytes and temperature compensation of reference channels. [ABSTRACT FROM AUTHOR]

• a novel cascaded long-period fiber grating sensor based on the modulation of both wavelength and intensity is demonstrated. • First application of slight deviation of resonance to cascaded fiber gratings. • The SRI sensitivities are −177.6 nm/RIU and −202.7 dB/RIU, and the temperature sensitivities are 0.1175 nm/℃ and 0.0997 dB/℃. • The SRI and temperature sensitivities of our proposed sensor are 30 times and 10 times higher than those of other sensors with dual-parameter modulation of wavelength and intensity. In this paper, a novel cascaded long-period fiber grating (CLPFG) sensor based on the modulation of both wavelength and intensity is demonstrated, which is able to simultaneously detect surrounding refractive index (SRI) and temperature. The appropriate grating periods are designed by theoretical simulations. One grating operate in single-peak resonance state and the other one operates at slight deviation of resonance (SDOR) state. When the SRI and temperature change, the wavelength of resonant peak in the single-peak resonance state shifts, and the intensity of the resonant peak at the SDOR state changes accordingly. In experiments, the CLPFG is inscribed based on the theoretical design and the responses of the sensor to SRI and temperature are investigated. The experimental results show that the SRI sensitivities are −177.6 nm/RIU and −202.7 dB/RIU, and the temperature sensitivities are 0.1175 nm/℃ and 0.0997 dB/℃. The SRI and temperature sensitivities of our proposed sensor are 30 times and 10 times higher than those of other sensors with dual-parameter modulation of wavelength and intensity. The proposed sensor with simplified structure, good linearity, and high sensitivity can be expanded to the area of multi-parameter sensing. [ABSTRACT FROM AUTHOR]

• A kind of SnO 2 nanomaterial sensitive to ammonia gas is prepared. • An optical fiber interference structure with fiber fusion taper is fabricated. • An ammonia concentration sensor based on optical fiber interference structure is fabricated. • The sensor has good selectivity and repeatability. This paper provides a quick and simple method for detecting ammonia gas concentration by coating microfiber interferometer (MFI) with SnO 2 sensitive material. The MFI sensor is fabricated by the method of fiber fusion and taper. Then the nanomaterial sensitive to ammonia is prepared by hydrothermal method and coated in the sensing area. When the sensor is in contact with ammonia the SnO 2 nanomaterial can adsorb ammonia molecules, which changes the refractive index of the MFI surface and causes the sensor transmission spectrum to shift. The real-time sensing of ammonia concentration can be realized by judging the magnitude of spectral shift. The sensor unit has only optical signal, simple structure, low cost and high sensitivity. The sensor can effectively detect the concentration of ammonia within the range of 0 ppm-10,476 ppm, and the sensor has a sensitivity of 0.58 pm/ppm. The experiment show that SnO 2 nanomaterial have high sensitivity to ammonia gas, and the interference sensor coated with SnO 2 nanomaterial has good repeatability and selectivity, so it has a broad application prospect in gas sensing in the future. [ABSTRACT FROM AUTHOR]

• A novel optical fiber sensor based on MMF-FCF-MMF structure is proposed. • The maximum fringe visibility of the interference resonance dips exceeds 26.3 dB. • The MMFs are used to improve the light coupling efficiency between the input SMF and the FCF. • The sensor has advantages of simple fabrication process, good stability, repeatability and high fringe visibility. In this paper, a novel optical fiber sensor based on multimode fiber (MMF) - four-core fiber (FCF)-MMF structure is proposed and experimentally demonstrated. The device consists of a segment of FCF sandwiched between two MMFs. The MMFs are used to improve the light coupling efficiency between the input single mode fiber (SMF) and the FCF, and the FCF is used as sensing arm. Due to the multipath interference configuration embedded in one fiber, higher phase sensitivity can be obtained in our scheme. The maximum fringe visibility of the interference resonance dips exceeds 26.3 dB. The interference fringe of the sensor would shift with the variation of liquid level and temperature. So it is possible to measure dual-parameter simultaneously by simply monitoring the different wavelength shift. For 10 pm wavelength resolution, the resolution of the sensor is 0.14 cm and 2.08 °C in liquid level and temperature, respectively. Moreover, the proposed sensor has the advantages of simple fabrication process, good stability, repeatability and high fringe visibility, which mean it has a broad application prospect in liquid level measurement. [ABSTRACT FROM AUTHOR]

• Demonstrate a novel passive optical-fiber sensor based on surface plasmon resonance (SPR). • The optical fiber sensor has a wide detection range of 1–1.4 RIU. • Comparing with the reported numerical and measurements based on an optical fiber structure, the experimental results based on fiber ring resonator present good agreement. In this paper a new passive optical fiber sensor depends on surface plasmon resonance (SPR) decorate by fabricating a configuration on metal-dielectric materials. A single-mode fiber is applied to harness the transmission light. In this work, FDTD method is introduced to control the injected refractive index on ring and fiber; hence, this leads to the shifting of the fiber SPR resonance wavelength. The advantageous of this mechanism is optical fiber sensor has a high sensitivity with the maximum of up to 1700 nm/RIU, wide detection range of 1–1.4 RIU (refractive index unit), and a nanometer resolution with a minimum of 4 nm. In addition, moreover we consider several performance parameters like sensitivity and FOM, interestingly it is shown applying silicon optical fiber improve the sensitivity. Comparing with the reported numerical and measurements based on an optical fiber structure, the experimental results based on fiber ring resonator demonstrate good ability and agreement, therefore the proposed structure pave the path new operation of optical fiber sensors. [ABSTRACT FROM AUTHOR]

• Recycling fused optical fiber to develop a miniature self-referred Fabry-Perot microcavity sensor. • The physical theory and Finite Element Method (FEM) of the Fabry-Perot microcavity were considered to fabricate an efficient sensor. • This sensor simultaneously measurements the refractive index and temperature. • This sensor has the advantages of low cost, high repeatability, and small dimension, making it a desirable sensor for chemical and biomedical applications. In this paper, a fiber-optic Fabry-Perot microcavity sensor is presented based on splicing a catastrophic fused fiber (CFF) to a single-mode fiber (SMF) for the simultaneous measurement of the refractive index (RI) and temperature. CFF formed by the catastrophic fuse effect consists of a periodic void in the fiber core that creates a teardrop shape or elliptical air microcavity after splicing. By simulation, the elliptical microcavity sensor is selected to measure external RI changes by Fast Fourier Transform (FFT) analysis. The RI sensitivity was obtained to be −2.5 ± 0.0074/RIU with a resolution of about 4 × 10−5 RIU. Also, the experimental results showed that temperature is obtained independently from the FFT parameter without being affected by the RI. The sensor temperature was determined by tracking the wavelength shift of the interference spectrum. Consequently, the temperature sensitivity of 7 pm/°C in the range of 20 °C to 95 °C was achieved. Therefore, the RI and the temperature were simultaneously measured by analyzing the spatial frequency and wavelength shift. The proposed dual-microcavity FP sensor can compensate for any power fluctuation in the input light source. [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 highly sensitive temperature sensor based on enhanced Vernier effect is proposed. • The enhanced Vernier effect is realized in the experiment. • Compared to the individual interferometer, the temperature sensitivity is improved. • No special sensitizing materials or desensitization is used to improve sensitivity. A highly sensitive optical fiber temperature sensor based on enhanced Vernier effect is proposed in this paper. The sensor is a cascaded structure which is composed of fiber optic Sagnac interferometer (FSI) and intermodal Mach–Zehnder interferometer (IMZI). Because the interference fringes of the IMZI and the FSI shift to opposite directions as ambient temperature varies, the enhanced Vernier effect is obtained. Compared to the temperature sensor based on the traditional Vernier effect, the proposed sensor realizes the high sensitivity temperature measurement and doesn't need any special sensitizing materials, complicated manufacture process or a reference interferometer which need to be desensitized. Experimental results show that the sensitivity of the sensor is 20.86 nm/°C. A good linear responses can be obtained in temperature measurement range from 25 °C to 33 °C. [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 novel optical fiber displacement sensing system based on Raman backscattering. • A recoverable displacement sensing unit were designed and built. • The proposed system can measure the displacement variation sensitively. Optical fiber displacement sensor has the advantages of high sensitivity, large dynamic range, long sensing distance and high electromagnetic interference resistance, which has been widely used in many industrial and civil fields. However, multi-parameter simultaneous sensing is the common requirement in many applications. In this paper, we demonstrated a novel and simple optical fiber displacement sensor based on the principle of Stokes Raman backscattering, and the Stokes Raman signal is used to sense the bending loss of the sensing fiber. An optical fiber displacement sensing system and a recoverable displacement sensor is designed and built to measure the optical power loss, which makes it possible to detect the temperature and displacement simultaneously use one optical fiber. The displacement measurement is carried out for five times at 0.25 mm intervals in the range of 0–5 mm, and the experimental results show that there is a good linearity between measured and applied displacement. Its measurement accuracy is 0.1 mm and maximum sensitivity is 0.343 dB/mm. This sensor has the potential to be applied in foundation settlement monitoring, safety pre-warning of subway on-site, soil deformation measurement, and civil structure crack monitoring. [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]

• Hydrogel sensing film was coted on the surface of plastic optical fiber. • The concentration of fluorescence indicator in the sensing film was optimized. • Sensing membrane related performance wsa studied. This paper proposes a dissolved oxygen sensing system based on PMMA plastic optical fiber covered at one end with biocompatible PEGDA (Polyethylene glycol diacrylate) hydrogel film. PEGDA, 2-hydroxy-2-methyl-propiophenone and DI water mixture was used as hydrogel precursors, and a certain concentration of oxygen-sensitive fluorescent indicator was incorporated into it. Fluorescence quenching method was used to detect dissolved oxygen in water, the doping concentration of fluorescent indicator in the hydrogel film was optimized. The experimental results indicate that the plastic optical fiber coated with hydrogel sensing film has good reproducibility, and the response time is within 10 s. The sensing characteristics of dissolved oxygen at 30 °C, 35 °C, and 40 °C were tested respectively, and the results showed that the linearity of the system is best at 35 °C. Within the range of pH 2–12, the fluorescence intensity of the fluorescent indicator is very stable. The sensor with biocompatible material is expected to be applied to the monitor of dissolved oxygen in the domain of biomedicine. [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]

• A novel temperature sensor based on microfiber interferometer was developed. • The MF interferometer was made into an unsymmetrical U-shape to ensure the stability. • Temperature sensitivity is 349.5 pm/°C for liquid temperature measurement. A novel temperature sensor based on microfiber (MF) interferometer with low-cost, long-term reliability and high repeatability was developed. The experimental results indicate that the unsymmetrical U-shaped MF has particularly advantages of a thinner waist diameter and better sensing stability compared with symmetrical U-shaped MF in terms of structure and spectrum. The air temperature sensitivity of 10 pm/°C was observed by the sensor with a diameter of 4.26 μm. In order to study the relationship between the structure parameters of transition region and the performance of temperature sensor, we fabricated another MF with a larger taper angle of transition region. The new sensor fabricated with this MF obtained a higher temperature sensitivity of 16.7 pm/°C. Considering the impact force from the temperature measurement environment (such as air and liquid environment), the MF interferometer was packaged in a special glass tube to enhance its impact resistance. After that this proposed sensing structure was applied to the measurement of liquid temperature. Results show that its temperature sensitivity is about 349.5 pm/°C when the liquid temperature changes from 18 °C to 30 °C. The sensor designed in this paper has the advantages of good resistance to liquid impact and high sensitivity. It is of great significance to the application of optical fiber sensor in the detection of liquid and atmospheric environment. [ABSTRACT FROM AUTHOR]

This paper proposes a biosensor based on side-hole fibers (SHF) formed by the structure of single-mode fiber (SMF)-coreless fiber (CLF)- SHF-CLF-SMF for the detection of bovine serum albumin (BSA). Among the fibers, the SHF's misalignment splicing makes one hole exposed, which is very sensitive to the change of the external environment. Due to the unique structure of the SHF, two Mach-Zehnder interferences (MZI) are generated in this fiber sensor, which further improves the sensitivity of the sensor. Experiments show that the refractive index (RI) sensitivity of this sensor reaches 13421.638 nm/RIU, and for the BSA concentration detection, the sensitivity reaches 2.413 nm/(mg/ml). The proposed biosensor is simple, easy to manufacture, cost-effective, and can be used for many chemical and biological analyses. [ABSTRACT FROM AUTHOR]

• Comprehensive review of field applications of distributed optical fibre sensor for various infrastructure health monitoring is provided. • Comparison between DOFS and other sensors in terms of type, measurand and operating condition is provided. • Methods of installation and optical fibre layout for efficient monitoring of different structures, including their advantages and disadvantages are thoroughly discussed. • Challenges and potential future works in implementing distributed optical fibre sensor for large infrastructure health monitoring are presented. For the past decades, the applicability of distributed optical fibre sensor (DOFS) technology has been widely explored to assess the structural health and integrity. The DOFS has distinctive features compared to traditional sensors, including the ability to provide long-distance monitoring of temperature, strain, and vibration with a single mode of cable. Moreover, it has high durability and low maintenance cost which reveals new possibilities for the health monitoring of large and spatially distributed infrastructures. This paper provides a general background and basic principles of the DOFS technology, followed by a comprehensive review and appraisal of DOFS applications for infrastructure health monitoring including bridges, dams, slopes, pipelines, tunnels, mining conveyors and pavement. The advantages and limitations of DOFS systems for health monitoring of various infrastructure, with emphasis on the installation methods, are presented. Furthermore, challenges and future research direction of DOFS implementation for large infrastructure health monitoring are discussed. [ABSTRACT FROM AUTHOR]