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• Novel Photonic Device Development : The paper introduces a groundbreaking approach to create a photonic device for chemical applications, utilizing an evanescent wave absorption technique-based optical fiber probe. • Free-base Porphyrin Monolayering: The sensing probe incorporates a Tetraamine porphyrin (TAPP) monolayer, exploiting the unique optical properties of TAPP to enhance chemical sensing capabilities through the optical fiber. • Distinct Optical Patterns: The study focuses on monitoring interactions between TAPP and nitrate and chloride ions in the solution phase, emphasizing on absorption spectra variations of porphyrin molecules significantly in Soret and Q-bands. Distinct optical patterns emerge from the interaction, and the paper thoroughly analyses ion-dependent shifts in both the bands across eight samples of nitrates and chlorides. • Spectral Pattern Recognition: It employs unsupervised learning algorithms for spectral pattern recognition, allowing the discernment of ion groups based on their unique optical signatures. The use of PCA and Dtrend techniques helps to extract significant data to cluster different compounds. The sensor probe however exhibits remarkable sensitivity to nitrate and chloride ions, showcasing an impressive rapid response time of 2-3 s. • Practical Implications: The findings highlight the potential practical applications of the developed photonic device, emphasizing its utility in environmental monitoring and chemical analysis. The study positions porphyrin-based optical fiber sensor as a valuable technology, demonstrating their efficacy in detecting and analysing chemical compounds, particularly nitrate and chloride ions. The paper introduces a novel approach to develop a photonic device for chemical applications using an evanescent wave absorption technique-based optical fiber probe layered with a Tetraamine porphyrin (TAPP) monolayer exploiting the optical properties of TAPP. The interactions between the TAPP and nitrate and chloride ions in the solution phase were monitored and the changes in the absorption spectra of TAPP molecules were recorded. Distinct optical patterns emerge, and ion-dependent shifts in the Soret and Q-bands are analyzed for eight samples of nitrates and chlorides. Principal Component Analysis (PCA) is employed to discern ion groups based on the spectral pattern recognition. These fiber probes exhibit sensitivity to nitrate and chloride ions, with enhanced selectivity, robustness, and a rapid response time of 2–3 seconds. The study demonstrates the emergence of porphyrin-based optical fiber sensors as a useful sensor technology in environmental monitoring. [ABSTRACT FROM AUTHOR]

• This paper introduces a strain monitoring method where optical fibers are embedded along a spiral path in composite cylindrical structures. • A monitoring method is proposed to judge the health state of the structure by using the strain data monitored in this direction. • The comparison between the measurements of the embedded fiber optic and the surface-bonded strain gauges under various working conditions demonstrates excellent consistency, validating the fiber optic sensor's measurement accuracy. Composite cylinder structures are widely used in various engineering fields. Monitoring the health state is an essential method for ensuring the safe operation of the structures. Embedded distributed fiber optic sensors have gained significant interest as they effectively address issues related to aesthetics and protection. This paper presents a strain monitoring method that involves embedding optical fibers along a spiral path within composite cylindrical structures. Additionally, a monitoring method is proposed to evaluate the structure's health state using the strain data collected in this manner. To validate the measurement accuracy of the fiber optic sensors, a comparison is made between the measurements obtained from the embedded fiber optic sensors and surface-bonded strain gauges under different working conditions. The results demonstrate excellent consistency, establishing the reliability of the fiber optic sensors. In this study, a distributed fiber optic sensor network and embedding scheme are designed for large composite cylinder structures, and strain monitoring experiments at various conditions, including pressure, bending, and hoisting loads, are conducted. Finite element simulations are also performed at corresponding working conditions, and the effectiveness of fiber optic sensor measurement is confirmed by comparing normalized simulation results with measured results. The embedded distributed fiber optic sensors effectively capture the strain changes at the reinforcement frame located in the middle of the cylinder structure, which aligns with the theoretical analysis of the cylinder structure under varying loads. This study on the embedding of fiber optics in large composite cylinder structures provides valuable insights for structural health monitoring purposes. [ABSTRACT FROM AUTHOR]

• Simple structure, micro size, and convenient fabrication. The sensor is created using a femtosecond micromachining system, which is used to etch miniature rectangular holes on a standard multimode fiber. These holes are then filled with a mixture of Er3+/Yb3+ co-doped TeO 2 -Na 2 CO 3 -ZnO powder and PDMS, resulting in a sandwich structure. • Accurate temperature measurement and rapid response. The maximum error is found to be ± 1 K in laboratory. The sensor is also applied to a GPU to monitor real-time temperature variations. In this application, the sensor demonstrates a maximum error of 0.9 K and a response time of 1.96 s. This paper introduces a miniaturized optical fiber temperature sensor based on Fluorescence Intensity Ratio (FIR) technology for real-time monitoring of graphics processing unit (GPU) temperature. Utilizing a femtosecond micromachining system, miniature rectangular holes are etched on a standard multimode fiber. These holes are then filled with a mixture of Er3+/Yb3+ co-doped TeO 2 -Na 2 CO 3 -ZnO powder and polydimethylsiloxane (PDMS) through capillary action, forming a sandwich structure. When illuminated by a 980 nm light source, upconversion (UC) fluorescence is generated, and a mathematical model correlating the fluorescence intensity ratio of two adjacent energy levels with temperature is established. The fundamental temperature sensing characteristics of the sensor are tested in a temperature-controlled chamber, with a maximum error of ± 1 K. The sensor is also applied to a GPU with real-time temperature variations, demonstrating a maximum error of 0.9 K and a response time of 1.96 s. The sensor not only possesses the advantages of a simple structure, micro size, and convenient fabrication but also exhibits immunity to electromagnetic interference, rapid response, good stability, and excellent repeatability in real-time monitoring of GPU temperature, showing potential for large-scale application. [ABSTRACT FROM AUTHOR]

• The maximum torsion response of 1.18 dB/° is achieved with the minimum wavelength shift of ∼ 0.012 nm/°. • The minimum range of π-phase jumping is obtained by the reduced length of polarization maintained fiber. • The ultralow axial-strain and temperature sensitivities of 0.0173°/με and 0.232°/°C are obtained. • The linear voltage response is about − 13.97 mV/° and the detection limit reaches 0.08°. In this paper, a fiber-optic directional torsion sensor based on the intensity modulated Lyot filter is completed to alleviate the crosstalk from ambient temperature and axial strain. The transmission is analyzed and compared with the varied twisted length. The experimental results show that the torsion response of 1.18 dB/° can be achieved by the 45° incident angle and 10-cm twisted length, with tiny wavelength shift (∼0.012 nm/°) and minimum range of π-phase jumping. Meanwhile, the crosstalk is well compressed within 0.006°/με and 0.23°/°C owing to ultralow axial-strain and temperature responses. In addition, an in-field test is performed electrically and the detection limit of 0.08° is witnessed experimentally. [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]

• Workflow algorithm and Fast Fourier transform (FFT) for mathematical interpretation. • Three simulations graph and waterfall display also Multiple intrusions detection. • SDH-based OFC network for Long-distance data provides intrusion data. • Thresholds in FFT for rapid detecting signals at higher speed. • Intrusion detection with machine learning techniques improve intruder detection capabilities. The intrusion detectors and warning devices have been aided by advancements in the fields of optical fiber, integrated circuit fabrication technology, and devices. This paper describes a system of long-distance intrusion and a warning system for monitoring third-party physical intrusion in long-distance sensitive Areas. It provides around-the-clock third-party intrusion detection. The signals of the external sensors are influenced by vegetation and external natural conditions, but not in the case of optical cable sensors. A workflow algorithm has been developed in this research paper where Fast Fourier transform (FFT) is used and mathematical interpretation is made. Long-distance data communication for optical perimeter intrusion detection and warning systems is demonstrated using a block diagram. Intrusion detection system range of detection has been demonstrated pictorial also. The higher the magnitude of vibration of equipment or human being, the higher will be the range of the detection by the system. Real-Time intrusion simulation performed during the research work is depicted for a better understanding. We have conducted three simulations namely manual digging, vehicle traversing the fiber and mechanical digging which is depicted with amplitude vs. channel graph and also by waterfall display. The controller employs the distributed acoustic sensing (DAS) principle which is based on interrogating the scattering of light from nanoparticles on the optical fiber in response to changes in refractive index or molecular binding events in the surrounding environment. [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]

• We developed a high-speed Raman temperature sensor with enhanced compatibility with existing strain sensors. • We revealed that the optimal Raman power measurement frequency is ∼195 THz, near the Rayleigh peak, not the traditional 206 THz. • We demonstrated effective real-time temperature sensing by monitoring Raman power variations at this specific frequency. This paper presents a high-speed Raman temperature sensor that is highly compatible with current strain sensors. Challenging conventional assumptions, we found that at a communication wavelength of 193 THz, the optimal frequency for measuring Raman scattering power is not the typically dominant 206 THz, linked to stimulated Raman scattering, but rather around 195 THz, in proximity to the Rayleigh scattering peak. Our study demonstrates the effectiveness of real-time temperature sensing through the monitoring of power fluctuations in Raman scattering at this particular frequency. © 2024 Elsevier Science. All rights reserved. [ABSTRACT FROM AUTHOR]

Fiber optic curvature sensors face several challenges that limit their application in structural health monitoring. These include high fabrication and demodulation costs, poor stability and robustness. In this paper, we propose a novel Mach–Zehnder interferometer (MZI) sensor that overcomes these limitations.The sensor was fabricated through the splicing of a seven-core fiber(SCF) and two single-mode fibers (SMF) with a common fusion splicer. The proposed sensor exhibited a good linear response, with a sensitivity of 22 nm/m − 1 in the range of 0–0.313 m − 1. The MZI sensor was demodulated by a neural network-based system and a data augmentation algorithm, reducing the need for large data samples and improving demodulation accuracy on sparse datasets. Overall, the sensor is characterized by fabrication simplicity, high mechanical strength, and compatibility with existing optical devices, making it applicable to various fields, such as aerospace, civil engineering, and mechanical manufacturing. [ABSTRACT FROM AUTHOR]

• Integration of Explainable Artificial Intelligence (XAI) models on fiber optic SPR sensors. • Insights into the intricate patterns and features learned by the model are attained. • A comparative analysis of XAI models, namely SHAP and LIME is carried out. • LIME and SHAP methods jointly find that metal layer thickness has considerably greater influence than wavelength on sensing performance. • This finding is consistent with the concept of radiation damping related to fiber optic SPR sensor. In recent years, there has been a significant surge in interest surrounding the utilization of machine learning techniques to optimize the performance of fiber optic sensors. This paper delves into the integration of machine learning methodologies, with a specific focus on Explainable Artificial Intelligence (XAI) models, within the domain of fiber optic surface plasmon resonance (SPR) sensors. We implement XAI techniques into a previously-trained Gaussian Process Regression (GPR) model to gain insights into the intricate patterns and features learned by the model, thereby providing valuable insights into its decision-making process. Furthermore, we present a comparative analysis of XAI models, namely SHAP and LIME , to elucidate the individual impact of light wavelength (λ) and metal layer thickness (d m) on the enhancement of sensor's figure of merit (FOM). The results obtained from both LIME and SHAP methods lead to a common finding that compared to λ, it is the value of d m that possesses a significantly greater influence on achieving maximum FOM. Furthermore, this finding is in complete agreement with the concept of radiation damping related to fiber optic SPR sensor structures. This study has the potential to contribute to the refinement of design and fabrication processes related to fiber optic SPR sensor for a wide variety of applications. [ABSTRACT FROM AUTHOR]

• A optical fiber sensor for detecting vitamin B1 has been proposed for the first time. • Accurately and efficiently detect vitamin B1 within a large concentration range. • Red and green dual channel detection of vitamin B1 reduces detection errors. • Optical fiber sensing based on smartphones has broad prospects in low-cost detection. This paper introduces an innovative optical fiber surface plasmon resonance (SPR) biosensor designed for smartphone assistance, aiming at detecting vitamin B1 in synthetic urine. The key novelty lies in the amalgamation of silver nanocoated cysteine with a gold-plated optical fiber probe, fostering efficient detection through the robust electrostatic interaction between negatively charged Cys-capped AgNPs and positively charged vitamin B1 molecules. Notably, this biosensor leverages the smartphone's digital camera capabilities to discern varying color intensities. A self-developed application ensures real-time monitoring of green and red channel intensities in the SPR signal, thereby mitigating the risk of false detections. Moreover, the integration of the fabricated optical fiber probe with the smartphone is achieved through a 3D-printed casing, endowing the biosensor with the advantages of portability, handheld operation, and cost-effectiveness. The proposed U-shaped optical fiber SPR biosensor, coupled with smartphone technology, demonstrates the ability to detect vitamin B1 within the concentration range of 2–10 μM, yielding a commendable R2 value of 0.9973. Experimental findings affirm that this novel biosensor, built on the U-shaped SPR optical fiber architecture and empowered by smartphones, is an economical, uncomplicated, and portable device adept at accurately and efficiently detecting vitamin B1 across a broad concentration spectrum. Its potential applications extend to the area of low-cost biochemical component identification. [ABSTRACT FROM AUTHOR]

• 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]

• The geometric dimension of the package shell of the sensing unit are studied by theory and experiment, and improper design of the package housing may degrade the frequency response characteristics of the sensing unit. • The influence of prestress on the frequency response characteristic of sensor is studied. • The geometric dimension of the package shell and the prestress of the sensing unit are reasonably optimized to make the working frequency of the sensor avoid the resonant frequency of the shell. • The paper provides a theoretical basis for the design and manufacture of the sensors with wide frequency response and high sensitivity in different engineering applications, such as high frequency small amplitude and low frequency large amplitude. In coal mine, military, oil well monitoring and other fields, the traditional electrical accelerometers based on piezoelectric and capacitive can not meet the needs of high-performance applications. The optical fiber acceleration sensor with high precision, large dynamic, wide frequency response, anti-electromagnetic interference and other unique advantages, can meet a variety of applications. The complete optical fiber acceleration sensor consists of a package housing and an internal sensing unit. Some factors, including the vibration and deformation of the housing itself, and the prestress of the sensing unit will directly affect the frequency response characteristics of the sensor. The experimental results show that the resonant frequency of the package housing is outside the operating frequency band of the sensor through reasonable design of shell parameters; in a certain torque range (0.2 N . m ∼ 0.8 N . m), the resonant frequency increases with the increase of the torque. The research provides a theoretical basis for the design and manufacture of the sensors with wide frequency response and high sensitivity in different engineering applications, such as high frequency small amplitude and low frequency large amplitude. [ABSTRACT FROM AUTHOR]

• Clean and symmetric tapered single mode and multimode fibers have been successfully prepared using CO 2 laser. • The controllable CO 2 laser tapering process could be done in just 5 s. • Different CO 2 laser powers influenced the final waist taper diameter. • Smaller diameter of tapers promoted more evanescence waves which enhanced the refractive index sensing sensitivity. • Au nanospheres immobilization on tapered multimode fiber could improve the sensitivity up to 122% due to localized surface plasmon resonance. Tapered optical fibers (TOFs) have been intensively used as sensors due to their high sensitivity, simple structure, and excellent coupling efficiency. In this paper, we reported the successful rapid tapering of single mode and multimode fibers using CO 2 laser beam at working wavelength of 10.6 μm. Clean and symmetric tapers could be produced in just 5 s using this method. Arduino-based microcontroller was included in the tapering set-up to precisely control the fiber pulling speed and distance. Smaller waist diameters that were observed in multimode than in single mode fibers generated more evanescent waves on the cladding of tapered multimode fibers (TMMFs). This implied that TMMFs performed better refractive index (RI) sensitivity, within the range of 1.3325–1.4266, than tapered single mode fibers (TSMFs). Furthermore, localized surface plasmon resonance (LSPR) effect was coupled with evanescent wave through AuNSs immobilization on the TOF surface. The AuNSs having average particle diameter of 12.70 nm was synthesized by citrate reduction route. The formation of AuNSs was confirmed by (ultraviolet–visible) UV–vis and high-resolution transmission electron microscopy (HRTEM) data. Self-assembly monolayer approach was conducted to immobilize AuNSs on the 3-mercaptopropyl trimethoxysilane (MPTMS)-functionalized surface of the TOFs. The AuNSs immobilization on TMMFs could improve the sensitivity up to 122 %. Besides sensitive to RI changes, the sensors were also excellent in terms of repeatability and reversibility performances. Finally, this paper described the amplitude modality-based RI sensing mechanism due to evanescent wave and LSPR effects. [ABSTRACT FROM AUTHOR]

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]

• A simple and low-cost optical fiber temperature sensing system is proposed, consisting only of an FP-LD and a Sagnac ring. • The built-in monitor PD of FP-LD is used to detect the beat frequency signal for temperature sensing. • The sensitivity of the sensor is 14.87 kHz/°C in the range of 25–125 °C. A novel, simple, and low-cost optical fiber temperature detection system is proposed and demonstrated in this paper, which uses the strong feedback of the Fabry-Perot Laser Diode (FP-LD) at 1550 nm to demodulate the temperature in real time in the resonator cavity. The experimental system consists of a commercial telecommunication laser source (FP-LD) and a fiber-optic Sagnac ring. In the sensing test, the change in temperature has a perfectly linear relationship with the shift of beat frequency signals (BFS). Measuring signals of different frequency will have different sensing sensitivities. System has an external photodetector (PD) in the first experiment. The temperature monitoring sensitivities is estimated to be 14.87 kHz/°C@2.49 GHz. In the second experimental system, there is no external PD, and the built-in monitor PD of FP-LD is used for photoelectric conversion. The sensitivity of the temperature sensor is 8.14 kHz/°C@1.382 GHz. In this paper, a fiber sensing system with the lowest cost and simplest structure is proposed, which consists of only a laser diode and a Sagnac ring. [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]

In the real environment, the carrier phase delay (θ) is difficult to accurately maintain at nπ (n is an integer), which results in the demodulation error of the phase generation carrier (PGC) demodulation algorithm. In this paper, the fast Fourier transform (FFT) technique is used to extract 1 θ 0 , 2 θ 0 , 3 θ 0 , 4 θ 0 from the first to fourth harmonic signals of the interference signal, then introduce 1 θ 0 , 2 θ 0 , 3 θ 0 , 4 θ 0 into the mixed signal as the initial phase, and finally perform a mixing operation to real time eliminate the carrier phase delay. The experiments show that when the carrier delay is in the range of 0 to π, the SINAD of the ameliorated algorithm reaches 43.66 dB, which is 19.31 dB higher than the PGC-Arctan algorithm on average. The THD of the ameliorated algorithm can be stabilized at 0.183 %, which always maintains the best level of the PGC-Arctan algorithm. The ameliorated algorithm eliminates the carrier phase delay and does not have the phase singularity, thus expanding the application range of the fiber optic interferometer sensor. [ABSTRACT FROM AUTHOR]

• A simple and efficient fiber optic sensor networking method that can be used for multiplexing a large number of sensors in mesh-grid topology. • Either discrete or long span distributed fiber optic sensors could be multiplexed in this network. • Self-reconfigurable topology provides sensors with more resilience in case of failures. • Centralized or discrete network control method could be determined based on actual sensor conditions. This paper presents a mesh grid topology for an optical fiber sensor network, which has an efficient structure for intelligent monitoring and reconfigurable protection operations. The integration of light sources and detectors within the network plays a crucial role in achieving comprehensive monitoring of the entire sensor network. Moreover, we have designed multiple nodes for sensing channels with distinct function to maximize the benefits of the topology in terms of network resilience. Finally, centralized and discrete signal processing schemes for network control systems have been demonstrated to be effective under various application conditions. [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]

• Present a ultraviolet (UV) detection method based on the all-fiber structure of zinc oxide (ZnO) composite graphene, and applied to the field of partial discharge detection. • The sensitive materials are tightly and uniformly combined with the single-mode conical, multi-mode single-mode (STMS) optical fibers by the surface functionalization method. • The response performance of the fabricated interference fiber sensor were explored and analyzed. • The UV sensitivity of STMS coated flower-shaped ZnO composite graphene is 857.41 pm/nW·cm−2, and the minimum detection voltage was 21.55 kV. The occurrence of partial discharge in high-voltage transmission equipment can lead to insulation breakdown, emphasizing the critical need for early fault detection. Traditional sensors are highly vulnerable to electromagnetic interference, which significantly impacts the accuracy and reliability of the obtained results. This paper presents an all-fiber ultraviolet (UV) sensor based on the STMS structure coated with zinc oxide (ZnO) composite graphene, realizing partial discharge detection based on optical theory. Firstly, using first principles calculation method, we conducted a comparative study on the ultraviolet-sensitive materials ZnO and ZnO composite graphene. The results demonstrate that the composite material exhibits optical response characteristics that are more suitable for partial discharge detection. Subsequently, sensitive materials with different surface morphology were prepared by hydrothermal method. These materials were uniformly coated on the waist cone region of the STMS optical fiber sensing unit using surface functionalization techniques. The UV test results showed that the flower-shaped ZnO composite graphene coated STMS optical fiber sensing has a higher sensitivity of 857.41 pm/nW cm−2, reaching 1.62 times that of rod-shaped sensitive materials. Finally, the test results of the partial discharge environment indicated that the sensor showed a good linear response when the partial discharge voltage was above 21.55 kV. The UV optical fiber sensor demonstrates promising potential for applications in the field of partial discharge detection. [ABSTRACT FROM AUTHOR]

• A physical stretching method for preparing PDMS-based fiber structure is reported. • The PDMS fiber is coupled with two multimode fibers by adhesion, forming an interferometer. • The PDMS fiber based interferometer sensor has good sensitivity to VOC gases. • The sensor has merits of simple preparation, high sensitivity, good tensile properties, and humidity resistance. In this paper, a method for preparing polydimethylsiloxane (PDMS)-based fiber structure was reported and its performance for volatile organic compound (VOC) gas sensing was demonstrated. The PDMS fiber prepared by the physical stretching method was coupled at the end faces of two multimode fibers by adhesion, forming an inter-mode interferometer. The PDMS was not easily dislodged after curing, which contributes to the strong robustness of the whole structure. When the PDMS optical fiber is exposed to VOC gas environment, PDMS will adsorb VOC gas molecules, resulting in changes in its refractive index and volume, which shifts the wavelength of inter-mode interference. Experiments show that the sensor has good sensitivity to VOC gases such as acetone, ethanol, and methanol. In the concentration range of 0–400 ppm, the sensitivity reaches 14.84 pm/ppm, 13 pm/ppm, 4.39 pm/ppm, 7.37 pm/ppm, and 5.99 pm/ppm for acetone, ethanol, methanol, toluene, and ether respectively. The proposed sensor has excellent characteristics such as simple preparation, high sensitivity, good tensile properties, and humidity resistance, which indicates that the sensor has some potential for VOC gas detection. [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]

Pressure sensing holds immense significance in the realm of fiber optic sensors, particularly in the context of industrial and environmental safety monitoring. In this paper, we propose and experimentally validate an optical fiber structural sensor based on Mach-Zehnder interferometer (MZI) for pressure measurement. The sensor adopts a cascaded spliced single-mode-multimode-tapered single-mode fiber (SMF-MMFTapered SMF, SMTS) structure, taking advantage of the mode mismatch effect between different fiber types and the high sensitivity of the evanescent field surrounding the tapered single-mode fiber. A segment of multimode fiber (MMF) is incorporated as the mode coupler to enhance the multimode interference (MMI). Experimental results demonstrated the sensor's robustness to pressure and temperature changes, with measurement sensitivities of the interference wavelengths Dip A and Dip B at 5.751 nm/KPa and 1.394 nm/KPa, respectively, within the pressure range of 0 ∼ 4410 Pa. The sensor also exhibited a response of 1.113 nm/°C and 0.857 nm/°C in the temperature range of 27.5 °C ∼ 41 °C. The sensor indicates a low temperature crosstalk of only 0.194 KPa/°C. By applying temperature compensation to correct pressure measurements, the accuracy of pressure sensing can be significantly improved. The sensor offers distinct advantages, including high sensitivity, low temperature crosstalk, compact structure, and simple fabrication, making it a promising candidate for applications in biochemistry and industrial production. [ABSTRACT FROM AUTHOR]

• This paper analyzes the state of mode coupling in a SI air-clad silica optical fiber. • The coupling coefficient of SI air-clad silica optical fiber is obtained. • At a length of 2100 m, the equilibrium mode distribution in the investigated fiber is established. • For the first time this paper reports on the strength of mode coupling in this fiber. • The results obtained can be exploited to employ SI air-clad fibers in broadband astro-photonics systems. This paper analyzes the state of mode coupling in a high numerical aperture (NA) step-index (SI) air-clad silica optical fiber using the power flow equation. The coupling coefficient of this equation was fine-tuned at first to allow it to correctly reconstruct previously reported experimental intensity distributions of output far-field (FF) patterns. At a length of 2100 m, the equilibrium mode distribution in the investigated fiber was obtained to be established. In our best knowledge for the first time, this paper reports on the strength of mode coupling and output beam quality of SI air-clad silica optical fiber. The results obtained can be exploited to employ SI air-clad fibers in broadband high-NA astro-photonics systems. [ABSTRACT FROM AUTHOR]

• An optical fiber device for detecting gas ethanol is proposed. • Gas detector based on surface plasmon resonance. • The device has a high repeatability and durability. • The device has a large detectable concentration range and an acceptable response time. Ethanol is an important material in for chemical, pharmaceutical or food industry. But ethanol gas in the air induces health danger and explosion risk. Therefore, the concentration of ethanol gas must be monitored. In this paper, an optical fiber device for ethanol gas detection is demonstrated. Glycerol aqueous solution is taken as the ethanol absorbent, a Teflon capillary is used as the gas permeable cell and a surface plasmon resonance (SPR) fiber sensor is employed as the receiver. When ethanol gas permeates into the capillary and be absorbed by the absorbent, the refractive index of the absorbent is modulated and then detected by the SPR sensor. With this principle, sensing devices were fabricated and tested. Within the range of 0–7534 ppm, the limit of detection (LOD) is obtained as 38.31 ppm. In cycle test, the excellent reusability, repeatability and durability are testified. In addition, with the large detectable range and low cost, the proposed device may realize wide applications in industrial process. [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 new nanocomposite material (CS/α-Fe 2 O 3) is proposed for the specific identification and detection of copper ions. • The specificity of the CS/α-Fe 2 O 3 nano-composite and the response performance of the fabricated interference fiber sensor were explored and analysed. • High sensitivity measurement of Cu2+ concentration (0–1 μM) was achieved. Instrument methods are essential for simple and rapid detection of copper(II) (Cu2+) in drinking water and healthcare. However, these methods suffer from complex sample pretreatment and long detection time. This paper proposed a highly sensitive fiber optic sensor for trace Cu2+ detection in aqueous solutions. Based on the principle of modal interference, by surface functionalization, a uniform chitosan (CS)/α-Fe 2 O 3 nanocomposite material was coated on a structure of no-core fiber–multimode fiber–no-core fiber (NCF1-MMF-NCF2). The amino and hydroxyl groups in the material have a specific recognition function for Cu2+, which can chelate with Cu2+ and change the refractive index of the material, leading to the shift of interference spectrum. Trace Cu2+ detection can be achieved by monitoring the displacement of the interference spectrum. The results revealed that within the range of 0–1 μM Cu2+ concentrations, with increasing Cu2+ concentration, the interference spectrum underwent a red shift and the maximum detection sensitivity of Cu2+ ions reached 456.69 nm/μM, with a good linearity of 0.989. Also, the sensor has the advantages of high stability, good repeatability, simple fabrication, and low cost. [ABSTRACT FROM AUTHOR]

This paper presents and experimentally validates a highly sensitive fiber-optic Fabry-Perot interferometer sound sensor based on a fiber-optic spherical structure. The Fabry-Perot interferometer comprises an aluminum foil diaphragm and a fused fiber-optic microsphere end face, both sealed in a structure made of a glass tube. An optical fiber microsphere structure is used for the first time in acoustic sensing. Based on the strong focusing capability of the microsphere, the high coupling efficiency to the reflected beam, and the extensive range of the focused convergent optical field formed, the focused convergent optical fields facilitate the sensing of an extensive range of motion or deformation of the cavity compared to the divergent optical field of a flat fiber end face, i.e., the acoustic signals at more minor sound pressures can also be well sensed, thus compared to a flat fiber end face fiber The acoustic sensor can monitor acoustic signals over a more extensive frequency range and a larger range of acoustic pressure variations, and the output voltage signal is of a larger amplitude at the same acoustic signal, which allows the monitoring of acoustic signals at acoustic frequencies within the hearing range of the human ear. Experimental results show that the sensor shows a flat frequency response in the range of 20 Hz–20 kHz, covering all frequency components of the human ear hearing range, and the signal-to-noise ratio of the proposed acoustic sensor in the range of 20 Hz–20 kHz is greater than that of the flat fiber end-surface based acoustic sensor and the reference electroacoustic sensor, especially The sensor has the advantages of good high sensitivity, excellent linearity, wide planar response range, and simple manufacturing process. It can potentially be used as a highly sensitive, high acoustic quality fiber optic microphone in practical applications. [ABSTRACT FROM AUTHOR]

This paper presents a novel design of an optical fiber sensor (OFS) utilizing a single-mode-coreless-multimode-single-mode (SCMS) fiber coated with Poly-Vinyl Acetate (PVAc) for humidity and moisture detection. PVAc, a highly sensitive synthetic polymer, exhibits changes in refractive index (RI) based on relative humidity (RH) variations in the surrounding medium, thereby affecting the output power of SCMS. Numerical analysis using COMSOL Multiphysics determined the optimal coating thickness. The fabricated PVAc-coated SCMS sensor enables RH measurement in air and moisture measurement in oil. The sensor demonstrated a sensitivity of 0.19 dBm/%RH and 0.16 dBm/%RH for increasing and decreasing humidity rates, respectively, within an RH range of 60–85% in air media. Additionally, the sensor achieved a sensitivity of 0.15 dBm/ppm for moisture measurement in oil media. [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]

The polarization characteristics of the optical fiber have spurred interest in a plethora of smart fiber optic sensors with extremely compact structures due to the capability of in-line configuration, except that the sensing sensitivity remains too low to meet the practical requirements. In this paper, fiber Lyot filter (FLF) is leveraged to generate Vernier effect for sensitivity enhancement of birefringence-based and versatile fiber optic sensor. The proposed sensing structure is composed of two sections of polarization maintaining fibers (PMF) with the reflective scheme, which can be applied to birefringence sensing scenarios such as strain and temperature measurement. The working principle analysis derived from Jones matrix indicates that the combination of Vernier effect and the reflective structure catalyzes a significant improvement in sensitivity. The length of the PMF used in this sensor, the fusing angle between two PMFs, and the birefringence properties of the PMF are all numerically investigated to generate better Vernier performance. Also, experiments are carried out for validation, and sensitivity as high as −12.96 nm/rad is achieved. The compact and in-line structure of the proposed sensor can enable itself to perform excellent and versatile sensing on various occasions. • Fiber birefringence sensors are widely used in many applications. Low sensitivity due to the slight birefringence change of the polarization maintaining fiber hinder their development, and the practicality and stability of existing structures (core-offset fiber, tapered fiber, etc.) used to generate Vernier effect are unsatisfactory. Therefore, we investigate the Lyot filter-induced Vernier effect for sensitivity improvement of fiber birefringence sensor. • The Lyot filter is used to generate Vernier effect for sensitivity improvement of fiber birefringence sensor. • This approach provides a practical and stable structure with high sensitivity and easy packaging. • Simulations and experiments show that the sensor is stable with high sensitivity, providing more opportunities for versatile sensing in various occasions. [ABSTRACT FROM AUTHOR]

The Structural Health Monitoring (SHM) of large civil infrastructures has become a priority due to economic advantages and early detection of structural failure. Distributed Fiber Optics (DFO) sensors are becoming widely used for SHM due to their immunity to Electromagnetic interference and the ability to collect multiple data points from a single fiber cable. However, its reliability for a long monitoring period is still an area of interest. This paper studied the long-term performance of DFO embedded into textiles for faster installation procedures. The aim was to understand if the textile distributed fiber sensor experience any signal degradation after multiple weather seasons. To perform this investigation a railway bridge was used as a case study. The data was collected using Brillouin Optical Time Domain Reflectometry (BOTDR) and Brillouin Optical Time Domain Amplification (BOTDA). Additionally, a temperature compensation technique was investigated to remove the cross-relation between temperature and strain. The final results show the sensor's effectiveness for long-term monitoring, where the data quality remained constant with an average standard deviation of 0.455 for the BOTDR and 0.208 for the BOTDA approaches. • Developed a Smart textile for Structural Health Monitoring (SHM). • Performed long-term monitoring of a Railway bridge under different weather conditions. • Utilized Brillouin Optical Time Domain Reflectometry (BOTDA) in combination with the smart textile to determine changes in the fiber strain distribution. • Determined the survivability of the Smart Textile under New England's harsh conditions. [ABSTRACT FROM AUTHOR]

• An ameliorated PGC demodulation scheme using Taubin least squares (LS) fitting of ellipse is proposed. • A combined internal modulation is used and optimized to guarantee the effectiveness of the ellipse fitting algorithm. • Compare with LS, Taubin LS has a high fitting accuracy, thereby reducing the extra low frequency modulation amplitude. • Quadrature signals are extracted from 0th and 1st harmonics of the interference signal, reducing the system complexity. • The scheme has a phase resolution of 3.16 μrad/√Hz, a dynamic range of 120 dB @ 1 KHz and a linearity of 99.996%. In this paper, we propose an ameliorated phase generated carrier (PGC) demodulation scheme using Taubin least squares (LS) fitting of ellipse to eliminate the nonlinear errors. Taubin LS has carefully chosen a normalization matrix and the ellipse fitting parameters are directly obtained by solving the generalized eigenvector problem, leading to a high accuracy result. Quadrature signals are extracted from the signal fundamental and the sidebands of the fundamental of the carrier, and then corrected by the ellipse fitting algorithm (EFA) based on Taubin LS. A combined internal modulation is used to ensure the proper operation of the EFA under small signals. And the desired phase signal is extracted from the corrected quadrature signals by the differential-cross-multiplying method and evaluated by the total harmonic distortion. Experimental results show that the proposed scheme is superior and highly stable, it has a phase resolution of 3.16 μrad/√Hz, a large dynamic range of 120 dB @ 1 KHz and a good linearity of better than 99.99%. [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]

• 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]

In this paper, a high-sensitivity magnetic field sensor is proposed. The sensor consists of a spindly optical fiber taper (SOFT) coated with magnetic fluid. The SOFT structure is made through fusing taper method and connected to a fiber loop for magnetic field sensing. The experiment results show that the sensor is barely dependent on laser wavelength within the range of 1520 nm to 1575 nm. When the magnetic field of 180 Oe is applied, the transmission change reaches up to 3.324 dB at 1550 nm. Additionally, the magnetic field sensitivity of the sensor is − 23. 4 μ s /Oe in the linear region of 10 Oe−80 Oe. Compared with the traditional fiber optic magnetic field sensor with the fiber loop ring-down (FLRD), the proposed SOFT sensor could achieve higher magnetic field sensitivity. • A spindly optical fiber taper (SOFT) structure is proposed to measure magnetic field. • The SOFT sensor has good magnetic field response ability while maintaining low loss. • The optimization of the fiber loop parameters is studied. • We could achieve higher magnetic field sensitivity compared with the previous sensors. [ABSTRACT FROM AUTHOR]

• A compact, high-sensitivity magnetic field sensor is experimentally demonstrated. • Higher M factor of the Vernier effect due to the usage of hybrid interferometers. • Experimental results show that a linear sensitivity of 811.53 pm/mT is reached. This paper presents and demonstrates an ultra-sensitive magnetic field sensor using a hybrid interferometer. The sensor is created by combining a Sagnac loop interferometer (SI) and a Fabry-Perot interferometer (FPI). The difference in the free spectrum ranges of the two cavities generates a Vernier effect, which amplifies the sensitivity. Experimental results indicate that the sensitivity of the single FP cavity is 25 pm/mT, while the sensitivity of the hybrid sensor is 811.53 pm/mT, which is about 32 times higher than that of a single individual FPI. The sensor can be applied in harsh environment such as mine, oil well and other fields. [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]

• A high sensitivity salinity sensor based on tapered optical fiber is proposed. • The tapered optical fiber salinity sensor based on SPR effect possesses the highest sensitivity of 0.708 nm/‰. • The protection of gold film makes the sensor have long-term effectiveness and high repeatability. High sensitivity detection of seawater salinity has important practical significance. SPR fiber optic sensors based on tapered fiber optic have high sensitivity to refractive index and are widely used in fields such as physics, chemistry, and biology. In this paper, a tapered fiber optic was prepared using a fusion tapering process, and the morphology of the SPR sensor was optimized by adjusting three important parameters: taper length, waist diameter, and coating thickness. The influence of these factors on the sensor performance was studied through simulations. Based on the quality factor as the main evaluation criterion, a tapered fiber optic SPR sensor with a taper length of 27 mm, waist diameter of 25 μm, and coating thickness of 40 nm (Au) was selected through experiments. This sensor was used to detect seawater salinity, and its sensitivity reached 0.708 nm/‰ with good linearity. The experiment also proved the long-term effectiveness of the sensor. [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]

• Two different structures of reflected fiber ethanol probes were compared. • Relationship of sensitivity and morphology parameters have been discussed. • Microfiber taper ethanol probe has been experimentally demonstrated. • Ethanol sensitivity of 7.18 × 10-4 dBm/ppm was obtained during 0–200 ppm. Reflected fiber probes can efficiently detect different environmental parameters in the narrow space. In this paper, the coreless fiber and the microfiber taper cascade-spliced structures were coated with the sensitive materials. Their refractive index sensing performance was studied and compared. The microfiber taper shows a sensitivity of −36.581 dBm/RIU, which is significantly higher than that of the coreless fiber structure with the same length (-9.482 dBm/RIU). The reflected microfiber taper ethanol gas sensor was made by surface-modified with the mixtures containing zinc oxide nanosheets and UV glue with low refractive index. In experiment, as the ethanol concentration increased, the intensity of the reflection spectrum increased, resulting in the sensitivity of 7.18 × 10-4 dBm/ppm. This miniature fiber structure can be integrated into the conventional optical fiber system to develop the miniature detectors with high performance. [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]

• Temperature-insensitive water content estimation in oil mixtures. • Root mean squared error in water content estimation of 1.36 M. • Compensation technique for the temperature effects in the sensor. • Temperature-induced errors as small as 0.32% are obtained. This paper presents the development of an optical fiber sensor for water content estimation in oil–water emulsions. The sensor is based on the surface Plasmon resonance (SPR) principle applied in a polymer optical fiber (POF). For the sensor fabrication, a D-shape is performed on the POF to expose its core region, which has a gold thin film (nanometer thickness) for the SPR signal. The sensor is analyzed as a function of the water content in different emulsions conditions (from 0 to 55.5 M of water content), where a sensitivity of − 0.0288 nm/M was obtained. In addition, the sensor presented a sensitivity of the transmitted optical power variation as a function of the water content. These results lead to the possibility of data fusion between optical power and wavelength responses to obtain a high accuracy in the water content estimation. In this case, a root mean squared error (RMSE) of 1.36 M was obtained on the emulsion characterization. As an important drawback in general sensors applications, the temperature cross-sensitivity was also analyzed, where a sensitivity of 0.118 nm/°C was obtained. Such temperature sensitivity of the sensor leads to errors in emulsion characterization as a function of the temperature. To address this issue, a compensation method based on direct difference between the sensor's sensitivities is proposed. By applying the proposed compensation method, a reduction of the temperature-induced errors in the water content estimation from 6.46 %±7.81 % to only 0.32 %±1.96 %. [ABSTRACT FROM AUTHOR]

• Polymer optical fibers with a ring-doped structure. • Light sources with tuning characteristics and optical sensors based on active polymer optical fibers. • Effects of dopant combinations, pumping conditions and light-propagation distance on the emission properties of polymer optical fibers doped with Lumogen Yellow 083 and Lumogen Red 305. • Time-resolved characterization of the doped fibers to obtain information about the energy transfer between the dopants. This paper employs the solution-doping technique for the fabrication of polymer optical fibers (POFs) doped with two perylene derivatives, Lumogen Yellow 083 and Lumogen Red 305, in different combinations. With the solution-doping technique is very easy to control the amount of dopant penetration into the core of non-doped uncladded fibers, allowing the fabrication of active POFs with a novel ring-doped structure. In addition to manufacturing the fibers, these have also been optically characterized. Specifically, the influence of the combination of dopants, pumping power and wavelength, as well as the light propagation distance, have been measured and analyzed. Furthermore, time-resolved emission characteristics have also been measured to determine the fluorescence lifetimes and to extract information about the energy transfer between the dopants. Finally, the aim of this work has been to investigate the performance of the aforementioned POFs for fluorescent lighting applications, with a special focus on tunable light sources, and also for sensing applications. [ABSTRACT FROM AUTHOR]

• PD of the proposed sensor is 1/10 of the other sensors. • Proposed sensor behaves as localized SPR sensor due to lowest PD and smaller PL. • Proposed sensor can selectively detect molecules of dimension ≤50 nm, e.g. virus. • Proposed sensor has highest sensitivity along with slightly higher spectral width. A D-Shaped fiber optic plasmonic sensor with silver grating coated with graphene is proposed in this paper. The proposed structure is compared to planar silver, grating silver, and planar silver with graphene grating. Performance defining parameters based on transmittance curves [resonance wavelength (λ r), shift in λ r (δλ r), minimum transmittance (T min), and bandwidth (BW)] as well as based on field intensity [penetration depth (PD) and propagation length (PL)] are determined and compared. The proposed structure has the highest λ r , δλ r , and lowest T min , indicating that it has a high sensitivity. However, its BW is slightly larger than that of other structures, which deteriorates its detection accuracy. Furthermore, the proposed structure's PD and PL are the lowest of all the other structures. At different refractive indices of sample, the PD of the proposed structure (33 nm–55 nm) is one significant digit lower than others (232 nm–299 nm). As a result, the proposed structure acts as a localized surface plasmon resonance (LSPR) sensor, which may be utilized to detect molecules with a dimension of less than 50 nm, such as viruses and cancer cells provided that these molecules should adhere on the graphene surface. [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]

• The temperature sensitivity can be improved by controlling the axial and radial TECs. • The axial TEC of NCF is increased and radial TEC is suppressed to improve sensitivity. • The temperature sensitivity is improved to −355 pm/°C. • The sensor has the potential of refractive index sensing and biosensing. In this paper, a liquid sealed temperature sensor is assembled based on single-mode-no-core-single-mode (SNCS) optical fiber structure. Theoretical calculations and experiments verify the sensing characteristics of this structure. The structure improves the temperature sensitivity to −355 pm/°C by encapsulating an organic solvent with large thermal optical coefficient and using packaging material with large thermal expansion coefficient. The theoretical results are in good agreement with the experimental results. Theoretical simulations are completed on MATLAB using far from the cut-off condition. The SNCS structure is encapsulated in a microfluidic chamber by applying microfluidic processing technology. The axial thermal expansion coefficient of the no-core fiber (NCF) is increased and radial thermal expansion coefficient is suppressed by the polydimethylsiloxane (PDMS) mold stretching NCF to improve the temperature sensitivity. The enhancement effect of PDMS stretching NCF on temperature sensitivity is verified by contrast experiment. Due to the wrapping of PDMS, the sensor has strong resistance to mechanical damage and external interference. [ABSTRACT FROM AUTHOR]

• NCF length which consist of even self-image index has higher sensitivity compared to odd self-image index. • However, there were no significant sensitivity changes between them. • The best sensor performance was achieved by the NCF length of 2.90 cm with two self-image indexes. No-core fiber has been widely demonstrated as an alternative refractive index sensor over conventional optical fiber sensors. However, previous works demonstrated that the self-image length optimization of this sensor only focused on simulation and formed a conclusion without data analysis. Hence, the sensitivity provided does not follow the standard unit, providing a limited evaluation of the sensitivity for refractive index measurement. This paper investigates the no-core fiber at a fixed diameter of 125 μm and varies the length based on the self-image index ranging from 1 to 4 indexes to evaluate the sensor sensitivity at the analyte refractive index ranging from 1.3000 RIU to 1.4000 RIU via experimental study. The results showed that the dip wavelength shift non-linearly increased for all the samples as the analyte refractive index increased. The overall analysis showed that there were significant changes in the sensitivity between the odd and even self-image indexes, and there were no significant sensitivity changes between them. The best sensor performance was achieved by the NCF length of 2.90 cm with two self-image indexes. The sensitivity achieved by the sensor was 104.64 nm/RIU and 241.30 nm/RIU for analyte refractive index ranging from 1.3000 RIU to 1.3600 RIU and 1.36000 RIU to 1.4000 RIU, respectively, via a linear fitting plot. Meanwhile, the figure merit value obtained by the sensor was 1.45 RIU−1 and 3.75 RIU−1 at analyte RI of 1.3600 RIU and 1.4000 RIU, respectively. In the future, this can be used as a reference for no core fiber length optimization. [ABSTRACT FROM AUTHOR]

• A large bandwidth optical fiber magnetic field sensor based on Sagnac interferometer for weak alternating magnetic fields measurement is proposed. • The sensing mechanism of this magnetic field sensor under an alternating magnetic field is theoretically derived and simulated. • The experimental system is built to verify the simulation results, and the magnetic field sensitivity is optimized by increasing the length of the delayed fiber ring. • The magnetic field sensor performances, such as frequency response, linear range, and noise, are analyzed theoretically and demonstrated experimentally. In this paper, a fiber magnetic field sensor is proposed based on Terfenol-D and Sagnac interferometer. The mechanism of the sensor is deduced theoretically. As the frequency of the magnetic field increases, on the one hand, the sensitivity is enhanced because the response to the high-frequency non-reciprocal phase differences is intensified in the Sagnac interferometer. On the other hand, the sensitivity is suppressed by the eddy current effect. Then it exhibits significant linear response to alternating magnetic field in a certain frequency range. The sensor performances, such as frequency response, linear range, and noise, are analyzed theoretically and demonstrated experimentally. Experimental results show that the sensor has a 3 dB bandwidth from 100 Hz to 2500 Hz, and the excellent linearity and reversibility from 0 to 274.05 μT (rms). The high sensitivity of 1.34 mV/μT (rms) and minimum detectable magnetic field of 25 nT/√Hz (rms) are achieved at 500 Hz. The sensor with large bandwidth and wide measuring range is suitable for detecting weak alternating magnetic field. [ABSTRACT FROM AUTHOR]