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• We propose an optical fiber magnetic field sensor based on Michelson interferometer. • High sensitivity, good linearity, temperature stability, sensitive response. • The magnetic field sensitivity:−62.8 pm/mT;The detection range:0–200 mT. We propose a new flexible optical fiber magnetic field sensor (FOMS) based on the combination of Michelson interferometer and the PDMS polymer-based optical fiber. The flexible optical fiber magnetic field sensor is made of stretchable elastomer. It uses a stepped refractive index core-cladding structure to achieve effective light confinement, thus enabling stable transmission of light in the waveguide. When the magnitude of the external magnetic field is changed, the micro bending of the sensing region of the optical fiber causes its equivalent refractive index to change, which leads to a drift in the interference spectrum. It is shown that the magnetic field sensitivity of this optical fiber magnetic field sensor reaches −62.8 pm/mT for wavelength detection in the measurement range of 0–200 mT. The FOMS has the characteristics of high sensitivity, good linearity, good temperature response capability, sensitive response, and resistance to electromagnetic interference. In the future, the sensor will have a broad application prospect in medical, geography, aerospace and other fields. [ABSTRACT FROM AUTHOR]

• Fiber sensor based on Michelson interferometric to achieve a leak signals detector in the pipeline. • Field experiments were conducted in gas valve chambers, to analyze the characteristics of leak signals in the pipeline. • The detection of the leak signals in far end depends on the propagation of the infrasound in pipeline. The advantages of fiber optic sensors based on fiber optic interferometers with explosion-proof safety in gas pipeline monitoring has gradually emerged. A monitoring system for natural gas pipelines was designed and implemented using a Michelson interferometer as the core structure of the fiber optic sensor and processed the collected data using phase carrier generation demodulation algorithms. Addressing practical issues such as phase wrapping and arm length difference losses, an improved phase carrier demodulation algorithm based on tangent transformation and arctangent-differential self-cross-multiplication was proposed. Field experiments were conducted in gas valve chambers, to analyze the characteristics of leak signals in the pipeline. This study lays the foundation for the application of fiber optic interferometer sensors in gas pipeline monitoring, promising enhanced safety and efficiency. [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]

In this work, a Fiber Bragg grating-based accelerometer design is presented. Different geometries (Double-L Cantilever, Single Cantilever, Triangular Cantilever, Clamped–Clamped Cantilever, Capillary Steel Tube, and Flexible Hinges) are analyzed using analytical models to select the one that meets the project requirements: natural frequency of at least 500 Hz and acceleration sensitivity of at least 100 pm/g. After this step, a multi-objective optimization approach is used to provide a geometric parameter combination to generate a Pareto front to analyze different sets of natural frequencies and acceleration sensitivities. Through this procedure, four combinations of geometric parameters were selected to provide one accelerometer with the maximum natural frequency, one accelerometer with the maximum acceleration sensitivity, and two accelerometers that fulfilled the project requirements. These structures were numerically evaluated using modal finite element analysis. Finally, the experimental characterization of the manufactured sensors was carried out at three different excitation frequencies: 17 Hz, 35 Hz, and 50 Hz. For 17 Hz, the experimental sensitivities were 180 pm/g, 690 pm/g, 380 pm/g, and 400 pm/g, for accelerometers 1, 2, 3, and 4, respectively. For 35 Hz, the experimental sensitivities were 150 pm/g, 510 pm/g, 290 pm/g, and 230 pm/g, for accelerometers 1, 2, 3, and 4, respectively. For 50 Hz, the experimental sensitivities were 120 pm/g, 410 pm/g, 150 pm/g, and 160 pm/g, for accelerometers 1, 2, 3, and 4, respectively. The multi-objective optimization procedure proposed in this work is a versatile tool for accelerometer design, where different geometric parameter combinations can be studied to meet different project requirements. In the case of quasi-distributed mechanical vibration monitoring, where project requirements vary depending on the sensor's location, this is an advantageous alternative. [ABSTRACT FROM AUTHOR]

• Machine learning approach for automatic estimation of responses using different TFBGs. • Estimation based on the optical spectral characteristics of TFBGs as a function of RI. • Root mean squared errors as small as 0.008 RIU for the trained samples. • The approach can be used to enhance the reproducibility of large set of TFBG sensors. This paper presents the development of an approach for automatic estimation of the regression coefficients as a function of the refractive index (RI) in Tilted Fiber Bragg Gratings (TFBGs) based on their optical spectral characteristics. The methodology is based on machine learning approach, where different samples are analyzed in various test conditions to obtain a dataset for the training of a kNN regression algorithm. In this case, the spectral characteristics related to the wavelength spacing between adjacent cladding mode resonances, number of valleys and amplitude of the normalized spectra are correlated with the linear regression algorithm for RI estimation using the machine learning approach. The analysis are performed as a function of the wavelength shift and optical power, considering two regions on the transmitted spectrum and the envelope of the curve. Furthermore, the number of valleys and the area of the envelope curve are also analyzed as a function of the refractive index, which result in six analyzed features. The Principal Components Analysis (PCA) is used to reduce the number of features to two principal components. The results of the proposed algorithm are obtained with different trained and untrained samples, which indicates a root mean squared error (RMSE) of around 0.003 RIU for the trained samples and 0.008 RIU for the untrained ones. Therefore, the results indicate the usefulness of the proposed approach, which can be used to enhance the reproducibility of large set of TFBG sensors in practical applications. [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]

This paper presents the development, signal analysis and application of an FBG array embedded smart rubber carpet for 2D shape reconstruction. The proposed sensor system is based on the optical fiber integration between two rubber layers, where the optical fibers have 7 FBGs for the quasi-distributed force assessment. The sensor system analysis is performed at room temperature in difference set of tests, namely the stability tests (without any applied force), the force characterization tests (forces applied directly on the FBG region) and the shape reconstruction tests (different forces applied at different positions in the carpet). The stability tests indicated a maximum variation below 0.02 nm considering all 7 FBGs, whereas the force characterization tests indicated a mean sensitivity of around 80 N/nm. However, it is worth to mention that there is a variation on the sensitivities considering all FBGs, which is related to minor thickness differences in the rubber layer that directly effects the strain transmission from the rubber layer to the optical fiber beneath it. Thereafter, the shape reconstruction occurs in two steps, the first one is the cubic interpolation between the FBGs, considering the boundary conditions on the carpet following the beam theory. Then, the k-nearest neighbors regression algorithm is used on the force distribution assessment with a dataset divided into training and testing with a 70/30 proportion. The shape reconstruction results indicate the feasibility of the proposed approach, where root mean squared errors below 0.65 N are obtained in the range of applied force between 3 N and 35 N. Therefore, the proposed smart carpet as a feasible option for different 2D shape reconstruction applications ranging from biomechanics to structural health monitoring. • Force sensor array using Fiber Bragg Gratings embedded in rubber carpet. • 2D Shape reconstruction using machine learning algorithms. • Relative errors on the shape reconstruction below 3%. [ABSTRACT FROM AUTHOR]

• We developed a fiber-optic temperature sensing probe for measuring temperature near the fiber tip without additional processing. • We utilized a low-coherence Brillouin optical correlation-domain reflectometry technique, eliminating the need for an independent reference light path. • We experimentally measured temperature within a 40-cm-long region near the fiber tip, achieving a theoretical spatial resolution of 26 cm. We present a new fiber-optic temperature sensing approach that enables temperature measurements near the tip of an optical fiber without requiring any special processing. By leveraging low-coherence Brillouin optical correlation-domain reflectometry, our proposed configuration eliminates the need for an independent reference light path and instead utilizes the Fresnel reflected light from the fiber tip as the reference. Through experimental validation, we successfully demonstrate the capability to measure temperature within a 40-cm-long region near the fiber tip. This achievement comes with a spatial resolution estimated at 26 cm, which is obtained from the measured laser linewidth. This method offers significant cost and labor savings due to its simplified implementation. © 2023 Elsevier Science. All rights reserved. [ABSTRACT FROM AUTHOR]

• The sensor was made up of only 1 cm long coreless fiber, and there was no taper. • The proposed SPR sensor can be used to detect a wide SRI range of 1.335–1.4019. • The average refractive sensitivity is 2836 nm/RIU in the range of 1.3335–1.4019. A compact and simple surface plasmon resonance sensor based on coreless fiber for refractive index measurement is developed. Compared with common SPR sensors, the proposed SPR sensor has a shorter length of 1 cm, avoids a taper and can be used to detect a wider range of 1.335–1.4019. The resonant wavelength shifts ∼200 nm to the longer wavelength in the refractive index range of 1.3335–1.4019. The average refractive sensitivity of 2836 nm/RIU is obtained in the range of 1.3335–1.4019, and the highest sensitivity can reach 4328.2 nm/RIU. Furthermore, our device has the advantages of small size, wide measuring range, excellent stability and short response time, which is appropriate for biological sensing. [ABSTRACT FROM AUTHOR]

• Tapered trench-assisted multicore fiber for refractive index sensing is proposed. • The sensor exhibits high sensitivity in a broad range of refractive indices. • Amaximum sensitivity of 35089.28 nm/RIU is obtained in the range of 1.4430–1.4442. • Flexible and precise detection of small increments of refractive index is achieved. • Easy fabrication and low cost of super sensitive device is demonstrated. In this study, a surrounding refractive index (SRI) sensor based on tapered trench-assisted multicore fiber (TAMCF) is proposed and experimentally demonstrated. A segment of TAMCF was spliced between two single mode fibers (SMF28). Electric arc discharge technique was employed to fabricate a tapered sensor with a SMF28–tapered TAMCF–SMF28 structure. Numerical beam propagation was used to investigate the benefit of tapering on TAMCF. Tapering allows coupling of light in the center and outer cores, thereby enhancing the sensitivity of the sensor. Two sensors were fabricated with waist lengths of 10 and 15 mm and a waist diameter of 25 µm. The sensitivity of sensors at different SRI ranges was investigated using a set of honey/water solutions with precisely controlled concentrations. The measured SRI sensitivity was up to 35089.28 nm/RIU (refractive index unit) in SRI range from 1.4430 to 1.4442, which is higher than previously reported studies using other kinds of MCF sensors. SRI sensitivity was enhanced with the increase in waist length of the tapered TAMCF. The fabricated sensors have demonstrated good linearity in terms of spectral wavelength shift versus SRI variations. [ABSTRACT FROM AUTHOR]

Highlights • A side-hole fiber (SHF) based temperature sensor is demonstrated. • The SHF is alcohol-filled and sandwiched between two single-mode fibers. • The so-formed Mach-Zehnder interferometer (MZI) is sensitive to temperature. • The sensitivity is up to 105 pm/°C. Abstract A temperature sensor by using an alcohol-filled side-hole fiber (SHF) based Mach-Zehnder Interferometer (MZI) is proposed and experimentally demonstrated. The SHF-MZI is fabricated by splicing one section of alcohol-filled SHF between two single mode fibers (SMFs). The interference pattern between the core mode and the cladding modes excited at the first fusion splicing point shifts with temperature, due to the temperature-dependent refractive index of alcohol in the SHF, as well as thermo-optic effect and thermal expansion effect of the SHF. Temperature measurement with sensitivity of 105 pm/°C is therefore achieved by detecting resonant wavelength shift of the interference pattern. Meanwhile, the interference properties of the alcohol-filled SHF is investigated and discussed. [ABSTRACT FROM AUTHOR]

Highlights • A novel displacement sensor based on Stokes parameters measured in fiber is proposed. • The performance of the sensor depends on incident SOP and the structure of the transducer. • The highest sensitivity of 4.0045/mm and resolution of about 25 nm have been achieved. • This kind of sensor will has a great practical value for weak vibration measurement. Abstract A novel displacement sensor based on polarization properties in fiber is demonstrated in this paper. The cantilever beam is used as a transducer to convert the displacement into a transverse force onto the fiber which results in the fiber birefringence and hence the amplitude of the Stokes parameters in fiber. The sensitivity and linear range of the sensor depends on incident state of polarization (SOP) and the mechanical and geometric parameters of the cantilever beam. The highest sensitivity of 4.0045/mm has been achieved in the experiments. [ABSTRACT FROM AUTHOR]

• Design of a multifunctional artificial tendon for deformation monitoring. • Two different configurations tested for curvature and strain monitoring. • Bioinspired structure with the combination of different resins and optical fiber material. Advances in materials development provide new insights on smart structures as the ones presented in this work, which consists of multifunctional artificial tendons with structural applications and the ability of measuring deformations. The tendons are produced using Flexible UV cured and Polyurethane (heat-cured) resins. Silica optical fibers were applied as intensity variation sensors to measure the deformations in the tendons. The optical fibers have an additional capability to provide a reinforcement to the bioinspired tendon structure. The mechanical capabilities of the tendons are also analyzed, with the PU tendon reaching 30% strain and withstanding stresses up to 1.0 MPa without suffering damage. The tendons are also subject in dynamic tests, where they present sensitivity to the strain rate. Using linear regressions, intensity variation rates into the 2 ∼ 2.6 × 104 a.u. × s−1 range are estimated with determination coefficient (R2) higher than 0.9. Similarly, in the static bending test, the variation of the power as a function of the flexion can also be approximated by a linear behavior with a R2 of 0.97. Thus, the proposed approach resulted in an optical fiber-embedded biomimetic artificial tendon that can perform not only the structural behavior, but also can measure the deformations in the structure, which are important data for the control of robotic devices. [ABSTRACT FROM AUTHOR]

[Display omitted] Strain and temperature sensing based on mode interference in a few-mode fiber is analyzed theoretically and experimentally. The effect of using different mode combinations is explored. A fiber guiding four LP modes is investigated as an example. Good agreement between modeling and experimental results is found for strain sensor. We demonstrate the sensors feasibility by measuring strain ranging from 0 mm to 16 mm with a sensitivity of 3.43 nm/mm and measuring temperature ranging from 30 °C to 100 °C with a sensitivity of 4.8 pm/°C. [ABSTRACT FROM AUTHOR]

• We report a refractive index sensor based on an inline etched tapered fiber. • The etching method allows enhancing the sensitivity of a ∼ 30 μm fiber taper. • We achieve a sensitivity of 110.9 GHz/RIU (refractive index units). • RI detection over 2.1 mm length, with 0.035 mm spatial resolution is reported. Reflector-less refractive index (RI) sensors employ simple optical fiber strictures for the accurate detection of RI changes. They operate in absence of any reflective element such as gratings, filters, interferometers while maintaining the form factor of a reflective probe; the RI dependence is encoded in the Rayleigh backscattering signatures, and is interrogated using optical backscatter reflectometry. In this work, we present a new reflector-less fiber-optic sensor based on two-steps: (1) tapering an optical fiber, followed by (2) rapid etching of the fiber. The combination of the two steps allows obtaining significant advantages in terms of manufacturing, making the process simpler and increasing the distribution of the refractive index detection. The sensitivity obtained is up to 110.9 GHz/RIU (refractive index units), with the possibility of detection over a length of 2.1 mm, with 0.035 mm average spatial resolution over distributed RI sensing. [ABSTRACT FROM AUTHOR]

Highlights • This paper present a novel refractive index sensor based on tapered optical fiber. • The sensitivity can be enhanced by using micro-tapered long-period fiber grating. • The sensing characterises has been indicated numerically as well as experimentally. • Proposed sensor features a low cost, a simple structure and a high sensitivity. Abstract A novel micro-tapered long-period fiber grating refractive index sensor is demonstrated in this study. Its transmission characteristics are investigated numerically and experimentally. The sensor is fabricated by inscribing periodical micro-tapers in a biconical single mode optical fiber (SMF-28) using a low cost electric arc heating technique. The periodical tapering in SMF enables the fundamental core mode to be coupled into the cladding modes resulting a resonant wavelength in the transmission spectrum. The resonant wavelength shifts linearly with the variation of surrounding refractive index. The refractive index sensitivity of 8188 nm/RIU has been experimentally achieved which is much higher than the previously reported refractive index sensitivity values for the sensors based on long period fiber grating. The effects of varying the number of periodical tapers, amount of grating period and waist diameter on the sensitivity have been analysed both numerically and experimentally. Our presented sensor features a low cost, a simple structure and a high sensitivity. The results presented in this study would be useful in both chemical and bio-sensing applications. [ABSTRACT FROM AUTHOR]

In this paper we focused on the experimental investigations for fabricating a surface plasmon resonance (SPR) based fiber optic sensor for the detection of the extent of adulteration in petrochemicals: petrol and diesel by kerosene. Primarily it is observed that the refractive index of the petrol and diesel changes if we mix kerosene in it. The variation in refractive index is linear in nature. Utilizing the phenomenon of surface plasmon resonance in Krestchmann configuration on optical fiber, the percentage of adulteration in petrol and diesel is detected. The detection level of adulteration is quantified systematically for both the petrol and diesel. The study carried out here explores the possibility of utilizing SPR technique for the detection of the level of adulteration in petrochemicals. The suitability of the optical fiber for remote sensing and its immunity towards electromagnetic interaction makes this probe very useful for such endeavor. High sensitivity, easy construction and its portability, makes this study important in the development of SPR based optical fiber sensors for petrochemical industries. Apart from this various aspects of environment polluting hazardous/toxic gases as an emission product of automobile fuels has also been discussed. [ABSTRACT FROM AUTHOR]

A high sensitivity optical fiber Surface Plasmon Resonance (SPR) sensor which based on coreless optical fiber, silver film and graphene, has been designed and implemented for liquid concentration detection. In this paper, Graphene is firstly verified that it can be used to enhance the evanescent field of traditional optical fiber and thus increasing sensitivity in experiment. The sensitivity of proposed sensor is 6.417 nm/%, which is higher than that of the traditional optical fiber SPR sensor according to the comparative experiments. In addition, the proposed sensor is extremely easy to make and the silver film could be protected from oxidation and damage due to the existence of graphene. Moreover, the sensor has pretty small size, immunity to electromagnetic interference, quick response speed and thus can suitable a variety of severe environments and real-time measurement. [ABSTRACT FROM AUTHOR]

This paper presents a low cost and highly reliable system for angle measurement based on a sensor fusion between inertial and fiber optic sensors. The system consists of the sensor fusion through Kalman filter of two inertial measurement units (IMUs) and an intensity variation-based polymer optical fiber (POF) curvature sensor. In addition, the IMU was applied as a reference for a compensation technique of POF curvature sensor hysteresis. The proposed system was applied on the knee angle measurement of a lower limb exoskeleton in flexion/extension cycles and in gait analysis. Results show the accuracy of the system, where the Root Mean Square Error (RMSE) between the POF-IMU sensor system and the encoder was below 4° in the worst case and about 1° in the best case. Then, the POF-IMU sensor system was evaluated as a wearable sensor for knee joint angle assessment without the exoskeleton, where its suitability for this purpose was demonstrated. The results obtained in this paper pave the way for future applications of sensor fusion between electronic and fiber optic sensors in movement analysis. [ABSTRACT FROM AUTHOR]

In this paper, we describe a novel method for spatially distributed temperature measurement with Chirped Fiber Bragg Grating (CFBG) fiber-optic sensors. The proposed method determines the thermal profile in the CFBG region from demodulation of the CFBG optical spectrum. The method is based on an iterative optimization that aims at minimizing the mismatch between the measured CFBG spectrum and a CFBG model based on coupled-mode theory (CMT), perturbed by a temperature gradient. In the demodulation part, we simulate different temperature distribution patterns with Monte-Carlo approach on simulated CFBG spectra. Afterwards, we obtain cost function that minimizes difference between measured and simulated spectra, and results in final temperature profile. Experiments and simulations have been carried out first with a linear gradient, demonstrating a correct operation (error 2.9 °C); then, a setup has been arranged to measure the temperature pattern on a 5-cm long section exposed to medical laser thermal ablation. Overall, the proposed method can operate as a real-time detection technique for thermal gradients over 1.5–5 cm regions, and turns as a key asset for the estimation of thermal gradients at the micro-scale in biomedical applications. [ABSTRACT FROM AUTHOR]

On the oil production also involves the production of water, gas and suspended solids, which are separated from the oil on three-phase separators. However, the control strategies of an oil separator are limited due to unavailability of suitable multi-interface level sensors. This paper presents a description of the multi-phase level problem on the oil industry and a review of the current technologies for multi-interface level assessment. Since optical fiber sensors present chemical stability, intrinsic safety, electromagnetic immunity, lightweight and multiplexing capabilities, it can be an alternative for multi-interface level measurement that can overcome some of the limitations of the current technologies. For this reason, Fiber Bragg Gratings (FBGs) based optical fiber sensor system for multi-interface level assessment is proposed, simulated and experimentally assessed. The results show that the proposed sensor system is capable of measuring interface level with a relative error of only 2.38%. Furthermore, the proposed sensor system is also capable of measuring the oil density with an error of 0.8 kg/m 3 . [ABSTRACT FROM AUTHOR]

In this letter, the sensitivity to strain, curvature, and temperature of a sensor based on in-line fiber Mach-Zahnder interferometer (IFMZI) is studied and experimentally demonstrated. The sensing structure is simply a section of single mode fiber sandwiched between two abrupt tapers to achieve a compact IFMZI. The phase of interferometer changes with the measurand interaction, which is the basis for considering this structure for sensing. The physical parameter sensitivity of IFMZI sensor has been evaluated using differential white light interferometry (DWLI) technique as a phase read-out system. The differential configuration of the IFMZI sensor is used to achieve a high phase resolving power of ±0.062° for read-out interferometer by means of omission of phase noise of environment perturbations. The sensitivity of the sensor to the strain, curvature, and temperature has been measured 0.0199 degree/μ ε, 757.00 degree/m −1 , and 3.25 degree/°C , respectively. [ABSTRACT FROM AUTHOR]

A high sensitivity measurement method for temperature has been proposed and investigated based on an isopropanol-filled photonic crystal fiber long period grating (PCF-LPG). Due to the high thermo-optic coefficient (TOC) of isopropanol, the sensitivity of the proposed temperature sensor could be effectively improved by filling isopropanol in the air waveguides of PCF. It can be found that the resonant dip will be split in two dips after filling isopropanol and the two dips have different sensitivities to surrounding temperature. Because of PCF-LPG is sensitive to the refractive index (RI) of internal filled liquid, the isopropanol-filled PCF-LPG temperature sensor has a high sensitivities of 1.356 nm/°C in the range of 20–50 °C. The simplicity and the excellent performance of our proposed device make it potential for the applications of high-precision temperature measurement is required. [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]

• Fiber-optic method for measuring the velocity of a liquid flow was presented. • An array of three FBGs was used as a sensitive element, which made it possible to study the flow direction. • Liquid flow velocity measurements were given for the range of 0.02–0.05 m/s. This work is devoted to the development and research of a new optical method for measuring the velocity of a fluid flow and determining the orthogonal directions of such a flow. Comparing with gas flow, measuring the fluid flow rate requires higher heating power and insulation, which is taken into account in this work. The standard hot-wire anemometry method is taken as a basis, but instead of a metal thread, optical fiber is used for measurements. The article presents the design of a sensitive element consisting of two optical fibers. The first fiber contains an array of fiber Bragg gratings and is used as a measurement element. The second fiber has a region with a tapered structure and is used to create a heated area. Velocity measurements are given for the range of 0.02–0.05 m/s, but are limited only by the capabilities of the measuring stand. A unique method for identifying the orthogonal flow direction, which has not been described in the literature before, is presented. Differences in the shapes of the curves for the dependences of flow rates moving in the direction and orthogonally to the axis of the sensitive element are presented. The stability of the device is also confirmed by continuous experiments for 70 min. [ABSTRACT FROM AUTHOR]

• An optical fiber in-line Mach-Zehnder interferometer (MZI) based on a pair inner microhole in the fiber core is presented. • The inner microhole is fabricated by firstly inscribing a short line structure by femtosecond, and then discharging the short line area by using a fusion splicer. • Being different from most of the fiber in-line MZIs, the light path via the fiber cladding experiences multiple reflections at the fiber surface before being collected by the second microhole. • The microhole separations can be flexibly controlled and hence MZI fringe pattern with different free spectral range (FSR) can be produced. An optical fiber in-line Mach-Zehnder interferometer based on a pair of inner microholes in the fiber core is proposed and demonstrated for refractive index and temperature sensing. The microhole is fabricated by firstly inscribing a short line structure by femtosecond, and then discharging the short line area by using a fusion splicer. The incident light beam on the first microhole is divided into two parts, one remains traveling in the fiber core and the other is directed into the cladding and then collected by the second microhole after multiple reflections at the air/cladding interface. The two parts recombines in the fiber core, thus forming a Mach-Zehnder interferometer. By controlling the separation of the two microholes, transmission spectrum with different free spectral range can be obtained. The sensitivities achieved for refractive index and temperature are ∼ 68.2 nm/RIU and 57.8 pm/°C, respectively. Such a device is robust in structure, easy in fabrication, and convenient in operation. [ABSTRACT FROM AUTHOR]

• The sensitivity of the bare FBG sensor in this study has improved a lot and is better than all the previously proposed bare FBGs, as shown in Table 2 of the manuscript. • Simple 3D printed structure with fast fabrication time is a great advantage compared to FBGs that have to be embedded inside another structure material. • The sensor can be widely used in industry with a pressure range from 0 to 0.45 MPa and can increase or decrease this pressure range based on the change in the size of the structure. A bare Fiber Bragg Grating (FBG) pressure sensor based on an oval-shaped 3D printed structure was proposed and experimentally demonstrated in this study. The whole structure was housed in a well-controlled pressure chamber, and the FBG signals were recorded by an Optical Spectrum Analyzer (OSA). Strains of the oval-shaped structure were simulated numerically using Ansys to optimize its performance. When under pressure, deformation of this 3D printed structure leads to response of FBG sensor, and the experimental results showed that Bragg wavelength shift is linearly proportional to the change in pressure. The proposed sensor shows a good sensitivity of 6.834 nm/MPa, corresponding to 4.43 × 10-3 MPa−1 in the pressure range from 0 to 0.45 MPa. The sensor's linear fit coefficient is also good, reaching a mean value of 98.57 % and the repeatability with negligible mean hysteresis error is ± 0.16 nm (equivalent to 2.3 %). The proposed pressure sensor has high potential to be used in related industrial applications and is highly customizable to meet the requirements of different pressure ranges. [ABSTRACT FROM AUTHOR]

• Optical fiber sensors for impact amplitude and localization. • Analysis of spectral features of FBG embedded in diaphragm. • Impact localization estimation in the x and y planes with errors below 5%. • Diaphragm-embedded FBG sensor for force assessment with errors below 0.47 N. This paper presents the development, numerical and experimental analyses of a fiber Bragg grating (FBG) based sensor for force amplitude and localization (in 2D plane) detection. The sensor system is based on a diaphragm-embedded FBG, where the spectral features, namely wavelength shift, optical power and full width at half maximum (FWHM) are analyzed as a function of the force application parameters (amplitude and position). The diaphragm results in a higher spatial range for the sensor as its larger dimensions (when compared with the optical fiber) enable the strain transmission to the optical fiber even when the force is not directly applied to the fiber. In order to verify the proposed approach, finite element method is applied on the strain analysis in the optical fiber when the diaphragm is subjected to forces at different positions. Then, the FBG reflected spectra are simulated considering the strain distribution obtained at each condition, where there are variations not only in the Bragg wavelength, but also in the FWHM and optical power as a function of the force amplitude and position. The experimental analysis confirms the results numerically obtained and shows the possibility of simultaneously measuring force amplitude and position (x and y components) using a single FBG. Results show the feasibility of the method, where root mean squared errors of 0.47 N (in 10 N range), 0.48 mm (in 10 mm range) and 0.33 mm (in 6 mm range) are found for the force, horizontal (x-) and vertical (y-) components estimations, respectively. Thus, the proposed approach shows relative errors below 5% and is an interesting approach for sub-millimeter resolution force and impact detection, which can also be used in multiplexed systems with the advantage of higher spectral efficiency, since a single FBG can provide multiple measurements that are commonly made employing a higher number of sensors. [ABSTRACT FROM AUTHOR]

Abstract: Recently developed micro- and nano-structured optical fiber sensors, with particular reference to surface plasmon resonance (SPR) fiber sensors and photonic crystal fiber (PCF) sensors are reviewed. SPR fiber sensors can have diverse structures such as D-shape, cladding-off, fiber tip or tapered fiber structures. Some of the recently developed novel structures include the use of various types of fiber gratings in SPR fiber sensors. PCF sensors cover diverse recent developments on photonic-bandgap fiber, holey fiber, hole-assisted fiber and Bragg fiber sensors. Major applications of these include gas sensors and bio-sensors. These micro- and nano-structured fiber sensors have attracted considerable research and development interest, because of their distinct advantages, which include high sensitivity, small sensor head footprint and the flexibility of the optical fibers. They are also of academic interest, and many novel ideas are continuously developed. [Copyright &y& Elsevier]

• Lossy mode resonance-based sensor coated with TiO 2 /PSS&Au Nanoparticles is proposed. • The shape, structure and size of Au-NPs is analyzed and optimized using FEM. • The spectra with different layer amounts of coatings are recorded and discussed. • The novel sensor is with 2.5 times higher sensitivity than that of only TiO 2 -coated. A novel lossy mode resonance (LMR) based sensor modified with TiO 2 /PSS&Au-nanoparticles (Au-NPs) bilayers is proposed for refractive index (RI) sensing. Three significant parameters of Au-NPs, including the shape, the interval between each other and the distance from the surface of LMR, are optimized using finite element method. Meanwhile, the resonance spectra of sensors with different amounts of bilayers are recorded and discussed. And then the sensitivity is studied and compared. Results indicate that Au-NPs significantly enhance the electric field strength (EFS) around sensors. In order to maximize EFS, the shape of Au-NPs is set as pyramid, the interval between each other is set as 0.1° and the distance from the surface of LMR is set as 5 nm. Meanwhile, as the number of bilayers increase, the valley in resonance spectra becomes more obvious. Because of the modification of bilayers, the sensitivity of the sensor we proposed reaches 6,405 nm/RIU when surrounding RI is from 1.3667 to 1.4140, which is 1.4 times higher than that of the sensor which only modified with TiO 2 /PSS. [ABSTRACT FROM AUTHOR]

Abstract: Fiber-optic extrinsic Fabry–Perot (EFPI) sensors have been successfully used for a wide range of applications. The recent progress in EFPI sensors is reviewed in this paper. First, the optical amplification technology is adopted into the EFPI sensor system to enhance the interferometric signal considerably. Second, both spatial-frequency multiplexing and coarse-wavelength-division multiplexing technologies are demonstrated for multiplexing of a modified EFPI sensor called the Fizeau sensor. Finally, the EFPI sensor is integrated with other fiber-optic sensing elements to realize the measurement of multiple parameters, for example, the EFPI is integrated with the in-fiber Bragg-grating (FBG) or the long-period-fiber-grating (LPFG) to achieve simultaneous strain and temperature measurement. Furthermore, simultaneous measurement of displacement, transverse load, static strain, temperature, and vibration can be achieved by using the combination of EFPI/FBG/LPFG. [Copyright &y& Elsevier]

• The signals employ pulse trains arranged at power intervals. • Obtained Brillouin spectra forms a comb-like shape. • The signals are generated and time gated by using a single intensity modulator. • The net gain can be enhanced to conventional signal formula. • Increasing the measurement distance does not increase the uncorrelation gain. We present a signal formula for correlation domain Brillouin analysis that uses short pulse trains arranged at different power intervals. The proposed signal can be realized by using only an intensity modulator, and the signal period (the measurement range) and the signal length can be independently realized without using an additional modulator for time gating. In the numerical analysis, we show that the proposed scheme obtains a gain advantage over the conventional scheme for fibers under test of approximately a few hundred meters when the available pump average power is limited. In the experiment, it was demonstrated that Brillouin frequency shifts can be discriminated with a spatial resolution of 5 cm over a 95-m-long fiber. [ABSTRACT FROM AUTHOR]

• The process of modal analysis based on AT-BOTDA and SSI-DATA is built up. • The strain-based state space model is derived. • Advantage of distributed vibration sensing on eliminating computational modes is discussed. Dynamic properties of structures, in terms of frequencies and mode shapes, represent inherent characteristics and health states of structural systems. A method for extraction of modal parameters of structures based on distributed measurements of dynamic strains is described in this article. AT-BOTDA (Amplitude Transfer-Brillouin Optical Time Domain Analysis) optical fiber sensor based on Brillouin scattering is employed for distributed measurements of dynamic strains in a laboratory beam. SSI-DATA (Data-driven Stochastic Subspace Identification) is selected in the formulation of the method, based on its capability for efficient analysis of large amounts of data. The analysis approach involves pre-processing of the distributed data based on basic principles of dynamics, determination of the sensing range, building the strain state space model, extracting modes and improving the results with selection of the sensing points. Laboratory experiments on a beam indicate that distributed vibration strains collected by AT-BOTDA are sufficient for convergence, resulting in an accurate determination of the first two mode shapes. [ABSTRACT FROM AUTHOR]

• A detailed analysis of the acoustic field dynamics resulting from the optical excitation in SBS is presented. • The resonant behavior of the phonon response guaranties the validity of the SVA in the subphonon lifetime regime. • We describe the detailed impact of the SVA for the phonon field in the modeling of SBS. • We set the precise limits of validity of the SVA. In this work, we address the phonon dynamics in the stimulated Brillouin scattering process. There exists a wide consensus on the fact that the slowly varying approximation for the acoustic field cannot be invoked in the transient subphonon regime. We present an analysis where we set the precise limits of validity of this approximation. Our study shows that the resonant behavior of the Stokes interaction permits, in general, a first order treatment of this process in the theoretical analysis of current practical applications. An improved-accuracy first order model is also put forward. Numerical calculations are used to support our conclusions. [ABSTRACT FROM AUTHOR]

• We present optical fiber ball resonators interrogated via optical backscatter reflectometry. • Ball resonators have polarization-sensitive spectral signature. • We present the result of 4 samples, having sensitivity ranging from 366.6 dB/RIU to 6691.6 dB/RIU. • Without any additional component, the sensitivity increase is 18.6%–47.7% over standard interrogation. Optical fiber-tip ball resonators are inexpensive devices characterized by a rapid, repeatable, and reliable fabrication process through a fiber splicer. The spectral features of a ball resonator appear as low-finesse interferometer, with an optical backscatter reflectometer (OBR) interrogator. The spectrum changes in amplitude when the external refractive index (RI) varies. In this work, we present a polarization-sensitive method for the detection of spectral changes in ball resonators. We achieved a sensitivity ranging from 366.6 dB/RIU to 6691.6 dB/RIU (refractive index units) on the most sensitive polarization, improving the sensitivity of a standard detection of 18.6%–47.7% without the need of any additional component. [ABSTRACT FROM AUTHOR]

We present an optical fiber gas pressure sensor based on two cascaded Fabry-Perot interferometer cavities, one is a section of capillary tube spliced with single mode fiber at its two ends and the other consists of a pair of in-fiber reflection mirrors inscribed by use of femtosecond laser micromachining. The cavity length difference of the two Fabry-Perot interferometers can be precisely controlled to achieve an extremely large sensitivity when vernier effect is utilized. The pressure sensitivity obtained is ~303.65 pm/kPa. Moreover, the temperature cross-sensitivity of the device achieved is as low as 0.124 kPa/°C. The device is flexible in design, easy in fabrication, convenient in operation and has high potential in medical, environmental monitoring and other industrial applications. [ABSTRACT FROM AUTHOR]

• Fiber Bragg grating (FBG) sensors are integrated into Kevlar fabric armor specimens. • FBG sensors measure transient fabric deformation during ballistic impacts. • FBG sensors provide in-situ strain when backing material is non-transparent. • Provides time-history of physical energy dissipation mechanisms in the fabric. In previous experiments, the authors demonstrated that strain values collected from fiber Bragg gratings (FBG) integrated into a single layer of Kevlar fabric, placed between a soft armor test specimen and backing material, could be related to the time dependent back-face deformation (BFD) of the armor sample. In this paper, we investigate the specific fabric deformation and failure mechanisms that cause observed events in the FBG measured strain behavior and the FBG spectral profile throughout the impact event. For these experiments, the standard clay backing material was replaced with a 20% clear ballistic gel to provide visual access to the back-face. The test specimen was impacted by an 8.24 g steel ball bearing travelling at 248.8 m/s, during which strain was calculated from the measured full spectrum response of the FBG using a high-speed optical interrogation system. The strain response was compared to the BFD of the Kevlar sample. The BFD was measured through the clear ballistic gel using two high speed cameras recording at 100,000 fps. The results from these tests can be used for future testing using a non-transparent backing material to obtain a detailed strain–time history, back-face deformation history and an understanding of the time sequence of physical energy dissipation mechanisms in the fabric. [ABSTRACT FROM AUTHOR]

• The recent advances in slope-assisted Brillouin optical correlation-domain reflectometry (SA-BOCDR) are reviewed. • Distributed strain and temperature along a sensing fiber can be measured on a real-time basis. • Other unique features are demonstrated, and the pseudo-field test results are presented. • Some technical issues to be resolved are discussed. Brillouin optical correlation-domain reflectometry (BOCDR) is one of the fiber-optic Brillouin-based distributed sensing techniques for strain and temperature with single-end accessibility. Since the first proposal of the basic concept in 2008, the performance of BOCDR has been drastically improved from a variety of aspects including high-speed operation. Here, we focus on one of the high-speed BOCDR configurations referred to as slope-assisted (SA-) BOCDR. In this configuration, instead of observing the whole Brillouin gain spectrum, only the spectral power at a certain frequency on the spectrum is detected, leading to frequency-sweep-free high-speed operation and some unique features. In this review paper, we describe the recent advances of SA-BOCDR technology from its principle to characterization and applications. We also discuss the current issues of SA-BOCDR and present a concluding remark. © 2020 Elsevier Science. All rights reserved. [ABSTRACT FROM AUTHOR]

• FBGs immersed in magnetorheological fluids for magnetic field estimations. • FBG arrays for simultaneous assessment of magnetic field distribution and intensity. • Machine learning algorithm for the classification of the magnetic field position. • Classification accuracy can be as high as 100%. This paper presents the simultaneous assessment of magnetic field intensity and position using a fiber Bragg grating (FBG) array immersed in magnetorheological (MR) fluid. The applied magnetic field leads to a variation of the MR fluid yield stress, which results in an axial strain on the FBG. As a well-known behavior of FBGs, the axial strain leads to a Bragg wavelength shift on the FBGs, which, in this case, is proportional to the magnetic field intensity and position. An array with 4 FBGs was used and characterized with respect to both magnetic field position and intensity. Then, a k-nearest neighbors' algorithm was proposed to classify the magnetic field position through the wavelength shift of the FBGs, where the magnetic field intensity is estimated from the FBG closest to the magnetic field position previously detected. Results show the feasibility of the proposed approach, where the algorithm accuracy is 100% for the best case and 86% for the worst case of magnetic field position, whereas a relative error lower than 5% was obtained on the magnetic field intensity estimation. [ABSTRACT FROM AUTHOR]

• A novel interferometric optical fiber sensing structure is proposed. • The real-time detection and location of disturbances have been realized. • The new positioning system has a wide frequency range and could achieve disturbance location above 400 Hz. • There is a large dynamic monitoring range and the maximum sensing distance can be reached as far as 50 km. • There is no detection blind zone and the repeatability of positioning error for human disturbances is stably below ±30 m. A novel interferometric fiber sensing system is presented and the real-time detection and location for disturbances have been realized. Dual Michelson interferometers are constructed to detect the interference signals with different wavelengths modulated by the same disturbance. After demodulation of the signals from both interferometers, a unique processing scheme is utilized to construct the signals with a fixed time delay. By cross-correlation to extract the time delay, the position of the time-varying disturbance can be determined. The vibration simulation experiments driven by PZT prove that this system could achieve disturbance positioning above 400 Hz. Moreover, there is no detection blind area within 50 km and the maximum positioning error for actual disturbance is stably below ±30 m. [ABSTRACT FROM AUTHOR]