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This paper presents a detailed analysis of a plasmonic sensor in which a plasmonic wire-grid structure is integrated into a hollow core fiber (HCF). The sensor has the capability to measure refractive indices (RIs) greater than that of the fiber material. The use of the HCF enhances the sensitivity and precision of the sensor as it allows for the efficient interaction of light with the sensing region of the wire-grid-based plasmonic structure, in comparison to a single plasmonic wire placed inside an HCF. We numerically investigate the optical properties of the sensor using the finite element method (FEM).The sensor exhibits high sensitivity, with an amplitude sensitivity of 673.97 RIU−1, a wavelength sensitivity of 15000 nm/RIU, and a resolution of 6.67 × 10−06 RIU, enabling precise detection of even small alterations in the RI of analyte. The sensor shows RI sensing range of 1.45 to 1.66, which is well-suited for a broad spectrum of uses, including the monitoring of biological and environmental samples. [ABSTRACT FROM AUTHOR]

Stress-strain response of optical fibers in direct tension is introduced in this article. The research involved direct tension tests of optical fibers and development of theoretical relationships describing the tensile behavior of the fibers. Two types of optical fibers, namely ribbon and standard single mode fiber (SMF-28) were included in the experimental and theoretical investigations. The impetus for the study was the need for the elastic properties of the optical fibers, such as the modulus of elasticity and the elastic limit for accurate interpretation of strains measured by optical fiber sensors. Ribbon fibers have proven to be robust for sensing applications in civil structures which prompted the research described herein. By employing the experimental data and by using the generalized Ramberg-Osgood law, it was possible to establish the theoretical stress-strain responses of the optical fibers. The experimental program allowed for distributed measurement of optical fiber strain by a Brillouin Optical Time Domain Analysis system (BOTDA). These results were compared with the direct measurement of the applied displacements at the fiber ends. [ABSTRACT FROM AUTHOR]

A temperature and strain dual-parameter sensor based on conical four-core fiber (FCF) combined with multimode fiber (MMF) is presented. A sensor based on Mach–Zehnder interferometry is prepared by using single-mode fiber (SMF)–MMF–FCF–SMF and tapering at the fusion point between FCF and SMF. When the temperature and strain change, the length and refractive index of the fiber change due to thermal and elastic effects. Based on the corresponding function relationship and coefficient matrix method, the temperature and strain dual-parameter matrix equation of the sensor can be obtained. The results show that the red shift of the transmission spectrum occurs with the increase of temperature in the range of 40 °C–90 °C, and the maximum temperature sensitivity is 85.45 pm/°C. At the strain range of 0– $800~\mu \varepsilon $ , the blue shift occurs in the transmission spectrum with the increase of the strain, and the maximum strain sensitivity is −6.76 pm/ $\mu \varepsilon $. Finally, the matrix equation of the dual-parameter sensor is obtained based on the experimental results. The sensor is simple to make and has high sensitivity. It can measure the temperature and strain parameters in the environment of the micro-magnetic field and micro-electric field. [ABSTRACT FROM AUTHOR]

A novel super-structure long-period fiber grating (SLPFG) is proposed for bending measurement with high sensitivity. The experimental and simulated results show that the number of multimode fibers (MMF) in one period influences the excited coupling mode and its coupling efficiency. Meanwhile, the bend sensitivity is increased by optimizing the number of MMFs and reaches $17.237\,\,\text {dB}/\text {m}^{-{1}}$. More importantly, the temperature cross-sensitivity is one order of magnitude lower than most fiber bending sensors. [ABSTRACT FROM AUTHOR]

This article highlights an intensity variation-based force sensor for polymer optical fiber (POF) that relies on the twisting structure. The sensing phenomena of the sensor rely on cladding mode frustrated total internal reflection (CMFTIR). A portion of two POFs is twisted in the middle where the light source is connected with illuminating fiber, and the power meter is connected with the forward end of the second fiber. Thus, the force measurement is taken from the second fiber by observing the variation of coupled intensity in which the applied force is up to 0–20 N. The sensor is analyzed on straight fiber, twisted fiber with bend, and without bend (considering the sensor’s sensitivity). The experimental results show that the sensor has higher sensitivity in the twisted with-bend structure, and the bending radius also affects the sensor sensitivity. Moreover, a multipoint and multiaxis force sensor is also presented. The POF is twisted up to 100 cm for multipoint sensing, and force is applied at five different points, whereas the cascading scheme is adopted for multiaxis to achieve the individual response of each axis. This study provides an alternative way to measure force based on intensity changes and has an additional advantage of being cheaper than fiber Bragg gratings and other systems. [ABSTRACT FROM AUTHOR]

Optical fiber structures for bending sensing have attracted extensive attention in recent years with their unique advantages. In this article, innovative C-shaped-core long-period fiber gratings (C-LPFGs) with two resonant peaks are proposed and experimentally tested. This special fiber microstructure is achieved by methods of CO2-laser polishing and oxyhydrogen-flame heating. During the process of heating, the core at polished area micro-bends to the surface of the structure due to the surface tension effect, which forms a “C”-shaped core. Each microbending Section of core results in partial energy leaking into the cladding to excite the cladding modes. As the number of periods increases, stronger effective refractive index modulation forms multiple resonant peaks. According to the experimental results, sensors with 17 periods represent excellent contrast and repeatability of two resonant peaks. The bending sensitivity of the proposed structure achieves −24.01/−21.3 nm/m−1 at 0° and −4.88/−10.17 nm/m−1 at 90° of Dip $_{1/2}$. Furthermore, the temperature sensitivity of both dips is 64/33 pm/°C, and the total length of the sensor is only 0.85 cm. With considerable bending sensitivity, the compact sensor possesses latent potential in fields of dual-parameter measurement in the future. [ABSTRACT FROM AUTHOR]

A kind of magnetic field sensor is proposed, which is based on symmetrically side-polished photonic crystal fiber (PCF) with regular hexagonal air holes. Magnetic fluid (MF) is used as the magnetic field-sensitive material. The polished surfaces are coated with an indium tin oxide (ITO) film to generate lossy mode resonance (LMR) for magnetic field sensing. The obtained magnetic field sensitivity is 466.81 pm/Oe, which can be further adjusted by changing the thickness of the ITO film. The comprehensive sensing performance is evaluated by defining the figure of merit (FOM). The proposed structure is simple and has the capability of mass production, which has great development space in the future. [ABSTRACT FROM AUTHOR]

Traditional optical voltage transformers (OVTs) based on electro-optical and inverse piezoelectric effects are gradually exposing their accuracy and reliability issues. In contrast, fibers for measuring electricity have unique properties and significant advantages in the high-voltage power industry, especially fiber optic current sensor (FOCS), which has been widely used in the field of high-voltage measurement. As a result, a novel fiber optical voltage sensor (FOVS) is proposed, which uses FOCS to measure the current flowing through the capacitive voltage divider (CVD) and then calculates the primary voltage proportional to it. In order to solve the inherent temperature- and wavelength -dependent errors and achieve high accuracy, a novel closed-loop signal processing method, which aims to minimize the influence of temperature-sensitive components such as CVD, quarter waveplate, and sensing fiber on the measurement accuracy of the system, is presented. A 110-kV FOVS prototype with a rated voltage of 110/ $({3})^{1/2}$ kV and a rated frequency of 50 Hz is developed and tested. The results show that after temperature and wavelength correction, the accuracy reaches the metering class of 0.2% and protection class of 3P specified in GB/T20840.7-2007 [ABSTRACT FROM AUTHOR]

The realization of multiple demodulation sche- mes of fiber Bragg grating (FBG) sensor is important for its engineering application. Here, we proposed a new edge filter that is formed by cascading dispersion compensation fiber (DCF) and no-core fiber (NCF) structure between two single-mode fibers (SMFs) for demodulation of quasi-distributed FBG sensor. The edge filter can be suitable for FBG with different center wavelengths by changing its structure parameters to achieve a wide range of FBG demodulation. In order to verify the performance of the edge filter, a demodulation system for quasi-distributed FBG sensor is successfully designed by introducing an optical time-domain reflectometer (OTDR), the wavelength shift of FBG caused by the change of temperature and strain is converted into power change through the edge filter, so as to realize temperature and strain demodulation. In the experiment, two FBG sensors separated by 500 m were set at the end of a 3000-m optical fiber for temperature and strain monitoring. The results show that the temperature sensitivity is 0.23 dB/°C from 50 °C to 100 °C, and the strain sensitivity is 0.01658 dB/ $\mu \epsilon $ from 0 to $669.6\,\mu \epsilon $. The edge filter for FBG demodulation has the advantages of high sensitivity, adjustable structural parameters, low cost, and independent multipoint/multiparameter demodulation. [ABSTRACT FROM AUTHOR]

In this study, gold nanorods (GNRs) are immobilized on the surface of fiber probe using a self-assembly method for a localized surface plasmon resonance (LSPR)-based optical fiber biosensor. In order to promote the performances of GNRs films, graphene oxide (GO) is used to encapsulate GNRs, which forms the GNRs@GO structure. Furthermore, the GNRs and GNRs@GO fiber probes are used to detect different refractive indices of surrounding environments from 1.33 to 1.38. The results show that the GNRs fiber probes have the sensitivity of 48 nm/RIU with the linearity of ${R}^{{2}}$ = 0.988. By comparison, for GNRs@GO with 0.2 mg/mL of GO fiber probe, the sensitivity is increased to 218 nm/RIU, and the linearity is improved to ${R}^{{2}}$ = 0.991. After that, anti-mouse IgG is immobilized on the GNRs and GNRs@GO fiber probes for immunoassay. The results show that the limit of detection (LOD) of GNRs fiber probe is 147.9 ng/mL, and the sensitivity is 26.5 nm/ $\mu \text{g}$ mL $^{-{1}}$ with the linearity of ${R}^{{2}}$ = 0.936 in the detecting range of 0.1– $0.5 \mu \text{g}$ /mL. By comparison, for GNRs@GO with 0.2 mg/mL of GO fiber probe, the LOD is improved to 32.7 ng/mL, and the sensitivity as well as the linearity are increased to 61.1 nm/ $\mu \text{g}$ mL $^{-{1}}$ and $R^{{2}}$ = 0.991, respectively. In addition, the optimal concentration of GO encapsulated with GNRs is 0.2 mg/mL, and the optimal response time is 20 min. The results also show that the GNRs@GO fiber probe has a good selectivity. Finally, the GNRs@GO structure shows the potential for applying in an optical fiber biosensor. [ABSTRACT FROM AUTHOR]

An in-fiber Fabry–Perot interferometer (FPI) is proposed and designed for performance improvement of a fiber Bragg grating (FBG) accelerometer. It consists of two FBGs which are inscribed using the femtosecond laser point-by-point technology. The spectral characteristics of the FBG Fabry–Perot (FBG-FP) were studied. Edge filter interrogation configuration was employed in the vibration measurement system. In contrast to the conventional FBG accelerometer used in wavelength interrogation or edge filter interrogation configurations, the scheme has a higher sensitivity and lower detection threshold which are achieved improving the optical properties of the accelerometer. We fabricated a high-frequency accelerometer and experiments were carried out to verify its performance. The sensitivity and detection threshold were 27.3 mV/g and 1.5 mg, respectively. This study presents a feasibility analysis of the short cavity FBG-FP, as opposed to a single FBG, for the vibration accelerometer. [ABSTRACT FROM AUTHOR]

A novel scheme for the measurement of an ultralow flow rate based on a fiber Fabry–Perot interferometer (FPI) and sensitivity-enhanced Vernier effect is proposed. The impact of water at different flow rates varies the cavity length of the sensor, resulting in a change of the free spectrum range (FSR) of the interferometric spectrum. The theoretical relation between the FSR and flow rate has been established by numerical simulation. The sensitivity can be further enhanced via the Vernier effect with a virtual reference cell and experimentally verified in the flow rate range of 0– $80\,\mu \text{m}$ /s. The proposed interferometric sensor with modified Vernier effect is used for ultralow flow rate measurements for the first time, which shows the potential for applications in microfluidic technology and biochemical reaction. [ABSTRACT FROM AUTHOR]

A high-sensitivity and high-resolution refractive index (RI) sensor with ultrawide measurement range based on no core fiber (NCF) with large offset splicing and microwave photonic filter (MPF) interrogation is demonstrated theoretically and experimentally for the first time. In the sensor, an inline Mach–Zehnder interferometer (MZI) based on NCF with large offset splicing is used as the sensing head, and a single-passband MPF is designed to interrogate the sensing signal. Compared with the traditional interrogation in optical domain, the MPF interrogation converts the periodic interference pattern in optical spectrum into the nonperiodic bandpass frequency response in radio frequency (RF) domain, so the measurement range is unlimited in principle. Owing to the large offset splicing in the inline MZI structure, the sensitivity of the sensor as high as −1.1054 GHz/RIU is obtained in the experiment. More importantly, MPF interrogation improves the resolution by an order of magnitude to $9\times10$ −7 RIU. Meanwhile, the sensitivity and the resolution of the sensor are easily adjusted by changing the length of dispersive media. The high sensitivity and high resolution of the sensor have potential applications in ultralow concentration biochemical detection field. [ABSTRACT FROM AUTHOR]

Distributed acoustic sensors (DASs) are able to monitor in real-time acoustic excitation and mechanical vibration of the external environment for many kilometers along the length of the optical fiber. In the oil industry, the technology allows the monitoring of well structural health, identifying flow profile, acquiring seismic data, updating reservoir information, and feeding geological models. The spatial resolution of DAS is a limiting factor on the quality of information gathered. In this article, we propose and analyze a novel method of measuring local dynamic strain of structures using DAS. The method uses serpentine configurations to measure the frequency spectrum of the in-phase strain signals at discrete regions of the structure, enabling DAS to determine, among other vibration parameters, the strain frequency response function (SFRF). An experimental study of the sensor application with different arrangements in a small structure has been performed. The structure in the experiment consists of a free–free aluminum beam of approximately 2.7 m. Experiment results show agreement with numerical results developed by the finite element method (FEM) and using reference sensors, demonstrating a significant improvement of the DAS SFRF accuracy using the serpentine configuration and up to 12.9 dB increase in peak-to-noise ratio (PNR) for low excitation forces. The recovered response enabled the reconstruction of mode shapes. [ABSTRACT FROM AUTHOR]

A fiber-optic hydrophone based on extrinsic Fabry–Perot (F-P) interferometer is proposed. The sensitive element of the hydrophone is an Archimedes spiral metal diaphragm, and the inner surface of the diaphragm and the end face of the optical fiber form the F-P cavity. Due to the grooved surface of the sensitive diaphragm, the hydrophone is minimally affected by hydrostatic pressure. The geometry structure of sensor and finite element method (FEM) modeling is presented. Experiments of underwater acoustic response and sensitivity calibration of the hydrophone are carried out and the experimental phase results are demodulated by the orthogonal phase demodulation method based on birefringent crystals and polarization technology. The experiment results show that the hydrophone has a relatively flat signal-to-noise ratio (SNR) and sensitivity within 1–20 kHz, and the average sensitivity is −172.52 dB (re. 1 rad/ $\mu $ Pa) @ 1–20 kHz. We believe that this type of hydrophone has the potential to be applied in the field of underwater acoustic communication. [ABSTRACT FROM AUTHOR]

A small-sized sensor combined with a diaphragm structure and a lever structure is proposed for application in high-temperature and high-pressure environments. The feasibility of the structure is verified from both theory and simulation, and the relationship between the change of the Bragg wavelength and the pressure is established. Differential measurements of temperature and pressure are achieved using two fiber Bragg gratings (FBGs). The experimental data show that the pressure sensitivity of the sensor is 28.86 pm/MPa in the range of 0–30 MPa, and the correlation coefficient is 0.9969. The pressure sensitivity of the improved structure is 29.76 pm/MPa and the correlation coefficient is 0.9999. The sensor’s temperature sensitivity is 32.66 pm/°C in the range of 50 °C–200 °C. The structure can be applied to the measurement of temperature and pressure in oil and gas wells. [ABSTRACT FROM AUTHOR]

Bolt joints are widely used in large-span truss and bridge structures. The integrity of bolt joint is paramount to the safety of the structure. For distributed multiple bolt joints in large-span structures, a multiplexed angle sensor based on fiber Bragg grating (FBG) for bolt loosening monitoring is proposed. Two FBGs characterized by different wavelength values are symmetrically arranged inside the specially designed sensor structure for the measurement of the bolt loosening angle in terms of their wavelength shift difference. The detail structure design of the FBG angle sensor with temperature self-compensation, automatic reset, and simple reuse is presented. The theoretical and numerical analyses are adopted to derive the measurement principle and optimize the structure of the sensor. Experimental results show that the measuring sensitivity of the FBG angle sensor prototype is about 109.6 pm/° with excellent linearity in the measuring range from 0° to 20°, and the maximum resolution of the bolt loosening angle obtained is 0.38°. The proposed sensor has stable static characteristics and can be expediently reused and developed into a quasi-distributed sensing network, which offers a new approach to real-time quantitative bolt loosening monitoring for distributed bolt joints. [ABSTRACT FROM AUTHOR]

Accurate and timely assessment of malignant tumors in the early stage and therapy process is critical for making clear guidance and prognosis for the treatment, which can significantly elevate the cure rate of cancer patients. Fluorescent sensing and imaging of tumors have attracted extensive attention because of their advantages of high sensitivity and spatial resolution. A “plug-and-play” fluorescent fiber-optic sensor was reported for the diagnosis of tumor in our previous work. However, the route to obtain the optimization parameters and the spatial resolving capability of the tumor sensor were still elusive. In this work, we have systematically investigated the optimized design of the fiber from the aspects of structural morphology of the fiber tip, incident light intensity, and the concentration of the fluorescent probes that renders a limit-of-detection of 3 ng/mL and a response time less than 20 s. Furthermore, the sensor can reveal the hypoxic gradient of the solid tumor by virtue of its spatial sampling ability, enabling disease classification and margin identification potential. This proof-of-concept offers great opportunities for application in oncology, such as preoperative diagnosis, clinical operation guidance, and postoperative re-examination of tumors. [ABSTRACT FROM AUTHOR]

We propose and experimentally demonstrate a distributed temperature and curvature sensing method based on spontaneous Raman backscattered lights in a few-mode optical fiber. A loop structure is used to demodulate the distributed temperature information using the Raman anti-Stokes (AS) scattering lights. And the fiber curvature is estimated by the loss of Raman Stokes (S) light. This system can measure temperature and curvature at the same position of the fiber simultaneously. The experimental results show that the temperature uncertainty and spatial resolution can reach up to 0.24 °C and 1.8 m on 6 km sensing fiber. Besides, the maximum measurement error is 0.11 cm within the curvature measurement range of 0.5–3 cm. [ABSTRACT FROM AUTHOR]

Multiplexing technology is important when realizing multipoint measurement. We propose a vector accelerometer based on a silicone rubber compliant cylinder, which makes it easy to realize multistage cascaded multiplexing. The proposed accelerometer has the natural frequencies of 19–39 Hz, which increase with an increased Shore A material hardness and a shortened length of the intermediate part of the compliant cylinder. In addition, at a random orientation, the maximum sensitivity reaches 340.6 pm/g when subjected to a 10-Hz vibration excitation. Moreover, the accelerometer can recognize the vibration orientation from 0° to 360°. With high integration, small size, and multistage cascaded multiplexing, the accelerometer is, especially, suitable for narrow spaces multipoint measurement. [ABSTRACT FROM AUTHOR]

In unmanned monitoring such as perimeter security, pipeline detection, railway security, and so on, it is still a challenging task to accurately locate the irregular vibration events. Compared with traditional vibration detection technologies, distributed optical fiber vibration sensors (DOFVSs) have superior advantages such as high spatial resolution, large monitoring range, good concealment, excellent flexibility, and simple sensing structure. The DOFVS can be mainly classified into two types, i.e., the interferometer-based DOFVS (IDOFVS) and the backscattering-based DOFVS (BDOFVS). In particular, IDOFVS has unique sensing advantages, such as high stability, simple structure, high sensitivity, and wide frequency response. In this article, the development of long-distance IDOFVS in recent years has been summarized. The sensing principle of IDOFVS has been theoretically explained. The specific classification of the IDOFVS and the corresponding sensing structures are described in detail. In addition, the long-distance distributed vibration sensing structure and processing algorithms, such as denoising and locating algorithms, are analyzed. Finally, the performance of recent long-distance IDOFVS has been compared and the development direction has been discussed. [ABSTRACT FROM AUTHOR]

A fiber surface plasmon resonance (SPR) curvature sensor has the advantage of being insensitive to axial strain and temperature, but there are still difficulties in multichannel cascade and curvature direction identification. In this article, a multichannel directional recognition SPR curvature sensor based on D-type double-clad multimode fiber was proposed. The asymmetric structure of D-type multimode fiber can well realize directional curvature sensing. When the light in the multimode core leaks to the silica clad, due to the existence of plastic clad, it can still continue to transmit. When the refractive index of UV curable adhesive coated in different sensing areas is different, wavelength-division multiplexing multichannel sensing can be realized. When the sensing probe is bent to be convex, the maximum intensity sensitivity is 0.0057 a.u./m−1, the resonance wavelength is redshift, and the wavelength sensitivity is 0.6114 nm/m−1. When the sensor probe is bent to be concave, the maximum intensity sensitivity is 0.0043 a.u./m−1, the wavelength is blueshift, and its highest sensitivity is 0.5831 nm/m−1. In addition, this article also realizes a three-channel directional recognition SPR curvature sensor based on a D-type double-clad multimode fiber, which provides a new scheme for multichannel directional curvature measurement. [ABSTRACT FROM AUTHOR]

Real-time monitoring of the dynamic vibrations of beam-type structures indicates important implications in aerospace engineering applications, in which the additional sensing information about vibration parameters, including displacements, rotational angles, angular rates, as well as accelerations, facilitates the improvement of the system’s safety. This article proposed a virtual sensing method with the usage of fiber Bragg grating (FBG) strain sensors to directly estimate the vibration parameters from the measured strains. The algebraic relationships of displacement–strain and rotation–strain on the sensor’s installing points as well as the boundary points are obtained based on the Birkhoff interpolation method, and meanwhile, the explicit expressions on angular rates and accelerations are derived. Furthermore, a tracking differentiator is used to recover the strain signals and their first- and second-order derivatives from the noisy measurements in order to real-time estimate the angular rates and accelerations of the vibrating beam. Differing from the classical modal-based method, complex modal decomposition is not required in the proposed method. The comparison of numerical simulation results reveals that the proposed method has advantages over the modal-based method in accuracy and efficiency. Furthermore, the laboratory experiments indicate that the proposed method is effective for real-time estimating vibration parameters from the noisy measured strains. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) is a common defect to determine the quality of high-voltage power equipment insulation systems. Localization of PD is very important for equipment maintenance and risk assessment. A localization algorithm of received signal strength (RSS) using demodulating PD optical signal is proposed. This is based on the optical effects caused by PD, which influence the optical signal that is transmitted through an optical fiber sensor placed inside the equipment. Compared with plastic fluorescent fiber, SiO2 cladding YAP:Ce crystal core fluorescent fiber (SYCF) with high luminous efficiency, low loss, and strong anti-electromagnetic interference capability is selected as the sensing unit to detect the received optical signal strength of PD, through the all-fiber optic sensing system to achieve the PD location in 3-D space. Based on the optical RSS localization method, the range of localization deviation of PD under 37-kV applied voltage is measured and studied. The average localization deviation and the error are 6.9 cm and 7.4%, respectively. Comparing the optical signal location method based on RSS with other methods of PD localization, the advantages and accuracy of the optical location method proposed in this article are outstanding. Therefore, the all-fiber system with SYCF via optical signal has a wide application prospect in the spatial localization of PD. [ABSTRACT FROM AUTHOR]

In fiber-optic vibration-sensing applications, intelligent sensing analysis is an important task for reducing costs and improving the overall quality of monitoring. Most existing methods show a low accuracy because their feature extractions significantly relied on the prior knowledge and the ability of the classifier designer. To deal with the challenges, an effective and scalable feature engineering method that uses vibration time-frequency imaging and deep learning has been proposed in this study. In order to improve the effectiveness and scalability, a Mel time-frequency-based vibration imaging approach is developed. This transforms the original time-domain data into a high-dimensional feature space and incorporates more feature information from data with various scales. Moreover, since the vibration-sensing recognition network is designed based on the deep-learning model, more discriminative features from the vibration image can be extracted automatically. The whole sensing recognition procedure includes four key steps: 1) original signal preprocessing of segmentation and denoising; 2) Mel time-frequency image generation; 3) representative feature extraction; and 4) vibration pattern-sensing classifier design. To verify the effectiveness and superiority of the proposed approach, eight datasets are collected, and three comparison schemes are investigated, using a distributed optical fiber interferometer-based vibration-sensing system. Our results demonstrate that the proposed intelligent sensing detection approach can provide higher accuracy and efficiency than other reported schemes. [ABSTRACT FROM AUTHOR]

In this article, we report a novel fiber-optic axial-strain sensor based on the bias-tapered micro-open-cavity (BT-MOC) structure. The comprehensive investigations of axial-strain response are theoretically and experimentally conducted with respect to the diameter and position of taper waist. The results show that the intensity variation is strongly related to the location of taper. When the diameter is ~ $30~\mu \text{m}$ , highly sensitive and linear intensity modulation (~0.043 dB/ $\mu \varepsilon)$ is gained by the bias position of ~ $210~\mu \text{m}$ , and for a small bias state, the wavelength response reaches 17 pm/ $\mu \varepsilon $ flatly and can be increased to 313.14 pm/ $\mu \varepsilon $ with the aid of Vernier effect. As a result, the detection resolution of ~ $0.2~\mu \varepsilon $ can be achieved in both intensity and wavelength modulated schemes. With the merits of compactness, repeatability, and low cost, our sensor is very promising and potential in the high-precision axial-strain-related engineering measurements. [ABSTRACT FROM AUTHOR]

We review recent advances in distributed fiber optic sensing (DFOS) and their applications. The scattering mechanisms in glass, which are exploited for reflectometry-based DFOS, are Rayleigh, Brillouin, and Raman scatterings. These are sensitive to either strain and/or temperature, allowing optical fiber cables to monitor their ambient environment in addition to their conventional role as a medium for telecommunications. Recently, DFOS leveraged technologies developed for telecommunications, such as coherent detection, digital signal processing, coding, and spatial/frequency diversity, to achieve improved performance in terms of measurand resolution, reach, spatial resolution, and bandwidth. We review the theory and architecture of commonly used DFOS methods. We provide recent experimental and field trial results where DFOS was used in wide-ranging applications, such as geohazard monitoring, seismic monitoring, traffic monitoring, and infrastructure health monitoring. Events of interest often have unique signatures either in the spatial, temporal, frequency, or wavenumber domains. Based on the temperature and strain raw data obtained from DFOS, downstream postprocessing allows the detection, classification, and localization of events. Combining DFOS with machine learning methods, it is possible to realize complete sensor systems that are compact, low cost, and can operate in harsh environments and difficult-to-access locations, facilitating increased public safety and smarter cities. [ABSTRACT FROM AUTHOR]

Optical spectroscopy supporting studies of the absorption and emission of light by matter is a widely used tool in analytical science. Recently, optical spectrometers have shown tremendous improvement on spectral resolution, accuracy, sensitivity, as well as miniaturization. However, most of them still only focus on single or few sensing modes, thus limiting their practical applications especially in complex sensing conditions. To extend applications, integrating with absorption, emission, and surface plasmon resonance (SPR) spectroscopy detecting systems, a multipurpose optical fiber smartphone spectrometer (MOSS) is designed to provide a point-of-care testing platform for on-site biochemical sensing. The MOSS is compact only with the size of $167\times 75\times85$ mm3 but has excellent sensing performance. It reaches extremely high spectral resolution as $\approx 10$ nm and has been successfully applied in nitrite, mercury ion, and sucrose sensing. In addition, the MOSS is equipped with inner power supply and user-friendly smartphone application, especially suiting for on-site applications. Since the MOSS can support high-quality multipurpose spectral detection but only using compact and cost-effective devices, it can be a potential point-of-care testing platform especially for biochemical sensing applications. [ABSTRACT FROM AUTHOR]

In this article, a dual-passband microwave photonic filter (MPF) is proposed and experimentally demonstrated for high-precision temperature sensing. A pair of parallel Mach–Zehnder interferometers (MZIs) sharing the same sensing arm is used to splice the broadband optical light source (BOS). Since the interference spectrum in the optical domain is a combination of two MZIs with different free spectrum ranges (FSRs), an MPF with two passbands is successfully generated in the microwave domain. The kernel of this scheme is precisely controlling the length of the sensing arm between the lengths of the upper arm and lower arm. When the temperature applied to the sensing arm changes, the FSR ${s}$ of two MZIs vary contrarily, resulting in the two passbands of the MPF shifting in the opposite direction. By tracking the central frequency difference between two passbands, the sensitivity can be enhanced. Experimental results show that with the increased temperature, the first passband moves to the lower frequency range, whereas the second passband shifts to the higher frequency range. The central frequency of an arbitrary passband is proportional to the temperature. The sensitivity is 16.81 ± 4.73 MHz/° by tracking the central frequency of a single passband, while, by recording the central frequency difference between two passbands, the sensitivity can be improved to 33.55 ± 4.17 MHz/°, which is twice over a single passband. [ABSTRACT FROM AUTHOR]

The ballistocardiogram (BCG) is a graphic representation of the movements of the body associated with cardiac activity. In this article, a 10-min BCG has been captured for ten different volunteers with a polymer optical fiber (POF) specklegram sensor. This transducer, which is composed of a charge-coupled device (CCD) camera, a laser emitting diode, and two meters of POF, allows capturing the BCG by analyzing how the induced speckle pattern changes over time. These changes are related to cardiac activity. Several processing methods have been compared to determine which method achieves the best performance: complex cepstrum, power of spectral density (PSD), Pam–Tompkins algorithm, wavelet, autocorrelation, Savitzky–Golay filter, mean absolute deviation, and Hilbert transform. Accuracy and resource consumption have been characterized and compared for these methods. Hilbert, PSD, and Savitzky–Golay exhibit both small errors and computational costs. This article describes a baseline for the main frequency determination of POF-based BCG signals in real-time. [ABSTRACT FROM AUTHOR]

Phase unwrapping is the prerequisite to obtain the true phase in interferometric fiber sensing. This article proposes a method based on an unscented Kalman filter (UKF) for interferometric fiber sensors. This method can break the limitation that the traditional phase unwrapping methods fail with a high probability when the differences in phase change are more than $\pi $ radians according to the Itoh criterion. Then, the measurement range of the phase signal can be expanded. This method combines the true phase and the phase gradient into the state vector estimation for UKF. The feasibility of this method is verified theoretically and experimentally. In the specific experiment, the phase signal with a maximum absolute phase difference of $1.5\pi $ (about 4.705) between adjacent sampling points can be recovered perfectly. The time and frequency domain results using this method show better performance. This method can effectively improve the detection range of the system and reduce the sampling rate to a certain extent, which is suitable for large-scale array systems. [ABSTRACT FROM AUTHOR]

Fiber-based surface plasmon resonance (SPR) sensors are the ideal configuration for rapid and label-free biomolecule detection. Gold-coated tapered optical fiber was used to excite the SPR mode that propagates at the metal–dielectric interface. By forming self-assembled monolayers (SAMs) of active chemical functional groups, the deoxyribonucleic acid (DNA) probe targeting the secY gene of Leptospira was immobilized onto the sensor surface. Red shifts in the SPR wavelengths were observed at different concentrations of leptospiral DNA target against the immobilized DNA probe due to the ability of plasmonic interaction to sensitively detect the binding of biomolecules on the sensor area. The developed sensor showed satisfactory linear responses at two target concentration ranges; low (1.0 fM to 1.0 pM) and high (0.1 nM to $1.0~\mu \text{M}$), with the highest sensitivity of 1.22 nm per log M. The negative control test also revealed high sensor specificity toward the complementary DNA strand. This sensor will represent a significant step toward laboratory-free diagnosis of leptospirosis. [ABSTRACT FROM AUTHOR]

This article describes a novel application of a piezoelectric sounder on an acoustic device as a respiratory detector. When human exhaled air at a temperature of 36.5 °C is blown onto the piezoelectric device in the sounder in a room of 26.4 °C, a peak voltage of 1–2 V directly appears without an amplifier. We herein present a theoretical model of how the device works, experimentally prove its validity, and show its potential to be used as a spirometry detector. The output voltage changes alongside the output of a medical spirometer for forced vital capacity test respiration. The device is noise robust under a variety of measurement conditions, including different masks, shields, snorkels, and sustained airway positive pressure devices. The S/N ratios for quiet sitting, walking, quick face swing, and mask movement were 38, 33.3, 32.3, and 30.9 dB, respectively. The piezoelectric sounder considered herein can return to the original function and generate an alarm if the measured reparation is abnormal. [ABSTRACT FROM AUTHOR]

A novel miniature magnetic-fluid (MF)-infiltrated eccentric fiber Bragg grating (EFBG) vectorial magnetic field sensor was proposed and experimentally demonstrated. An EFBG with a length of 10 mm and an eccentric distance of $2~\mu \text{m}$ was written in a standard single-mode fiber by a point-to-point femtosecond laser (FSL) writing process, which was then sealed in an MF-infiltrated glass capillary (GC) to form the magnetic-field-sensitive structure. Through the refractive index and absorption modulation of the MF under the influence of external magnetic fields, the transmission intensities at the resonant wavelengths of cladding modes are accordingly changed. Transmission at these resonant wavelengths can be used as the magnetic field sensing quantity. Experiments show that the sensor can sensitively respond to both strength and direction of the magnetic field. A sensitivity of about 0.042 dB/Oe in a magnetic field strength range of 0–120 Oe was found. [ABSTRACT FROM AUTHOR]

In this article, a compact fiber-optic current sensor (FOCS) based on an in-fiber Fabry–Pérot interferometer (FPI) and a magnetostrictive transducer is presented. The in-fiber FPI sensor employed in the proposed FOCS was fabricated with a 273- $\mu \text{m}$ Section of a capillary fiber spliced between two pieces of a standard single-mode fiber (SMF). The in-fiber FPI sensor and a commercial fiber Bragg grating (FBG), which was used as a reference sensor for sensitivity comparison, were bonded to a small magnetostrictive actuator (Terfenol-D) and assembled in the proposed FOCS for electric current monitoring up to $800 {A}_{\text {rms}}$ using a commercial FBG interrogator. The in-fiber FPI sensor presented a sensitivity 84.4% higher than that of the FBG sensor. An alternative interrogation system based on an edge-filter technique with temperature compensation was also developed and employed for the characterization of the proposed FOCS under the same range of electric current. Using the edge-filter interrogation system developed, the in-fiber FPI sensor presented a sensitivity 169.8% higher than that shown by the FBG sensor. The results show that the proposed FOCS using an in-fiber FPI sensor presented high sensitivity to electric current sensing and, therefore, is a potential candidate for current monitoring in complement to conventional current transformers (CTs) as well as to conventional optical CTs (OCTs). [ABSTRACT FROM AUTHOR]

A surface plasmon resonance (SPR) dual-parameter sensor is designed and fabricated. It was electroless plated with silver on the surface of the core of the plastic-clad optical fiber and a gold film was plated near the end face side to produce the double SPR resonance valleys. 4-mercaptophenylboronic acid (4-MPBA) and $\beta $ -cyclodextrin ($\beta $ -CD) were coated separately on the surface of the silver and gold films. 4-MPBA can selectively recognize the structures of cis-vicinal diols, and thus can capture glucose that contains such structures. $\beta $ -CD is a class of cyclic oligosaccharides whose unique hydrophilic outer surface and lipophilic central cavity help facilitate the formation of inclusion complexes with cholesterol through multiple interactions. The sensor can detect 0–4 mM glucose and 0–500 nM cholesterol, and it has a linear response in the concentration range of 0–0.1 mM glucose and 0–15 nM cholesterol. The sensitivity of glucose and cholesterol concentration of the sensor is 89.6 and 0.994 nm/nM, respectively. The detection limits for quantifying glucose and cholesterol are $1.12~\mu \text{M}$ and 0.1 nM, respectively. The experimental results also show that the sensor has good selectivity and stability. The proposed technique can be further applied in medicine, environment, and biology monitoring. [ABSTRACT FROM AUTHOR]

Biogenic amines (BAs) are compounds found in a vast range of food products. In recent years, there has been a crescent awareness toward food safety, followed by an increase in food regulations. Long-period fiber gratings (LPFGs) coated with titanium dioxide (TiO2) were used to monitor the optical properties of a layer of poly(ethylene-co-vinyl acetate) (PEVA) doped with maleic anhydride (MA), which was polymerized on top of TiO2. This hydrophobic polymeric structure is permeable to BA, which causes a steady increase in its effective refractive index (RI) causing a wavelength shift in the coated LPFG attenuation band. LPFG wavelength shift was observed and measured for the monoamine tyramine (TYR), to the diamines, putrescine (PUT), cadaverine (CAD), histamine (HIS), and tryptamine (TRYP), and to the polyamines, spermidine (SPED), and spermine (SPEM). It was determined that, while PEVA-coated devices present a residual sensitivity to BA, the MA greatly increases it. In fact, for PEVA only coated LPFGs, the sensitivities of 1.45 ± 0.11, 0.97 ± 0.05, 0.46 ± 0.08, and 0.94 ± 0.09 nmM−1 for PUT, CAD, HIS, and TYR, respectively, were measured. However, for PEVA-doped MA-coated LPFGs, the sensitivities are 3.34 ± 0.13, 3.06 ± 0.11, 2.62 ± 0.14, and 3.65 ± 0.23 nmM−1 for PUT, CAD, HIS, and TYR, respectively. Thus, the RI of PEVA increases with BAs in- diffusion, and MA doping further enhances the PEVA sensitivity to BA. The proposed sensor is expected to play a part in the further development of a biosensor for the quantification of BA in real foodstuff, providing a methodology for quality control. [ABSTRACT FROM AUTHOR]

Recognizing the patterns of partial discharge (PD) is the key to assess the hazard level of PD. This article proposes an oil-immersed transformer PD pattern recognition method based on a multifrequency fiber-optic Fabry–Perot (F–P) ultrasound sensing array and deep learning (DL). Three F–P ultrasonic probes with different resonance bands were used to form a sensing array for collecting ultrasonic signals excited by PD. Five types of PD models were prepared, including metal tip (MT) discharge in oil, PD in the air cavity (AC), and surface discharge (SD) on the pressboard, in addition to SD after pressboard blocking or after pressboard plus transformer winding blocking, to study the influence of ultrasonic attenuation on recognition accuracy. The collected PD ultrasound signal time-frequency matrix was obtained using adaptive optimal-kernel time–frequency representation to reinforce the differences in PD patterns. Then, based on the features of the PD ultrasonic signals, the signal time and frequency band for analysis were determined. The intercepted time–frequency matrix of the signals from the three F–P probes was formed into a (3, 300, 375) tensor. A modified ResNet-18 net was used for PD pattern recognition, which achieved a 98% recognition accuracy. The probabilities given by the softmax function were used to study the confidence of the model’s predictions of signals belonging to known and unknown types. [ABSTRACT FROM AUTHOR]

An optical fiber sensor based on the parallel Vernier effect for temperature and transverse load measurement is reported in this paper. The sensor is composed of a novel compact fiber Michelson interferometer (MI) and a closed-cavity Fabry–Perot interferometer (FPI) connected in parallel. The fiber MI consists of a bent single-mode fiber (SMF) structure and a short length of hollow core silicon tube (HCST). A section of HCST is fused between two segments of SMF to form the fiber FPI. Two sensors in parallel can easily measure different physical parameters independently, thus realizing multi-parameter sensing. The fiber MI and FPI are used for temperature and transverse load measurement, respectively. Fiber optic MI and FPI are Vernier effect mutual reference interferometers. When MI is used for temperature sensing, FPI is the reference interferometer. When FPI is used for transverse load sensing, MI is the reference interferometer. It can improve the sensitivity of temperature and transverse load. Two samples were made and the sensing experiment was carried out. Through the sensing experiment, we get the maximum temperature sensitivity of 85.1 pm/°C in the temperature range of 30–80 °C and a transverse load sensitivity of −3.15 nm/N in the temperature range of 0–1.96 N. In addition, the sensor has the advantages of tiny size, low cost, and easy manufacture, which is suitable for temperature and transverse load sensing applications. [ABSTRACT FROM AUTHOR]

This work reports a simple optical fiber Surface Plasmon Resonance (SPR) sensor with ultra-high sensitivity for simultaneous measurement of the dual-parameter. The sensor is based on a D-shaped fiber with a nanolayer coating of Silver (Ag) and Hematite (α−Fe2O3). The Ag/α−Fe2O3 layer is deposited on the longitudinal surface of residual cladding, and a Fiber Bragg Grating (FBG) is inscribed on the single-mode fiber (SMF) for temperature compensation. The α−Fe2O3 layer protects the Ag layer from oxidation and effectively enhances the surface plasmon wave while interacting with free electrons. Finite Element Method (FEM) modeling is employed to investigate the refractive index (RI) and temperature response of the SPR sensor. The SPR sensor exhibits an ultra-high RI sensitivity of 8,518 nm/RIU (refractive index unit) in the RI range of 1.33 to 1.40, and a temperature sensitivity of −52 pm/°C in the temperature range of 20 °C to 60 °C. The SPR performance suggests that it is a potential candidate for various applications of dual-parameter sensing. [ABSTRACT FROM AUTHOR]

A D-shaped fiber-optic surface plasmon resonance (SPR) sensor with MXene (Ti3C2Tx) is proposed as the sensing enhancement material. A spin coating method was used to cover the gold film D-shaped fiber SPR sensor with a layer of MXene. Experimentally, the sensor's sensitivity, full width at half maximum (FWHM), and figure of merit (FOM) are discussed. The effect of various MXene concentrations on D-shaped fiber SPR sensors was investigated. The results demonstrate that as the MXene concentration increases, the sensitivity, FWHM, and FOM value of the D-shaped fiber SPR sensor increase as well. The MXene-based D-shaped fiber SPR sensor exhibits the highest sensitivity, measuring 3143 nm/RIU, and the highest FOM value, 15.26. Compared to the Au-film D-shaped fiber SPR sensor, Sensitivity is increased by 53.6%. The results indicate that MXene could be used to enhance the performance of SPR refractive index sensors. [ABSTRACT FROM AUTHOR]

We propose and demonst rate multi-parameter sensors based on optical reflective coupler probe (ORCP), for detection of liquid level, temperature, and refractive index (RI). Single-mode fiber ORCP (S-ORCP), few-mode fiber ORCP (F-ORCP) and multi-mode fiber ORCP (M-ORCP) are fabricated by adiabatically tapering and weak fusion technology. The highest liquid level sensitivity is 363.64 nm/mm, achieved by the M-ORCP, with detection limit of as low as 0.06 μm. In addition, multi-parameter measurement of liquid level, temperature and RI can be achieved by the M-ORCP, with temperature and RI sensitivities of −0.327 nm/°C and 7859.27 nm/RIU, respectively. With the merits of high sensitivity, stability, and compact half-coupler probe structure, the proposed ORCP sensors of ∼ cm length can measure the liquid level range of ∼3 mm, which have broad application prospects in the fields of fuel storage and chemical liquid level measurement, especially for chemical reagents, biological tissue liquids and flammable and explosive liquids. [ABSTRACT FROM AUTHOR]

This article presents a sapphire-based Fabry–Perot acoustic sensor with potential for application in harsh environment acoustic sensing. The key component of the sensor is a thin sapphire diaphragm with a diameter of 20 mm and thickness down to 10 proposed Au–Au bonding-associated lapping and polishing technique. Microfabrication characterizations have corroborated the availability of realizing wafer-class thin sapphire diaphragm fabrication by employing the Au–Au bonding-associated lapping and polishing technique, and the diaphragm can be then easily transferred and assembled as a sensor. The strategy for optimizing the stripping phenomenon during lapping is also proposed. Acoustic calibration experiment through a plane wave tube indicates the sensitivity of 13.58 mV/Pa at 10 kHz and the frequency response range of 100250 kHz. The temperature tolerance experiment through the muffle furnace shows that no structural damage or optical spectrum invalidity occurs even if the environmental temperature reaches 800°, which indicates a high working temperature of the sensor up to 80°. Major findings and future application directions of this article are concluded as: 1) realization of an innovative sapphire-based Fabry–Perot acoustic sensor that shows significant potential in applications of numerous key industrial domains, such as aerospace engineering, and 2) a novel approach for achieving wafer-class sapphire ultrathin diaphragm fabrication, which might offer innovative methods for the optimization of sapphire-based harsh environment sensors and give reference to the sapphire lapping process as demanded in the gallium nitride (GaN)-based light-emitting diodes (LED) industry. [ABSTRACT FROM AUTHOR]

In this article, a fiber optic sensor based on double-cladding fiber (DCF) composite tapered structure cascaded long-period fiber gratings (LPFGs) for measuring refractive index (RI), temperature, and axial strain is designed. The two ends of a Section of DCF are, respectively, fused with single-mode fiber (SMF) and LPFG1 to form two up-tapers, and then the DCF is formed into a down-taper. Another end of LPFG1 is cascaded with LPFG2, and the center wavelengths of the two LPFGs are different. LPFG1, LPFG2, and tapered DCF generate three resonance peaks, namely, dip1, dip2, and dip3, respectively, whose sensitivity in response to different parameters is different. When external parameters change, the dips will have spectral shifts. The experimental results indicate that, when the RI is changed within 1.3355–1.3715, the sensitivity of dip1, dip2, and dip3 is –52.12, – 132.58, and 37.61 nm/RIU, respectively. When the temperature varies from 30 °C to 80 °C, the sensitivity of dip1, dip2, and dip3 is 74.05, 98.02, and 26.33 pm/°C in turn, respectively. When the axial sensitivity of dip1, dip2, and dip3 is – 1.95, – 2.58, and –3.24 pm/ $\mu \varepsilon $ , respectively. The resolutions for RI, temperature, and axial strain are 0.0005 RIU, 1.3375 °C, and $14.0886 \mu \varepsilon $ , respectively. The three parameters can be monitored simultaneously using the coefficient sensitivity matrix. The designed sensor has high repeatability and good linearity, and it is practical in the field of multiparameter detection. [ABSTRACT FROM AUTHOR]

Fe-C-coated single-mode–multimode–single-mode (SMS) optical fiber sensor is fabricated and characterized for steel corrosion monitoring in 3.5 wt.% NaCl solution. The multimode fiber (MMF) is a none core fiber and has a length of 30 mm. A 100-nm-thick layer of the gold film is first sputter deposited on the surface of the SMS fiber and then the Fe-C layer is electroplated with a thickness of around 20 $\mu \text{m}$. The microstructure of both the inner gold film and the outer Fe-C layer is characterized by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy. The corrosion evolution of the Fe-C layer over time in 3.5 wt.% NaCl solution is measured with open circuit potential (OCP), linear polarization resistance (LPR), and electrochemical impedance spectroscopy (EIS). In the meantime, the change in the transmission spectrum is recorded with an optical spectra analyzer during the corrosion test. The shift in the characteristic wavelength of the Fe-C-coated SMS fiber sensor is extracted and correlated with the corrosion-induced mass loss of the Fe-C layer. Regression analysis demonstrates that the corrosion-induced mass loss of the Fe-C layer exponentially increases with an increase in the characteristic wavelength of the Fe-C-coated SMS optical fiber sensor. [ABSTRACT FROM AUTHOR]

In this article, a high-sensitivity surface plasmon resonance (SPR) sensor based on multimode fiber (MMF)-tapered hollow core fiber (THCF)-MMF structure is fabricated and proposed for refractive index (RI) sensing. As the measurement RI increases, the SPR peak right shifts, a multimode interference (MMI) spectrum will also appear in the visible wavelength in front of the SPR peak, and the MMI resonance wavelength left shifts with the RI increases. The RI changes can be detected by monitoring the resonance wavelength difference between the MMI wavelength and the SPR peak, which is a new method used to measure the RI. The RI sensing performance for the proposed sensor probes with different taper ratios and different HCF core diameters is explored. The experimental results show that the sensitivity could be significantly improved at large RI for the MMF-THCF-MMF probe with a large taper ratio. For the probe with an HCF core diameter of $30 ~\mu \text{m}$ and a taper ratio of 3.3, a high RI sensitivity of 7592.25 nm/RIU is obtained at the RI of 1.40. The proposed SPR sensor is easily fabricated and has a simple structure, which has potential applications in biochemical sensing fields. [ABSTRACT FROM AUTHOR]

The development of a p-cresol biosensor for aquaculture, marine life, and healthcare applications is described in this study. The biosensor is based on localized surface plasmon resonance (LSPR) and employs a novel tapered-in-tapered fiber (TiTF) probe. Gold nanoparticles (AuNPs) and copper oxide nanoflowers (CuO-NFs) on TiTF structures are immobilized on this probe. The developed probe’s performance was evaluated by measuring the response to a variety of p-cresol solution concentrations. Combining AuNPs and CuO-NFs improves the sensitivity and anti-interference capability of the sensing probe. When the p-cresol solution reacts with the tyrosinase enzyme, the refractive index on the sensing region of the probe changes, and the resulting spectrum varies. The linear range, the limit of detection (LoD), and sensitivity of the proposed sensor are 0–1 mM, 0.14 mM, and 3.8 pm/mM, respectively. The probe is subjected to extensive testing, including LoD, repeatability, reusability, stability, selectivity, and pH testing, that all obtain satisfactory results. [ABSTRACT FROM AUTHOR]

Regenerated fiber Bragg grating (RFBG) sensors for thermal aging at high temperatures from 600 °C to 1000 °C are produced and analyzed in this work. Seed FBGs are regenerated at 850 °C–900 °C and thermal characterized at the increasing steps of 50 °C–100 °C. Signal decay is measured from the reflected spectra until imminent erasure. The objective is to analyze the RFBG sensors lifetime above the regeneration temperature for long periods after annealing cycles with increasing thermal levels. Lifetime results from RFBG were reported to be a few hours above their regeneration temperature. Our results are significantly better. The sensors resisted for the time intervals of one to eight days up to 100 °C above the regeneration temperature. In addition, some sensors remained stable in terms of the spectral amplitude for more than four days in that thermal condition. [ABSTRACT FROM AUTHOR]

Fabric-based sweat sensors based on colorimetric analysis have the characteristics of simple structure, small size, convenience, and comfort, which are more suitable for vigorous exercise. However, the realization of on-demand detection of colorimetric sweat sensors is difficult. It will be invalid immediately once the detection process is activated, which may easily lead to ineffective detection. Therefore, the design and controllable fabrication of the sweat sampling layer is the key to improving the practicality of colorimetric sweat sensors. In this article, the hydrophobic thermoplastic polyurethane (TPU) flexible microfibrous mesh was controllably constructed by melt electrowriting and used as a sweat sampling layer to realize on-demand detection of colorimetric sweat sensor. The gating of different amounts of water can be achieved by precisely and orderly depositing the TPU microfiber meshes with different pore sizes and stacking layer numbers. Simultaneously, the optimal parameters of the TPU mesh were determined by the calibration experiment of the running test, which greatly improved the use efficiency (22 times) of the colorimetric sweat sensor.Moreover, the stability of the optimal sweat sensor was improved by 418% compared with the unoptimized one. The comfort and applicability of wearable sweat sensors in vigorous sports can be enhanced greatly by the controllable preparation of the TPU flexible microfiber meshes, which is helpful to promote the development of sweat sensors. [ABSTRACT FROM AUTHOR]

Optical fiber surface plasmon resonance (SPR) sensors are widely used for refractive index (RI) sensing, thanks to their high sensitivity. However, with the increasing demand for multiple analytes detection and sensor integration, a traditional single-channel optical fiber SPR sensor is outdated. We propose and demonstrate a novel triple-channel optical fiber SPR sensor with high sensitivity. The sensor is fabricated with side-polished single-mode fiber (SMF), where the sensing film consists of a layer of gold nanofilm and a layer of TiO2 nanofilm. The resonance wavelength range of the SPR sensing zone can be adjusted significantly by changing the TiO2 nanofilm thickness. Experimentally, by cascading three sensing zones whose TiO2 nanofilm thicknesses are 10, 30, and 60 nm, respectively, a triple-channel SPR sensor is realized. The average sensitivities of three sensing channels reach up to 2637.8, 3962.2, and 6097.3 nm/RIU in the RI range of 1.333–1.370 RIU, which is superior to most current multichannel SPR sensors. Besides, three sensing channels are independent without crosstalk. The proposed triple-channel SPR sensor has great potential in biological and chemical sensing fields, especially in multiple analytes detection applications. [ABSTRACT FROM AUTHOR]