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This article reports on optical time-of-flight (ToF) as a detection mode for soft optical deformation sensors. In this work, low-cost miniature light detection and ranging (LiDAR) chips recently developed for consumer mobile device applications perform real-time optical reflectometry at cm scales in soft optical waveguide networks. Our methods complement existing work by capturing spatial information that is lost with amplitude-only measurements. We demonstrate its application in a multitouch button system capable of extracting displacement values from soft optical switches with average error in the range of 100–200 microns on 1.75 mm of the switch travel. Because making reliable electronic connections to soft materials is a challenging task, we focus on minimizing connector count. In a tracking application, we find the centroid of an object on a three-point supported plate using three optical pressure switches connected to a single LiDAR sensor. [ABSTRACT FROM AUTHOR]

Fiber optic current transformers (FOCTs) have advantages over conventional current transformers. However, the application of FOCTs is significantly limited by the birefringence effect caused by temperature and vibration. Thus, this study proposed a sensing scheme of coupling a fiber polarization rotator to improve the robustness of FOCTs. First, the mathematical model of the proposed FOCT was derived using the Jones matrix, and the error caused by the birefringence effect was computed. Then, the FOCT was modeled by applying the finite element method to verify the effectiveness of the proposed FOCT in improving the temperature and vibration robustness. Finally, experiments were conducted, including temperature, vibration, and frequency response characteristics. The proposed scheme was compared with the basic polarimetric detection (PD) scheme and several state-of-the-art schemes. Results show that the maximum error of the proposed scheme is 1.7% in the temperature range from $- 20 ^{\circ }\text{C}$ to $80 ^{\circ }\text{C}$ (that of the basic PD scheme ranges from −7.05% to 9.75%). In the vibration test, the maximum output error of the proposed scheme is 0.72%, while that of the basic PD scheme reaches 4.89%. Meanwhile, the proposed FOCT meets the requirements of the measurement of high-amplitude and high-frequency transient currents, such as lightning impulse current, in power systems. [ABSTRACT FROM AUTHOR]

The major advantage of products made from composite materials is given by their superior weight-specific mechanical properties. These can be weakened by defects induced in the manufacturing process. Therefore, online detection and analysis of the processed fiber bundle geometry is a key factor for the quality assurance of the final part. In this article, the instrumentation and data evaluation for determining the surface geometry of fiber bundles by means of light sectioning was examined. Bundles of glass and carbon fibers were measured continuously on an inspection test rig. Different background materials have been used in order to validate the applicability of the approach. By utilization of a polynomial fitting algorithm, data segmentation of object and baseline was robustly achieved. By means of cross correlation, the data alignment could be evaluated faster and more reliable compared to a method previously presented by us. The information could then be used for the determination of the fiber bundle width, centerline, spatial changes, and oscillations. In addition, unwanted defects as well as lateral movement of the fiber bundles were reliably detected. The information revealed by the proposed algorithm provides the basis for robust online monitoring of fiber bundle geometry in highly automated composite manufacturing processes. [ABSTRACT FROM AUTHOR]

Dynamic response of the seismic sensor is crucial, particularly for ultra-weak vibration detection. The related researches almost focus on traditional electrical ones. However, for fiber-optic seismic sensor (FOSS), the dynamic response, as well as the on-site results, is rarely reported. This study thoroughly evaluates the theoretical mechanism for improving the dynamic response of FOSS and its applications in the oilfield. The Ricker wavelet is utilized to simulate the applied seismic signal, and its dynamic response output is investigated. The influences of peak frequency and damping ratio on resonance wave suppression and correlation coefficient improvement are discussed. Besides, the perforation shot detection based on ten-level 3-component FOSS arrays is demonstrated in the oilfield. The basic FOSSs and damping-optimized FOSSs are both manufactured for dynamic response comparison. The signal features, including attenuation time, correlation coefficient, and principal frequency component distribution, all confirm the dynamic response improvement of the proposed damping-optimized FOSS array. [ABSTRACT FROM AUTHOR]

There is an increasing interest in the use of surface plasmon resonance tilted fiber Bragg grating (SPR-TFBG) as a sensor for various chemical applications. Signal demodulation is an important process for extracting useful parameters from the unique optical spectra for an accurate measurement. In this work, we proposed a global-based estimator for SPR-TFBG based on a spectral subtraction approach. The estimator is generated based on the root sum squared (RSS) error of the spectral intensity difference between the tested spectrum and the reference spectrum. In the comparison with the other established methods, the proposed method has the lowest standard deviation, the maximum absolute deviation, the finest measurement resolution, and the fastest processing speed. Besides, the proposed method is immune to the interference of white noise in the transmission spectra and capable of producing a high-quality signal with the lowest noise level. [ABSTRACT FROM AUTHOR]

In this article, a reflective-type optical fiber probe, which can measure salinity and temperature simultaneously, is proposed. The proposed probe integrates Mach–Zehnder interferometer and Fabry–Perot interferometer in the same channel skillfully and effectively, which makes it possible to integrate more information into the sensor. The reflected spectrum is improved greatly by adopting special sputtering technology. The spectrums of Mach–Zehnder and Fabry–Perot are separated by applying the frequency division multiplex technique. The simulation and experimental results prove that the proposed probe has a highly sensitive response to temperature and salinity. The experiment result verifies that the salinity sensitivity is up to 2.7 nm/‰ and the temperature sensitivity is better than 0.4 nm/°C. The reflective probe makes the sensor easy to use and paves the way for practical application. [ABSTRACT FROM AUTHOR]

A compact vector magnetometer based on balloon-like fiber structure functionalized by magnetic fluid (MF) was proposed. To fabricate the sensor, the single mode fiber (SMF) was bent into a balloon-like structure, and then the shape was fixed by using the alcohol lamp flame. Because of the bending, the fiber core mode leaks into the cladding, thereby forming higher order cladding modes that are sensitive to the variations of external refractive index. When the structure is encapsulated in MF, due to the anisotropic distribution of ferromagnetic nanoparticles under magnetic field, the refractive index around the fiber structure also presents anisotropy. By measuring the refractive index of the MF under different magnetic-field directions and intensities, both the measurement of the magnetic field intensity and direction can be achieved. The experimental results show that a high magnetic field intensity sensitivity of 0.78 nm/mT and a high magnetic-field direction sensitivity of 0.094°/mT were obtained. Due to the simple fabricating method, low cost, and highly sensitivity, the proposed fiber vector magnetometer has potential applications in the navigation, aviation, and electric power industry. [ABSTRACT FROM AUTHOR]

This article reports a technique to embed optical fiber into a copper mold plate for generating high-density thermal maps of the mold during the process of metal casting. The temperature measurements were based on acquiring and interpreting Rayleigh backscattering (RBS) signals from embedded fiber, using the interrogation technique of optical frequency domain reflectometry (OFDR). The instrumented mold plate was used to perform a cast-iron dip test and a steel dip test in a 200 lb induction furnace. The maximum temperatures recorded by the embedded fiber-optic sensors were 469 °C and 388 °C in the cast-iron and steel dip tests, respectively. The closely spaced and rapidly fluctuating temperature features that were imparted to the mold wall during solidification were successfully mapped with a high spatial resolution (0.65 mm) and a fast measurement rate (25 Hz) using a commercial OFDR interrogator (LUNA ODiSI 6108). Moreover, the thickness of the solidified steel shell was measured, and a thickness map of the shell was generated. A good correlation was observed between the thickness of the solidified shell and the temperature of the mold, as regions with higher and lower temperatures in the thermal profile of the mold corresponded to thicker and thinner areas on the shell, respectively. The dip testing experiments demonstrate that RBS-based fiber-optic sensing is a feasible and effective method for generating information-rich thermal maps of caster molds. The information obtained from thermal maps can be useful for improving the quality of the metal and productivity of the metal casting process. [ABSTRACT FROM AUTHOR]

A refractometer based on an etched fiber Bragg gratings (EFBGs) sensor has been developed for monitoring beer fermentation in real time. The sensor was calibrated using synthetic solutions composed of water, maltose, and ethanol with alcohol by volume (ABV) ranging from 4.0% to 12.0% and Brix (sugar content) varying from 5% to 25%. Calibration results showed that the EFBG sensor responds with high sensitivity even to minor density variations less than 0.5 Brix ± 0.03 Brix in a quasi-linear monotonic fashion with respect to sugar and ethanol concentration, which allowed fitting an empirical model with good adherence to experimental data (low model variance) and with well-defined parameters (parameters covariance matrix with elements exhibiting small absolute values). Finally, the developed sensor was tested in a typical fermentation scenario. A rigorous material balance framework was established to relate the fit calibration model to actual density and alcohol content in real time, which is of assistance for process control strategies. As the proposed sensor can be fit in fermentation vessels for real-time (online) fermentation monitoring, it becomes a technological improvement to traditional offline measurements in terms of both biosafety and process control. [ABSTRACT FROM AUTHOR]

In this article, we propose a temperature-based proportional–integral–derivative (PID) controlling algorithm using highly dense fiber Bragg grating (FBG) arrays for laser ablation (LA) of ex vivo pancreatic tissues. Custom-made highly dense FBG arrays with a spatial resolution of 1.2 mm were fabricated with the femtosecond point-by-point writing technology and optimized for LA applications. In order to obtain proper PID gain values, finite element method-based iterative simulation of different PID gains was performed. Then, the proposed algorithm, with numerically derived PID gains, was experimentally validated. In the experiments, the point temperature was controlled at different distances from the laser fiber tip (6.0, 7.2, 8.4, and 10.8 mm). The obtained results report robust controlling and correlation between controlled distance and the resulting area of ablation. The results of the work encourage further investigation of FBG array application for LA control. [ABSTRACT FROM AUTHOR]

A novel magnetic field measurement method based on the magneto-volume effect is proposed. The magnetic field sensing structure fabricated by the proposed measurement method is made by the magnetic fluid (MF) partially filled into the hollow core fiber (HCF). The presence of air provides conditions for the change of MF volume with the magnetic field in the proposed sensing structure. Therefore, the magneto-volume effect is introduced into the magnetic field sensing mechanism. From 109.6 to 125.8 Gs, the peak wavelength shift is −65480 pm and the sensitivity is −4219.15 pm/Gs, which is 28.16 times higher than that of the fabricated contrast sensing structure. The sensitivity of the magnetic field sensing structure is greatly improved by introducing the magneto-volume effect compared with the contrast sensing structure. The response time and recovery time of the sensing structure are equally 0.2 s. Therefore, the sensing structure has high sensitivity, good linearity, and short response time, which has potential application prospects in magnetic field sensing. [ABSTRACT FROM AUTHOR]

A fiber ring cavity laser temperature sensor is presented and demonstrated. The sensor probe consists of a fiber Bragg grating (FBG) and a heterostructure based on polymer-coated no-core fiber (NCF), and this probe also functions as a wavelength-selective component in the fiber laser system. There are multimode interference and antiresonance effects in the NCF coated with high refractive index polymer. Embedding this sensing head into an Er-doped fiber ring cavity laser system is helpful to improve the spectral quality and the detection limit (DL). The passband peak position of the heterostructure filter and FBG filter varies with the temperature, which leads to the corresponding shift in the output laser wavelengths. Experimental investigation achieves a maximum sensitivity of −3.155 nm/°C in the temperature range from −4 °C to 47 °C, and the DL is calculated as 0.005 °C. It should be noted that there are several measurement intervals in this temperature range, which can be distinguished by the cascaded FBG to obtain the accurate temperature value. Moreover, the sensor with a smaller NCF diameter has a higher temperature sensitivity. The proposed sensor can meet the actual demands of mesoscale ocean phenomena and microregion temperature detection in seawater, and it provides references for improving the sensing capability of optical methods in ocean detection and other practical application. [ABSTRACT FROM AUTHOR]

This article proposes an integrated high-sensitivity temperature sensor based on SiO2-asymmetric Mach–Zehnder interferometer (AMZI) and thermal-expansion-induced stress of aluminum-SiO2. Mechanical analysis of this structure is carried out. The result shows that the SiO2 chip is compressed, and the waveguide length is decreased, leading to the central wavelength shift of the sensor. The sensitivity of the sensor can be customized by different arm lengths of the AMZI. The chip is fabricated using a standard planar lightwave circuit (PLC), and the aluminum alloy plate is glued to the chip using $\alpha $ -cyanaloc acrylic resin adhesive. The sensitivities of two sensors with different arm lengths are tested. The test results show the sensitivity reaches 6.86 and 54.2 nm $\cdot \text{K}^{-1}$ , respectively, which coincides with the theoretical analysis. With low cost and customizable sensitivity, this new device is applicable in many fields. [ABSTRACT FROM AUTHOR]

The strain- and temperature-sensing characteristics of fiber ring laser (FRL) sensors are studied comprehensively. The reflected sensing head, made up of spheroidal Fabry–Perot interferometer (FPI) concatenated with fiber Bragg grating (FBG), is inserted in the cavity of FRL acting as the laser intensity modulator and wavelength selector. The laser intensity is sensitive to temperature change while insensitive to strain, making the dual-parameter demodulation realizable without multivariate matrix by detecting the laser wavelength shift and the laser intensity variation, for the first time in our best knowledge. The comparable sensitivities of 1 pm/ $\mu \varepsilon $ and 24.9 pm/°C are detected in experiment for strain and temperature, respectively, with low detection limit (DL) of 0.87 $\mu \varepsilon $ and 0.035 °C, high $Q$ factor of 1140 and 7.07 $\times \,\,10^{5}$. The high intensity and narrow linewidth of the sensor make it potentially great for application in long-distance sensing systems. [ABSTRACT FROM AUTHOR]

A reflective optical fiber nickel ion (Ni2+) sensor based on surface plasmon resonance (SPR) and surface ion imprinting was proposed and demonstrated. The sensor is made of multimode optical fiber with a core diameter of 600 $\mu \text{m}$ and coated with a gold film for an exciting SPR effect. To realize highly sensitive and highly selective detection of Ni2+, graphene oxide (GO) and Ni2+-imprinted Chitosan (IP-CS) are subsequently combined on the sensor surface. The GO can not only act as the intermediate binder between the Au film and the CS but also increase the sensitivity of the sensor. The Ni2+ holes on the IP-CS can realize the specific capture of Ni2+. The experimental results show that the sensitivity of the sensor to Ni2+ is 5.220 nm/[lg(mg/L)] with a limit of detection of $2.393\times 10^{-3}$ mg/L, and the selectivity of the sensor is significantly improved after imprinting. Besides, the sensor has good stability and repeatability, and fast responsiveness. [ABSTRACT FROM AUTHOR]

Partial discharge (PD) detection and localization is one of the most effective methods in gas-insulated transmission lines (GIL) insulation fault diagnosis, which is important to the early troubleshooting and safe operation. Compared with the widely used ultrahigh frequency (UHF) and acoustic PD localization methods, the PD localization method based on optical signals has good resistance to electromagnetic and acoustic interference. However, the current optical localization method comes with several shortcomings: narrow detection range, requiring field experiments to accumulate data, and installing a large number of sensors, which renders low feasibility. Therefore, this article proposes a PD localization method utilizing virtual sensors (VSs) and optical simulation fingerprint. Based on the idea of the digital twin, this method constructs a fingerprint database through optical PD simulation in an equal-sized GIL simulation model, which solves the difficulty of fingerprint collection in actual equipment. Meanwhile, this article predicts the detection value of the VS through the adaptive neuro-fuzzy inference system (ANFIS) to greatly reduce the installation of the actual sensor (AS). Finally, through the support vector machines (SVMs) algorithm, the detection fingerprint that includes the virtual and actual detection values matches with the fingerprint database. The location corresponding to the optimal matching result is recorded as the localization result. As experiments have shown, the average localization error of the proposed method is 19.69 mm, which is 54.8% lower than the one without VSs under the same conditions. The results reveal that this method compensates for the loss of localization accuracy caused by the reduction of ASs, which embodies value in practice. [ABSTRACT FROM AUTHOR]

Micrometeoroid and orbital debris (MMOD) can cause serious impact damage to long-term on-orbit spacecraft. MMOD impact detection and location are essential to improve the safety and reliability of the on-orbit spacecraft. In this article, a combined artificial neural network (ANN) and Mahalanobis distance method for MMOD impact detection and location based on fiber Bragg grating (FBG) sensor network is proposed. The inputs of the proposed ANN are the Mahalanobis distances determined by the wavelength changes of the FBG sensors at impact points and reference points. A finite element model is developed to explore the relationship between the Mahalanobis distance and the actual distance, which proved to be nonlinear based on the finite element simulation results of MMOD impacts. Four FBG sensors were installed on the back of the test board and sampled at a frequency of 100 Hz using a four-channel FBG demodulator. A total of 360 impact experiments were carried out on this experiment system. The average location error of the proposed method is 0.89 cm, which is 39% and 31% lower compared with the Mahalanobis distance discriminant analysis method and the ANN method, respectively. The combined ANN and Mahalanobis distance method reduces the location errors and has higher accuracy of position prediction. [ABSTRACT FROM AUTHOR]

A near-infrared ammonia (NH3) sensor system based on a novel fiber-coupled off-axis cavity-enhanced thermoelastic spectroscopy (OA-CETES) was demonstrated for the first time. A quartz tuning fork (QTF) with a high Q-factor of ~12 000 was served as a detector and placed after a 6-cm-long off-axis integrated cavity with a finesse of ~482. The proposed fiber-coupled scheme was adopted exploiting a 100-m-long single-mode and multimode optical fiber for long-distance and multipoint sensing. Dual-laser modulation schemes of wavelength scanning and locking were comparably used for performance improvement. By targeting the NH3 line at 6612.71 cm−1, the developed OA-CETES sensor achieved a minimum detection limit (MDL) of 8.5 parts per million (ppm) for a 100-ms integration time and a normalized noise equivalent absorption (NNEA) coefficient of $1.7\times 10^{-9}$ cm−1 WHz−1/2. A fast-response NH3 leak monitoring with single-channel and multichannel cycling schemes was separately performed to verify the reliability of the proposed sensor system for field-sensing applications. The realization of the proposed OA-CETES with a small-size QTF (1–2 cm long) and fiber-coupled method allows a class of optical sensors for long-distance (up to kilometers) and multiplexed multipoint measurement and distributed sensing. [ABSTRACT FROM AUTHOR]

For the purpose of monitoring seawater temperature, a novel Mach–Zehnder (MZ) temperature-sensing structure with a wedge-shaped open cavity is presented and experimentally studied in this article. The open-cavity structure realizes the direct contact between the sensing optical path and the external environment, and ensures the high sensitivity of the structure. At the same time, it achieves not only low loss by direct fusion splicing instead of dislocation fusion splicing, but also low cost by hydrofluoric (HF) etching instead of femtosecond processing. In addition, the temperature sensitivity of the sensor structure proposed in this article can reach 1.5 nm/°C without temperature-sensitive material filled, and 6 nm/°C with silicone oil filled. Moreover, the resolution of the sensing structure is 0.24 °C based on the criterion of $3\sigma $. [ABSTRACT FROM AUTHOR]

Localized surface plasmon resonance (LSPR) sensor with hybrid structure of dielectric thin film and bilayered gold nanoparticles (AuNPs) on an optical fiber is proposed to enhance the sensitivity. The increased number of nanoparticles per unit plane due to the bilayer structure contributes to the upgrade in the sensitivity by improving the LSPR effect and extending the sensing area. The bilayered AuNPs will be particularly important in the application of fiber optic LSPR (FO LSPR) sensors because the optical fiber has a small cross-sectional area. The Al2O3 thin film by atomic layer deposition is inserted between the first and second layers of AuNPs. The dielectric spacer can also help to improve the sensitivity by concentrating the electric field due to its high refractive index and ensuring a nanometer gap between nanoparticles. The monolayer and bilayer with AuNPs are prepared on the optical fibers, respectively. In each FO LSPR sensor, refractive index sensitivities are also analyzed and compared. In the sensor with bilayered AuNPs, a sensitivity improvement of approximately 2.3 times is confirmed. This strategy can be a breakthrough to increase the sensing area of the FO LSPR sensor without modifying the structure of the optical fiber. [ABSTRACT FROM AUTHOR]

In measurement systems, it is essential to have a procedure to obtain not only the calibration curve but also its associated uncertainty for the entire operating range. To achieve it, different alternatives should be taken into account according to the characteristics of the measurement, the way they are taken, the repeatability and reproducibility of the experiments or the nature of the measurement itself. This work presents a step-by-step procedure, to obtain simultaneously the calibration and uncertainty curves of a measurement system, which takes advantage of the maximum likelihood estimation (MLE), the weighted least squares (WLS) method, and the ISO-28037 recommendations. As an application, a U-Bent plastic optical fiber (POF) sensor measuring the sucrose solution density related to the refractive index (RI) is implemented and calibrated according to the proposed procedure. [ABSTRACT FROM AUTHOR]

A high-sensitivity temperature sensor based on reflective Solc-like filter with Vernier effect has been proposed and experimentally investigated in this article. The reflective Solc-like filter is composed of a polarizer and the cascaded polarization maintaining fibers (PMFs) of similar length, which is a kind of all-fiber polarization interference filter. The gold film is coated in the end of the longer PMF to improve the reflectivity of the filter. The mathematical model of the reflective Solc-like filter has been built by the Jones matrix. The simulation results show that the main factor affecting the temperature sensitivity is the length of the PMF fiber, and the fringe visibility of the spectral fringes is only related to splice angles. Experiment results indicate that the Solc-like filter exhibits a high sensitivity of −44.27 nm/°C, which is enlarged over 25 times compared with that of the reflective Solc filter with single PMF, and the temperature resolution can reach $4.5\times 10^{{-4{ }}\,\circ }\text{C}$. To the best of our knowledge, this is the first demonstration that the cascaded PMFs have been applied to the reflective filter, which provides high reference value for the application of polarization interference filters in the sensing field. [ABSTRACT FROM AUTHOR]

According to Faraday rotation effect and Fresnel reflection theory, a novel magnetic field intensity sensor is theoretically proposed and experimentally demonstrated by introducing a 6.4-cm-long tellurite (76.5TeO2-6BiO3-11.5Li2O-6ZnO) no core fiber (TBLZ NCF) into the loop of Sagnac interferometer. The large Verdet constant of TBLZ NCF will bring about an apparent Faraday rotation angle in magnetic field sensing, and the large refractive index difference between TBLZ and silica fibers makes the Fresnel reflection more obvious. The designed sensor demonstrates a sensitivity of 13.51 pm/Gs with an accuracy of 1.47 Gs in the measuring range of 0–380 Gs. It has satisfactory linearity, repeatability, and stability, and does not require additional utilization of expensive magnetic field sensitive materials, exhibiting great application potential. [ABSTRACT FROM AUTHOR]

Real-time identification of threats has been the focus of attention in the security field in recent years. Researchers have aimed to build a fast, efficient, and accurate model to address risks in threat event identification. This study proposes a fast, effective deep learning model—the squeeze and excitation WaveNet (SE-WaveNet). WaveNet is a 1-D convolutional neural network (CNN) model with a large receptive field. It is small, fast, and highly accurate. Thus, it has a great advantage in processing time series data. On this basis, the squeeze and excitation (SE) structure of the attention mechanism is introduced to adapt to matrix data with complete information. To ensure the integrity of information in the experimental verification, the distributed optical fiber vibration sensing (DVS) system is used to collect the data near the vibration point and form the time–space matrix as the input of the model. SE-WaveNet is compared with WaveNet and related models. Results show that the accuracy of SE-WaveNet based on WaveNet is increased by approximately 3.5%, the model size is reduced by 39%, and the speed is increased by 32%. Compared with the five other related models, the average performance is 3%–9% higher, and the final test accuracy rate can reach approximately 97.73%. [ABSTRACT FROM AUTHOR]

Obtaining a continuous relationship between input and output is essential during sensor characteristic determination. However, it is a little challenging for complicated input data, for example, image data. In this article, a sensor characteristic determining method using deep learning Compact yet Efficient Cascade Context-based Estimation (C3E) network, which can establish the continuous relationship between the input and the output, was proposed. A UV sensing system based on this C3E method was presented. The sensor after algorithm optimization has a sensitivity of $1~\mu \text{W}$ /cm2. The limit of detection is $9~\mu \text{W}$ /cm2. The network test time is close to 3 s, and the error rate is 5%, which is very suitable to transplant to the embedded platform and provides solutions for compact instruments. [ABSTRACT FROM AUTHOR]

Gas detection plays an important role in many fields, such as atmospheric environment monitoring, biological monitoring, gas leakage monitoring, gas decomposition products detection, and so on. Nowadays, there are many kinds of gas sensors. Compared with normal gas sensors, optical sensors have the advantages of antielectromagnetic interference, high sensitivity, and compact size. Especially in certain extreme conditions, such as in aerospace applications and oil exploration application, optical fiber sensors are more attractive. This article presents a critical review on fiber loop ring-down spectroscopy (FLRDS), which can be used for quantitative detection of concentrations of trace gases. Fiber cavity is the core component of FLRDS system. The principles, loss characteristics, and application ranges of different types of fiber cavities are discussed. This article brings a variety of FLRDS techniques and introduces the basis for each technique, recent developments in methods, and performance limitations in detail. Finally, the existing technical barriers and future prospects of the FLRDS system for gas sensing are presented. [ABSTRACT FROM AUTHOR]

This article reports on the first demonstration of in situ, real-time dosimetry realized with an enhanced backscattering optical fiber, and a high-resolution optical backscattering reflectometry measurement. This work is devised to overcome the current problems in monitoring radiotherapy treatments, in particular, the difficult evaluation of not only the actual X-ray dose that is accumulated on the target volume but also the distribution profile of the ionizing radiation beam. Overall, the research aims at developing a dose sensor with the most demanding features of small form factor, spatial profiling, and remote interrogation. The experiments have been conducted by evaluating the spatial profile of radiation-induced spectral shift of the Rayleigh backscattering along an optical fiber exposed to X-rays. The sensing element is a section of specialty optical fiber whose Rayleigh backscattering signature changes under ionizing radiation. The specialty fiber is designed to exhibit an enhanced backscattering, in order to overcome the poor sensitivity to radiation of standard optical fibers that are normally, used in telecommunications. The enhanced sensitivity is achieved by doping the core with either aluminum or magnesium nanoparticles, and two different fibers have been fabricated and tested. The experimental results show the capability of real time detection of the radiation profile from high-dose rates (700 Gy/min) to low-dose rates (2 Gy/min). Moreover, different sensing mechanisms and responses to high- and low-dose rates are evidenced. A comparison with a quasi-distributed sensing system based on an array of fiber Bragg gratings (FBGs) is discussed, highlighting the superior performance of the backscattering approach in terms of sensitivity and spatial resolution, whereas the array of FBGs exhibits an advantage in terms of sampling speed. [ABSTRACT FROM AUTHOR]

A low cost, wearable textile-based respiratory sensing system is proposed and experimentally demonstrated. A highly sensitive D-shaped plastic optical fiber (POF) sensor that responds to bending is integrated into an elastic band structure to form a respiratory sensing system. The curvature sensing experiments were conducted on the D-shaped POF sensor, which has a coefficient of determination (R2) of 0.9977. The system can be used to monitor not only the respiratory rate (RR) of the human body under different movement states (resting, walking, and running) but also the RR of steady and unsteady respiratory signals due to different physiological states. In addition, using the proposed signal processing technique, the interference of motion noise can be removed and the influence of body movement on the sensor response can be eliminated. The advantages of the system are its low cost, compactness, and simplicity in design. Thus, the application of the proposed respiratory sensing system provides a simple and inexpensive optical solution for wearable health. [ABSTRACT FROM AUTHOR]

In this work, a parallel polished plastic optical fiber (POF)-based surface plasmon resonance (SPR) sensor is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The sensor is fabricated by double polishing the POF symmetrically and depositing a layer of gold film on both sides of the polished regions. By coating the polydimethylsiloxane (PDMS) layer on one of the polished sides, another resonance peak for the temperature sensing can be observed in the transmission spectrum. When the RI and temperature vary, the two resonance peaks will change, and the RI and temperature could be measured simultaneously by monitoring the wavelength shifts of the two peaks. Experimental results show that the sensor has an RI sensitivity of 1174 nm/RIU in the RI range of 1.335–1.37 and a temperature sensitivity of −0.7 nm/°C in the temperature range of 30 °C–80 °C. In addition, the results show that the crosstalk between the two sensing channels of the proposed sensor is negligible. The parallel polished structure is helpful to develop the multiparameter measurement sensors, which has potential application prospects in the biochemical sensing fields. [ABSTRACT FROM AUTHOR]

Nowadays there is an increasing request to realize green, eco-friendly, and biodegradable electronic devices for biosensor implementation. In this context, we have conceived and realized a green sensor based on localized surface plasmon resonance (LSPR) phenomenon in a thin-slab waveguide of bacterial cellulose (BC). These LSPR sensors can be obtained simply by gold sputtering on the slab BC waveguides. The performances have been studied investigating the presence of ionic liquids (ILs) inside and in absence of ILs with various thicknesses of the BC substrate. Depending of the thickness of the BC layer, the ILs effect on the LSPR can be constructive or destructive. In this work, we present a study of the sensor performances, in terms of bulk sensitivity and resolution by changing the aforementioned parameters. Analyses in terms of BC geometry are pursued in order to improve the interaction between the light and the LSPR phenomenon. The experimental setup used for this kind of extrinsic optical fiber LSPR sensor is based on two optical fibers used to connect a white light source and a spectrometer with the green LSPR sensor chip. Results evince the suitability of the proposed approach in order to realize sensors and biosensors with several intriguing properties and features. In fact, these LSPR platforms could be used to realize disposable biosensors, when a specific bioreceptor is covalently bonded to the gold. [ABSTRACT FROM AUTHOR]

Rehabilitation of stroke survivors can be expedited by employing an exoskeleton. The exercises are designed such that both hands move in synergy. In this regard, often, motion capture data from the healthy hand is used to derive control behavior for the exoskeleton. Therefore, data gloves can provide a low-cost solution for the motion capture of the joints in the hand. However, current data gloves are bulky, inaccurate, or inconsistent. These disadvantages are inherited because the conventional design of a glove involves an external attachment that degrades overtime and causes inaccuracies. This article presents a weft knit data glove whose sensors and support structure are manufactured in the same fabrication process, thus removing the need for an external attachment. The glove is made by knitting multifilament conductive yarn and an elastomeric yarn using WholeGarment technology. Furthermore, we present a detailed electromechanical model of the sensors alongside its experimental validation. In addition, the reliability of the glove is verified experimentally. Finally, machine learning algorithms are implemented for classifying the posture of hand on the basis of sensor data histograms. [ABSTRACT FROM AUTHOR]

A magnetic field measurement method based on the magneto-volume effect of magnetic fluid (MF) and Fabry–Perot (FP) interferometer (FPI) is proposed, which has higher magnetic field sensitivity and shorter response time than the traditional optical fiber methods. The MF is partially injected into the fabricated FP cavity composed of two sections of SMFs and a capillary glass tube (CGT), where the FPI is made up of optical fiber end face and MF end face. When the external magnetic field is applied to the sensor, the volume change of MF will cause the change of air cavity length. Therefore, the reflection spectrum changes with external magnetic field due to the magneto-volume effect of MF. Experimental results show that the magnetic field sensor shows a sensitivity of 268.81 pm/Gs from 15.5 to 139.7 Gs even with a small amount of MF. The response time is 0.2 s at a step magnetic field of 7.7, 31.0, 46.5, and 139.7 Gs. In this work, the magnetic field sensor boasts high sensitivity, good repeatability, short response time, and low cost, which shows a great potential in applications. [ABSTRACT FROM AUTHOR]

Optoelectronic oscillator (OEO) has received growing attentions from researchers in the field of demodulations for optical sensors because of its excellent performance of combining optics and electronics in signal processing. In this article, we present a wideband-tunable OEO for angular velocity measurement. The OEO consists of two cascaded microwave photonic filters (MPFs), namely, a dispersion-induced MPF for mainly determining the OEO’s oscillating frequency and a two-tap MPF for the fine selection of the oscillating frequency. With proper control of polarization of the optical signal, the angular velocity is first mapped into the central frequency shift of the dispersion-induced MPF, which will then cause an oscillating frequency shift of the OEO. This approach ensures a large-scale factor for the angular velocity measurement due to the fact that a small phase change in the optical domain causes a large frequency shift in the electrical domain. An experiment is conducted to validate the scheme. The experimental results show that the oscillating frequency shift can reach nearly 700 MHz when the system rotates at about 1 rad/s by using a 560-m Sagnac loop. In addition, the measurement range can be achieved to 5.24 rad/s with a 2.5-GHz oscillating frequency tuning range and the maximum measurement error is 0.06 rad/s. [ABSTRACT FROM AUTHOR]

This article investigates a real-time nondestructive measurement method of the seam strength using the optical spectroscopy and ensemble learning in laser beam welding. First, a linear positive relationship between the seam strength and acquired optical spectrum signal was established. Then, a new feature reduction method was proposed to extract the independent feature set with low correlation and high diversity. Finally, a hybrid classification model is proposed based on fast independent component analysis (ICA) and extremely randomized trees (ET). The model was thoroughly verified with various welding experiments and careful comparison with decision tree (DT), extreme gradient boosting DT (XGBoost), and support vector machine (SVM). More importantly, driven by the monitoring data and random forest with fast independent component analysis (RFICA)-ET model, it was found that Al I at 669.8673 nm and Ar I at 610.5635 nm were the two key elements in the dynamic plasma for accurate seam strength measurement. [ABSTRACT FROM AUTHOR]

A strain sensor based on cascaded Sagnac loops with two polarization-maintaining photonic crystal fibers (PMPCFs) has been experimentally demonstrated. Air holes distributed on the cross section of PMPCF go through the whole fiber, which makes the special fiber more sensitive to ambient force compared with conventional optical fiber. The lengths of the two PMPCFs are close, but not the same, to form envelopes on the total output spectrum and then obtain high resolution to the detected parameter by the Vernier effect. The influences of the difference in length of the two PMPCFS on spectral envelopes are discussed. As the length of the PMPCF in reference loop increases, free spectrum range of the cascaded Sagnac loops (or envelope period) and magnification decrease. The sensitivities of this strain sensor are calculated by tracking upper and lower envelopes. Results reveal that its average sensitivity is up to 45.15 pm/ $\mu \varepsilon $ and the corresponding resolution is $0.44~\mu \varepsilon $ as strain varies from 0 to $2000~\mu \varepsilon $. Compared with a single Sagnac loop, the sensitivity magnification is about 28.94. The sensing characteristics of a conventional polarization-maintaining fiber (PMF) are studied to compare with the PMPCF. Moreover, reversibility has also been proved to possess low error rate. In short, the fiber strain sensor possesses high sensitivity and great reversibility. [ABSTRACT FROM AUTHOR]

Accurately detecting man-made intrusion from different events is of great significance for distributed optical fiber sensing perimeter security system. Most traditional algorithms lack the ability to reject various events of unknown class which are mainly from natural disturbance, and greatly decline the accuracy of intrusion recognition in field application. In order to solve this problem, we proposed a novel robust intrusion event recognition scheme based on convolutional prototype network (CPL), which realized end-to-end feature extraction and recognition based on the similarity of intrusion signals by integrating relevant variables of prototype learning into the training process of multiscale convolutional neural network (MSCNN) as trainable parameters, and had the ability to recognize and reject the unknown disturbance events. In field experiments, the average recognition accuracy of intrusion events as known class can reach 84.67%, with the rejection rate of disturbance events as unknown class is about 83.75%, which ensure the accuracy of intrusion events monitoring in complex field environments. And the recognition response time is about 17 ms, which also meets the need of real-time monitoring. [ABSTRACT FROM AUTHOR]

A fiber-optic pH sensor, based on layer-by-layer (LBL) self-assembly and surface plasmon resonance (SPR) technology, is proposed and experimentally confirmed. This article proposes poly (diallyldimethylammonium chloride) (PDDA) as the pretreatment solution for the surface of the gold film and the highly sensitive polyacrylic acid/chitosan (PAA/CS) self-assembled nanomembrane as the pH-sensitive film. It has been experimentally verified that the sensor can detect pH from 3.18 to 11.84 and it exhibits a high average sensitivity of 32.31 nm/pH in this range. In addition, experiments have confirmed that the sensor has attractive stability, repeatability, and hysteresis characteristics. [ABSTRACT FROM AUTHOR]

In this work, we report a self-compensating, label-free, and anti-interference surface plasmon resonance (SPR) fiber biosensor based on a cascaded U-shaped multimode fiber and a single-mode fiber and both coated with gold film. In the traditional dual-channel SPR sensor structure, in order to obtain two independent SPR signals on one probe, different metal films need to be coated on two sensing channels. We proposed a U-shaped cascade dual-channel SPR optical fiber sensor, two channels are covered with gold film, and two independent SPR signals can be obtained by controlling the U-bend radius. In order to achieve self-compensating in detection of biomarkers, the U-shaped multimode fiber area of sensing probe is biologically functionalized as a detection channel, and the straight single-mode fiber without functionalized used as a reference channel. During the process of measuring biomarkers concentration, the existence of reference channel can be used to eliminate the effect of impurities’ physical adsorption in the detection channel on the result. This sensor was used to detect the IgG concentration in complex samples, achieving the high precision detection with a limit of detection of $0.104\mu $ g/mL. A small-in-size, simple-to-manufacture, dual-channel SPR sensor with self-compensation was proposed and demonstrated in this article which has an excellent application prospect in the clinical monitoring of various biomarkers. [ABSTRACT FROM AUTHOR]

We report, for the first time, the development of a rectangular fiber with two cores consisting of different refractive indices for the discrimination of temperature effects during strain or curvature measurements using fiber Bragg gratings written in a single segment of fiber. Two cores with a diameter of $8~\mu \text{m}$ and a rectangular cladding with dimensions of $120\,\,\mu \text{m} \times 255\,\,\mu \text{m}$ permit convenient splicing to conventional single-mode fiber and support easy identification of the orientation of the fiber. When placed in the desired orientation, each core of the two-core rectangular fiber experiences a different spectral shift in the presence of applied strain or curvature. The sensitivities of the two cores are 0.87 and 1.08 pm/ $\mu \varepsilon $ for strain, 126 and −128 pm/m−1 for curvature, and 10.05 and 10.40 pm/°C for temperature, respectively. In addition, the flat cladding structure of the fiber provides a large physical contact area between the sensing fiber and the substrate than a circular fiber and allows better strain coupling between the object and the fiber. [ABSTRACT FROM AUTHOR]

A highly sensitive humidity sensor with low-temperature sensitivity is proposed and demonstrated using a polyvinyl alcohol (PVA) coating tapered fiber. The taper fiber is highly sensitive to the environmental medium due to its microscale diameter. Thus, a humidity sensor can be prepared by plating a PVA moisture-sensitive film in a tapered region. The experimental results show that the sensor has a high humidity sensitivity of 0.1194 nm/%RH but with a relatively low-temperature sensitivity of 0.029 nm/°C. Thus, the sensor can eliminate temperature interference and achieve accurate measurement of environmental humidity. At the same time, the sensor has excellent reversibility, stability, and advantages common to optical fiber sensors, such as small size, antielectromagnetic interference, and remote measurement. [ABSTRACT FROM AUTHOR]

We present a novel methodology for magnetic field sensing by exploiting surface plasmon resonance (SPR) sensors based on D-shaped plastic optical fiber (POF) and magnetic fluids (MFs). The proposed methodology adopts a multimode POF patch covered with ferrofluid placed between the light source and the SPR-POF platform. The working principle exploits the changing of the SPR spectra due to variations of the SPR resonance conditions generated by the changing in the input light in the SPR-POF sensor due to the attractive force exerted by the target magnetic field on the POF covered with ferrofluid. A model showing a good fitting between the simulated and measured values of the magnetic field is also presented. A proof of concept for this novel sensing approach has been obtained exploiting a prototype of a sensor that has been characterized in the range of values between 0.15 and 1.2 mT. Particularly, we have estimated, in the linear range of the sensor response, sensitivity equal to about 6800 pm/mT and resolution equal to about 0.029 mT. Moreover, the comparison between the expected and the experimental behavior has shown a very good match with a mean squared error of about 5%. [ABSTRACT FROM AUTHOR]

The inherent advantages of fiber optical current transformer (FOCT) are very attractive for high-voltage applications when compared with conventional transformers. However, the accuracy and long-term reliability of FOCT can be greatly affected by temperature. Fiber temperature sensors are suitable for FOCT temperature compensation, especially in ultrahigh voltage (UHV) applications. As a result, a fiber temperature-sensing scheme based on the temperature birefringence effect of polarization maintaining fiber (PMF-TS) was proposed. A high integration real-time temperature correction FOCT based on PMF-TS (PMFTS-FOCT) was designed, and a prototype with rated current of 5000 A and rated voltage of 1100 kV was developed. The accuracy, temperature, frequency response, step response, and transient response of the system was tested. The results showed that within the temperature range of −40 °C to 70 °C, the ratio error (RE) of 500–5500 A was less than ±0.2% after temperature correction, and in the frequency range of 50–1200 Hz, RE and phase error (PE) were less than ±0.75% and $110~\mu \text{s}$ , respectively. The step response time was less than $125~\mu \text{s}$ , the overshoot was less than 1.5%, and the composite error was 0.807% for the 50 000-A transient current. We proved that PMFTS-FOCT had good steady-state and transient performance and the possibility for measuring dc and ac harmonics, whose performance meets the error limit requirements specified in GB/T 26216.1-2010. [ABSTRACT FROM AUTHOR]

This article presents an ultrasensitive open-ended hollow coaxial cable resonator (OE-HCCR) that was trained to detect impact events that occurred in the surrounding space through machine learning (ML) techniques. First, inspired by the well-known mass-based accelerometer, for the first time, we propose an OE-HCCR-based accelerometer with the addition of an inertial mass block as the element responsive to acceleration. The OE-HCCR is formed by a metal shorting post welded at the RF input end and the open end of the cable. The instantaneous displacements of the mass block mounted at the endface of the cable due to accelerations lead to variations of the gap distance between the mass block and the endface of the coaxial line, which can be accurately metered by the coaxial resonating structure. Impact events that occur in the surrounding space of the OE-HCCR result in vibrations transmitted to the OE-HCCR system and thereby measured by the OE-HCCR. Second, we demonstrated that employing ML algorithms to analyze the transient responses from the OE-HCCR makes unambiguous mappings between the transient responses of the OE-HCCR and the sources of vibrations possible, due to the ultrahigh sensitivity of the OE-HCCR and the spatial anisotropy of matter in the surrounding space. The preliminary results demonstrate the feasibility of using ML-assisted OE-HCCR system for locating impact events and recognizing different impactor implements that caused the impact events. [ABSTRACT FROM AUTHOR]

We propose a bidirectional flowmeter based on specialty fiber interferometry by controlled mode excitation for measuring the frequency of the vortices induced by a bluff body. The proposed arrangement of the cylindrical bluff body with a slit and the interferometric system based on modal excitation allows the bidirectional flowmeter to determine the flow velocities in either direction precisely and effectively. We have encapsulated the specialty fiber interferometer in a protective metal casing, which not only allows the interferometer to respond to the vortex frequency but also provides mechanical stability to the whole flowmeter so at to operate in a harsh environment. Experiments are conducted by introducing the modal interferometer in the slit of the cylindrical bluff body at various flow rates ranging from Reynolds number (Re) = 7200 to 56 600. A linear response is observed between the flow velocity and the vortex frequency from both directions. Thus, this work provides a reconfigurable bidirectional flowmeter, which is sensitive even at low flow rates and demonstrates a large operating range. [ABSTRACT FROM AUTHOR]

Measurement of optical transfer delay (OTD) is crucial to applications such as fiber-distributed multiantenna systems, fiber-optic sensors, and high-capacity optical fiber communications. However, present OTD measurement techniques are inadequate for the demands of high accuracy, high speed, and large measurement range, simultaneously. Here, we propose a novel method based on nonlinear frequency sweeping and phase derived ranging to achieve all the above-mentioned performance. A continuous-wave light modulated by a microwave signal propagates in a device under test. Then, the OTD is mapped into the phase variation of the microwave signal by photodetection. A microwave phase discriminator is used to extract the phase variation from the microwave signal, while the nonlinear frequency sweeping and a novel phase unwrapping algorithm are proposed to resolve $2\pi $ phase ambiguity caused by phase detection. Frequencies of the microwave swept signals are set at four selected points in a range of 10 MHz, which ensures high speed and large measurement range. Our experiment results verify an accuracy of ±0.05 ps in measuring an ultrahigh-accuracy optical delay line. In addition, long fiber is also tested, which proves that a measurement range of at least 37 km (theoretically 100 km) can be achieved. Moreover, the measurement speed reaches milliseconds per measurement. [ABSTRACT FROM AUTHOR]

High-temperature measurements are of significant importance in various harsh-environment engineering fields, such as fossil fuel production, and the metallurgical and aviation industries. In recent years, there is a steady trend to shift from conventional electronic sensors to optical fiber sensors for high-temperature applications. In particular, optical fiber sensors are small in size, immune to electromagnetic interference, readily applicable for remote sensing, have high elasticity, and incorporate capabilities for multiplexing and distributed sensing. However, commonly used fused silica optical fiber sensors exhibit severe limitations at ultrahigh temperatures due to significantly degraded optical and mechanical properties at temperatures >1000 °C. The excellent optical transparency, thermal and chemical stability, mechanical robustness, and high melting temperature (~2040 °C) of single-crystal sapphire fibers (SFs) make them a strong candidate for sensing applications in high-temperature environments. Translation of the sensing schemes from mature silica fiber sensors to SF sensors has undergone tremendous growth and advancements in the past two decades. However, hurdles to the development of a near-term deployable SF sensing system have proven persistent due to the highly multimodal nature of SFs. This article reviews sensing techniques that have been implemented with SFs recently. The aim is to provide a comprehensive summary of past research on SF sensing systems. Perspectives on further research into the challenging yet promising arena are also discussed. [ABSTRACT FROM AUTHOR]

Fabrication and experimental validation of a newly developed copper oxide nanoparticles (CuO-NPs) and silver nanoparticles (AgNPs) immobilized SM-MM-SM-MM-SM (SMSMS) in-line Mach–Zehnder interferometer (MZI) fiber structure-based sensor using localized surface plasmon resonance (LSPR) technique are reported in this article. The AgNPs and CuO-NPs are used to improve the biocompatibility and sensitivity of the sensor. The nanoparticles (NPs) and immobilized probe were characterized by UV-Vis spectrophotometer, HR-TEM, SEM, and EDX, revealing superior performance over the existing uric acid (UA) sensors. The proposed optimized structure-based optical fiber sensors (OFSs) exhibit excellent specificity, sensitivity, and limit of detection (LoD) for the detection of UA. Moreover, the developed sensor exhibits linear profile over a very wide range: 1) $10~\mu \text{M}$ –1 mM for detection over UA presents in serum, while the normal range of UA in serum is 100– $400~\mu \text{M}$ and 2) 0.4–10 mM for detection over UA presents in urine, while the normal range of UA in urine is 1.5–4.4 mM. In addition, the developed sensor has a distinguished advantage, such as multicompatibility over the detection of UA presents in both serum and urine along with its high selectivity, reusability, reproducibility, and lowest LoD. The proposed AgNPs and CuO-NPs immobilized and the uricase functionalized SMSMS structure-based LSPR sensor has better compatibility and are potent candidates for medical applications. [ABSTRACT FROM AUTHOR]

We present an all-optical photoacoustic (PA) multigas analyzer by using a digital fiber-optic acoustic detector, which consists of a spectrum demodulation based fiber Fabry–Perot (F–P) pressure sensor and a digital virtual lock-in amplifier. The concentration of multigas is quantified by using an intensity-modulated infrared thermal source and two wavelength-modulated laser diodes. Light beams from the three sources are coupled to a single nonresonant PA cell. The generated PA pressure is detected by the fiber F-P pressure sensor, which is demodulated by a near-infrared spectrometer. A lock-in white-light interferometer is developed for detecting first-harmonic ($1f$) and second-harmonic ($2f$) signals. The noise equivalent detection limits with a 100-s averaging time are 37, 9, 6, 17, 4, and 60 ppb for CH4, C2H2, C2H4, C2H6, CO, and CO2, respectively. [ABSTRACT FROM AUTHOR]

A biocompatible and robust fiber surface plasmon resonance (SPR) temperature sensor was fabricated based on an alcohol-filled hollow-core fiber. The fabrication process of this designed sensor included Ag film coating, liquid injection, and fusion splicing, which was low cost and efficient. Due to the high refractive index sensitivity of the SPR effect and the high thermal optical coefficient of alcohol, the designed sensor performed well in temperature sensing, whose linear sensitivity reached as high as 1.16 nm/°C in the range of 35.5 °C–70.1 °C. Being small in size, low in fabrication cost, and highly sensitive in performance, this sensor is suitable for temperature detection during biological and chemical reactions and has the potential to realizing multiparameter or distributed temperature measurement. [ABSTRACT FROM AUTHOR]

The selective detection of pollutants in water in a laboratory scenario has been presented by authors exploiting low-cost optical biosensors based on plastic optical fibers (POFs) and biological or biomimetic receptors. So, for instance, the detection in water of naphthalene, perfluoroalkyl substances (PFAs) and polyfluoroalkyl substances have been investigated with interesting detection limits when compared to those obtained by using different expensive traditional approaches (e.g., liquid chromatography-mass spectrometry with high performances). In this article, we have developed and tested a novel approach used in a smart measuring system to use POF sensors in situ for the remote measures of pollutants in water for smart cities applications. More specifically, we have used different water–glycerin solutions to test the novel sensor system based on a Raspberry Pi connected to the Internet and to a spectrometer, a light source, a POF sensor, and two computers connected to Internet used as client and server. [ABSTRACT FROM AUTHOR]