Due to the benefits of the high sensitivity, real-time response, no labeling requirement, and good selectivity, fiber optic sensors based on surface plasmon resonance (SPR) have gained popularity in biochemical sensing in recent years. The current research on such sensors is hot in enhancing sensitivity, improving detection accuracy, and achieving the detection of biochemical molecules. The goal of this work is to present a thorough overview of recent developments in the optical fiber SPR biosensor research. Firstly, it explores the basic principles and sensing structures of optical fiber SPR biosensors, focusing on four aspects. Subsequently, this paper introduces three fiber optic surface plasmon biosensors: SPR, localized surface plasmon resonance (LSPR), and long-range surface plasmon resonance (LRSPR). Each concept is explained from the perspective of the basic principles of fiber optic SPR biosensors. Furthermore, a classification of fiber optic SPR biosensors in health monitoring, food safety, environmental monitoring, marine detection, and other applications is introduced and analyzed. Eventually, this paper summarizes the current research directions of SPR biosensors. Meanwhile, it provides a prospective outlook on how fiber optic SPR sensors will develop in the future.

Miniaturized fiber-Bragg-grating (FBG) interrogators are of interest for applications in the areas where weight and size controlling is important, e.g., airplanes and aerospace or in-situ monitoring. An ultra-compact high-precision on-chip interrogator is proposed based on a tailored arrayed waveguide grating (AWG) on a silicon-on-insulator (SOI) platform. The on-chip interrogator enables continuous wavelength interrogation from 1 544 nm to 1 568 nm with the wavelength accuracy of less than 1 pm [the root-mean-square error (RMSE) is 0.73 pm] over the whole wavelength range. The chip loss is less than 5 dB. The 1 × 16 AWG is optimized to achieve a large bandwidth and a low noise level at each channel, and the FBG reflection peaks can be detected by multiple output channels of the AWG. The fabricated AWG is utilized to interrogate FBG sensors through the center of gravity (CoG) algorithm. The validation of an on-chip FBG interrogator that works with sub-picometer wavelength accuracy in a broad wavelength range shows large potential for applications in miniaturized fiber optic sensing systems.

An acidic gas is an important basic chemical raw material used for synthesizing fertilizers, insecticides, explosives, dyes, and salts. Alternatively, inorganic acidic gases that leak into the air have harmful effects on the human health, infrastructure, and cultural relics. Therefore, the demand for inorganic acidic gas sensors for air quality monitoring and management has continuously increased, enabling the development of various sensing technologies. Among them, fiber-optic sensors are promising for acidic gas detection because of their excellent in-situ measurement, resistance to corrosion, anti-electromagnetic interference, long service life, and smart structure. In particular, fiber-optic sensors have proven to be very useful for the in-situ detection and distributed monitoring of multiple gas parameters. However, the sensitivity, selectivity, repeatability, and limits of detection of these sensors can be improved to achieve acceptable performance levels for practical applications. In this review, we introduce fiber-optic sensors based on structured optical fibers and fiber gratings for detecting H₂S, SO₂, NO₂, CO₂, and N₂O. The structures of the sensing regions, gas-sensitive materials, and measurement principles of these sensors are presented. The sensitivity, selectivity, limit of detection, and response time of the sensors are summarized. Finally, the future of fiber-optic sensors for the detection of inorganic acidic gases is discussed.

For expanding the amplitude-frequency response range of the differential cross-phase multiply (DCM) algorithm in the φ-OTDR system, a temporal spline interpolation (TSI) method is proposed to pre-process Rayleigh backscattering (RBS) signals. Through the TSI method, the discrete temporal signals characterizing RBS traces are subjected to interpolation, facilitating a reduction in differential approximation errors. This, in turn, establishes a heightened level of precision in phase demodulation, especially relevant across extensive sensing distances. By comparing the recovered time-domain waveforms and the corresponding power spectral densities without and with the TSI, the above improvement effect has been experimentally validated by utilizing the TSI. The results show that, with the TSI, the amplitude-frequency response range of the DCM algorithm is enlarged by 2.78 times, and the new relationship among f _pulse, f, and D under the root mean square error (RMSE) tolerance less than 0.1 can be expressed as 1.9(D+1)f ≤ f _pulse. This contribution underscores a substantial advancement in the capabilities of the DCM algorithm, holding promise for refined performance in optical fiber sensing applications.

Cancer has been one of the most serious diseases, resulting in more than 10 million deaths every year. Fiber-optic sensors have great potential for diagnosing and treating cancer due to their flexibility, precise positioning, real-time monitoring, and minimally invasive characteristics. Compared to traditional central laboratory examination, fiber-optic biosensors can provide high sensitivity, miniaturization, and versatility, which feature the point-of-care diagnostic capability. Herein, we focus on recent advances in fiber-optic biosensors for cancer theranostics. It is primarily concerned with advancements in the design of various fiber sensing approaches, fiber cancer sensing, and therapy sensors. With fiber-optic biosensors, cancer marker detection, cancerous cell differentiation, ex vivo tumor model validation, and in vivo tumor detection can be achieved. And the medical fiber also can be used to provide photothermal therapy, photodynamic therapy, and combination therapy for solid tumors. Additionally, cancer sensing and therapy can be integrated into the fiber, which demonstrates the multiplexing capabilities of fiber-optic biosensors. Lastly, we systematically summarize the fiber biosensor applications from in vitro to in vivo, and conclude with the challenges in development and prospects.

This article proposes a line scanning chromatic confocal sensor to solve the problem of limited chromatic confocal measurement due to the small measurement range and low measurement efficiency in the industrial inspection process. To obtain an extensive dispersion range, the advantages of a simple single-axis structure are combined with the advantages of a large luminous flux of a biaxial structure. Considering large-scale measurement, our sensor uses off-axis rays to limit the illumination path and imaging path to the same optical path structure. At the same time, the field of view is expanded, and a symmetrical structure is adopted to provide a compact optical path and improve space utilization. The simulation and physical system test results shows that the sensor scanning line length is 12.5 mm, and the axial measurement range in the 450 nm to 750 nm band is better than 20 mm. The axial resolution of the detector is ±1 µm combined with the subpixel centroid extraction data processing method, and the maximum allowable tilt angle for specular reflection samples is ±7°. The thicknesses of transparent standard flat glass and the wet collagen membrane are measured. The maximum average error is 1.3 µm, and the relative error is within 0.7%. The constructed sensor is of great significance for rapidly measuring the three-dimensional profile, flatness, and thickness in the fields of transparent biological samples, optics, micromechanics, and semiconductors.

Localized surface plasmon resonance (LSPR) biosensors, which enable nanoscale confinement and manipulation of light, offer the enhanced sensitivity and electromagnetic energy localization. The integration of LSPR with the fiber-optic technology has led to the development of compact and versatile sensors for miniaturization and remote sensing. This comprehensive review explores various sensor configurations, fiber types, and geometric shapes, highlighting their benefits in terms of sensitivity, integration, and performance improvement. Fabrication techniques such as focused non-chemical bonding strategies and self-assembly of nanoparticles are discussed, providing control over nanostructure morphology and enhancing sensor performance. Bio-applications of fiber-optic LSPR (FOLSPR) sensors are detailed, specifically in biomolecular interactions and analysis of proteins, pathogens and cells, nucleic acids (DNA and RNA), and other small molecules (organic compounds and heavy metal ions). Surface modification and detection schemes are emphasized for their potential for label-free and real-time biosensing. The challenges and prospects of FOLSPR sensors are addressed, including the developments in sensitivity, fabrication techniques, and measurement reliability. Integration with emerging technologies such as nanomaterials is highlighted as a promising direction for future research. Overall, this review provides insights into the advancements and potential applications of FOLSPR sensors, paving the way for sensitive and versatile optical biosensing platforms in various fields.

In recent years, detecting and quantifying multiple gases have garnered widespread attention across various fields, particularly in volatile organic compound (VOC) detection, which holds significant importance for ecosystems and the medical field. The Raman spectroscopy has been widely used in multi-gas detection due to its advantages in fast response speed and non-destructive detection. This paper reviews the latest research progress of the multi-gas sensing technology in the Raman spectroscopy, focusing on using the hollow-core fiber to enhance the gas signal intensity. The basic principles of the fiber-enhanced Raman spectroscopy are introduced. The detailed discussion includes the system architecture, parameter configuration, and experimental results. Then, the latest advances in the coherent anti-Stokes Raman scattering multi-gas detection technology are reviewed. Finally, the challenges faced by the hollow-core fiber in practical applications are discussed.

Biochemical sensors have important applications in biology, chemistry, and medicine. Nevertheless, many biochemical sensors are hampered by intricate techniques, cumbersome procedures, and the need for labeling. In the past two decades, it has been discovered that liquid crystals can be used to achieve the optical amplification of biological interactions. By modifying recognition molecules, a variety of label-free biochemical sensors can be created. Consequently, biochemical sensors based on the amplification of liquid crystals have become one of the most promising sensors. This paper describes in detail the optical sensing principle of liquid crystals, sensing devices, and optical detection technologies. Meanwhile, the latest research findings are elucidated. Finally, the challenges and future research directions are discussed.

Vacuum ultraviolet (VUV) light sensing shows great potential applications in the space science, materials, biophysics, and plasma physics. In this work, an all-optical detection method is proposed for VUV sensing by constructing an optical fiber-end Fabry-Pérot interferometer based on a single aluminum nitride (AlN) microwire. Compared with the traditional electrical devices, this all-optical detection method overcomes the difficulties like the fast response and electromagnetic interference immunity in detecting VUV bands at the present stage, and improves the response speed. The proposed device shows the excellent performance of VUV detection, with the static sensitivity of 1.03 nm/(W·cm⁻²), response rise time of down to 10 µs, and decay time of 0.64 ms. Beneficial from the excellent radiation resistance of AlN microwires and UV resistance of silica fibers, the proposed device is expected to have the good stability and potential applications in the fields of the solar physics and space exploration.

Fabry-Pérot (FP) microcavities have attracted tremendous attention in recent years due to their favorable optical characteristics of the high quality (Q) factor and small mode volume. In this work, we presented a novel approach that utilized the soft lithography and imprinting technology to incorporate the convex micro-lens array structure into the FP (FP-lens) cavity. A strong mode-profile restriction of the micro-lens simultaneously reduced the mode volume and enhanced the Q factor, exhibiting high tolerance to non-parallelism of mirrors compared with that of the plane-plane FP (PP-FP) microcavities. In the experiment, the Q factor of the FP-lens cavity was measured to be 8.145×10⁴, which exhibited a 5.6-fold increase than that of the PP-FP cavity. Furthermore, we experimentally measured the refractive index sensing performance of the FP-lens cavity with the sensitivity of 594.7 nm/RIU and a detection limit of 4.26×10⁻⁷ RIU. On the basis of this superior sensing performance, the FP-lens cavity has the great potential for applications in biosensors.

Latent fingerprints (LFPs) at the crime scene are served as important clues to locate the trajectory of criminal behavior and portray the characteristics of the suspect. Therefore, visualizing LFPs is of considerable significance. In this work, the europium metal-organic framework (Eu-MOF) sensor was successfully constructed for sensitive detection of gallic acid (3,4,5-trihydroxybenzoic acid, GA) and visualization of the sweat LFPs. The boric-acid-modified Eu-MOF was prepared by using the simple one-pot solvothermal method using Eu as the metal ion center and 3,5-dicarboxybenzeneboronic acid (BBDC) as the organic ligand. The sensor showed desirable photoluminescent performance through the chelating of BBDC with Eu³⁺. The sensor exhibited the satisfactory linear relationship to GA in the range of 1 nM to 20 nM with a low detection limit of 0.34 nM under the optimized conditions. The prepared sensor with ideal selectivity to GA was successfully applied for visualizing LFPs on porous substrates with the high contrast and superior stability. Given the good performance of the sensor, all fingerprint images obtained from 1 200 samples presented clear friction ridges and met the identification criteria. Notably, the sensor had less impact on the subsequent deoxyribonucleic acid (DNA) detection, displaying a promising perspective for applications in extracting physical evidence of site investigation.

The illegal water injection into meat not only breaks the market equity, but also deteriorates the meat quality and produces harmful substances. In this work, we proposed a fiber Bragg grating (FBG) sensor that enabled fast, quantitative, and in-situ detection of the moisture content of water-injected meat. The FBG was written in the erbium-ytterbium (Er/Yb) co-doped fiber, which could perform the self-photothermal effect by injecting the near infrared laser into the fiber. As the heated fiber sensor probe was inserted into the meat sample, the temperature decreased due to the heat dissipation mediated by moisture. The intracore Bragg grating could monitor the temperature loss by recording the Bragg wavelength shift, which reflected the water content quantitatively. The results revealed that the sensor could complete the detection within 15 s. The sensor’s sensitivity to detect changes in the pork water content was theoretically calculated to be 0.090847%. The proposed sensor is expected to provide a novel approach for examination of the meat moisture.

A new type of human immunoglobulin G (IgG) sensors based on the surface plasmon resonance (SPR) in the low refractive index (RI) plastic optical fiber (POF) and an antibody immobilization method is presented. A 50-nm-thick gold film was formed on the polished D-shaped fiber surface by magnetron sputtering. The RI response of the POF sensor is 30 049.61 nm/RIU, which is 26.5 times higher than that of single mode fiber (SMF) SPR sensors. The proposed SPR biosensor can be developed by simple and rapid modification of the gold film with 11-mercapto undecanoic acid (MUA). Upon immobilization of the goat anti-human IgG antibody, the resonance wavelength shifts by 11.2 nm. The sensor can be used to specifically detect and quantify the human IgG at concentrations down to 245.4 ng/mL with the sensitivity of 1.327 7 nm per µg/mL, which offers an enhancement of 12.5-fold compared to that of the conventional SMF based SPR sensors. The proposed device may find the potential applications in the case of use at the point of care.

Benefiting from the great advances of the femtosecond laser two-photon polymerization (TPP) technology, customized microcantilever probes can be accurately 3-dimensional (3D) manufactured at the nanoscale size and thus have exhibited considerable potentials in the fields of microforce, micro-vibration, and microforce sensors. In this work, a controllable microstructured cantilever probe on an optical fiber tip for microforce detection is demonstrated both theoretically and experimentally. The static performances of the probe are firstly investigated based on the finite element method (FEM), which provides the basis for the structural design. The proposed cantilever probe is then 3D printed by means of the TPP technology. The experimental results show that the elastic constant k of the proposed cantilever probe can be actively tuned from 2.46 N/m to 62.35 N/m. The force sensitivity is 2.5 nm/µN, the Q-factor is 368.93, and the detection limit is 57.43 nN. Moreover, the mechanical properties of the cantilever probe can be flexibly adjusted by the geometric configuration of the cantilever. Thus, it has an enormous potential for matching the mechanical properties of biological samples in the direct contact mode.

Since the discovery of the extraordinary optical transmission phenomenon, nanohole arrays have attracted much attention and been widely applied in sensing. However, their typical fabrication process, utilizing photolithographic top-down manufacturing technologies, has intrinsic drawbacks including the high costs, time consumption, small footprint, and low throughput. This study presented a low-cost, high-throughput, and scalable method for fabricating centimeter-scale (1×2 cm²) nanohole arrays using the improved nanosphere lithography. The large-scale close-packed polystyrene monolayers obtained by the hemispherical-depression-assisted self-assembly method were employed as colloidal masks for the nanosphere lithography, and the nanohole diameter was tuned from 233 nm to 346 nm with a fixed period of 420 nm via plasma etching. The optical properties and sensing performance of the nanohole arrays were investigated, and two transmission dips were observed due to the resonant coupling of plasmonic modes. Both dips were found to be sensitive to the surrounding environment, and the maximum bulk refractive index sensitivity was up to 162.1 nm/RIU with a 233 nm hole diameter. This study offered a promising approach for fabricating large-scale highly ordered nanohole arrays with various periods and nanohole diameters that could be used for the development of low-cost and high-throughput on-chip plasmonic sensors.

The monitoring of an individual’s hydration levels is a vital measurement required for the maintenance of a healthy skin barrier function as well as the avoidance of dehydration. The current commercial devices for this measure are typically based on electrical methodologies, such as capacitance, which allows for the extraction of skin hydration using the ionic balance deviations in the stratum corneum. The use of optical-based methods such as near-infrared spectroscopy (NIRS) has been recently explored for the measurement of skin hydration. Optical approaches have the ability to penetrate deeper into the skin layers and provide detailed information on the optical properties of present water bands. This paper presents the development of a multi-wavelength optical sensor and its capability of assessing skin hydration in an in vitro experiment utilizing porcine skin. Regression analysis of the results showed to be in line with standard reference measurements (R ² CV=0.952257), validating the accuracy of the developed sensor in measuring dermal water content. A Monte Carlo model of the human skin was also developed and simulated to predict the optical sensor’s performance at variable water concentrations. This model serves as a tool for validating the sensor measurement accuracy. The output from this model gave a standard expectation of the device, which agreed with trends seen in the in vitro work. This research strongly suggests that non-invasive (wearable) NIR based sensors could be used for the comprehensive assessment of skin hydration.

Excessively tilted fiber gratings (ExTFGs) are a type of special optical fiber grating device different from traditional fiber Bragg gratings, long period fiber gratings, and tilted fiber Bragg gratings. Due to the excessively tilted fiber fringe structure in the fiber core, ExTFGs could couple the light of the core mode into the high-order forward-propagating cladding modes, which would split into two sets of polarization dependent modes resulting in dual-peak resonances in the transmission spectrum. ExTFGs have the properties of the high refractive index sensitivity and low thermal crosstalk, which makes them very suitable for biochemical sensing applications. This paper will review the development of ExTFGs in terms of the mode coupling behavior, spectra characteristic, especially the refractive index sensitivity enhancement, biochemical modification methods of the sensor, and their applications in the bio-chemical sensing area, including pondus hydrogenii (pH) heavy metal ions, humidity, glucose, and immune sensing for various animal virus and biomarkers. Moreover, several composite sensing structures based on ExTFGs will be summarized.

In this paper, a theoretical analysis of how the excitation conditions affect the sapphire fiber Fabry-Perot interferometer (SFPI) visibility was performed. The conditions were considered, in which an SFPI was excited by a single-mode fiber (SMF), a multimode fiber (MMF), and a fiber collimator. The finite difference method (FDM) was used to realize the numerical solution of the modal electric fields, and then, the modal excited distributions in the sapphire fiber and the SFPI visibility were calculated. The results showed that different numbers of modes were excited in sapphire fibers under different excitation conditions and finally affected the fringe visibility of the SFPI. The fiber collimator excited the fewest modes and the visibility remained at the highest level. Finally, an experiment was performed, and the experimental results agreed well with the theoretical results.

This study presents the two-dimensional (2D) image of a subsurface structure reconstructed using an imaging method based on the photothermal effect. The photothermal imaging method is based on the deflection method using two lasers: pump and probe lasers. A continuous scanning technique is proposed for 2D (x- and y-directions) surface scanning. The continuous scanning method is compared with the conventional point-by-point scanning technique, and a low-pass fast Fourier transform filter and a Marr-Hildreth detector are found to produce significant results. The photothermal imaging method with continuous 2D surface scanning is performed on three copper-resin double-layer samples with different subsurface structures. The subsurface structures of the copper-resin double-layer samples comprise a square block of 5×5 mm² area and blocks shaped as the alphabet letters “T” and “F”. The letters are 3 mm wide and 10×13 mm² in area. All three shapes are 1 mm thick and located at a depth of 0.5 mm from the surface of the copper block. The reconstructed photothermal images show an absolute error within 0.122 mm compared with the actual subsurface structure, equivalent to a 2.3% relative error.

Since photo-induced polymerization of the ultra-violet (UV)-curing adhesive from a fluid state to a solid state is an exothermic process, the UV curing exothermic behavior can be regarded as a potential evaluation methodology to analyze UV-curing kinetics. Herein, a fiber Bragg grating (FBG)-based UV curing exothermic behavior monitoring is proposed to evaluate the UV-curing dynamic process and analyze a series of thermal and mechanical properties changes during curing. The exothermic behavior of the UV curing adhesive during curing and the feasibility of FBG-based curing kinetic analysis scheme are verified experimentally, full cycle cure monitoring of the UV curing adhesive can be realized by this FBG-based curing kinetic analysis scheme, and the UV-curing kinetics of four different types of the UV curing adhesive are corresponding to different exothermic behaviors. Compared with curing process evaluation based on refractive index variation, this FBG-based exothermic behavior monitoring has the ability to extract more details of the curing process, and some curing stages with negligible refractive index changes also can be distinguished. By using this proposed scheme, the UV-curing dynamic process and multiple characteristic parameters, such as curing time, time constant, transient temperature rise, and residual stress, can be evaluated, which may contribute to evaluating and analyzing UV-curing kinetics more comprehensively.

Herein, an innovative glyphosate imprinted poly(hydroxyethyl methacrylate-N-methacroyl-(L)-phenylalanine methyl ester nanoparticles (MIP@NPs) based plasmonic nanosensor featured with high sensitivity and selectivity was constructed by using the molecular imprinting technique and used for real-time glyphosate detection. The characterization of nanoparticles was performed by the nano Zetasizer and scanning electron microscopy (SEM), while nanosensors were characterized by the Fourier transform infrared-attenuated total reflection (FTIR-ATR) and contact angle measurement. Control experiments were conducted to evaluate the imprinting efficiency on the signal response using a non-imprinted surface plasmon resonance (NIP SPR) nanosensor prepared without adding glyphosate pesticide into the polymerization mixture. The MIP@NPs integrated molecularly imprinted surface plasmon resonance (MIP SPR) nanosensor having synthetic molecular recognition elements yielded a novel biosensing platform for label-free detection and real-time monitoring of glyphosate pesticide. The MIP SPR nanosensor detected the target glyphosate molecule 4.950 times more selectively than the competitor molecule malathion while 3.918 times more selectively than the competitor molecule malaoxon. In addition, the imprinting efficiency factor was found to be 6.76, indicating that the molecular imprinting process was successful. In addition, the imprinting factor was found to be 6.76. Kinetic studies and adsorption characteristics of glycosate adsorption were carried out to assess adsorption dynamics. The linear concentration range for glyphosate detection was 0.001 ppm–10.000 ppm of pesticide, and the detection limit was found to be 0.120 ppb. Studies on the repeatability of the MIP SPR nanosensor revealed that even after five cycles, the signal response for glyphosate detection did not change significantly with relative standard deviation, RSD<1.5 value. The artificial urine selected as the real sample was spiked with glyphosate at a final concentration of 10.000 ppm to evaluate the matrix effect, and the glyphosate amount was reported.

In this paper, a graphene oxide (GO) composite film-coated humidity sensor is proposed based on the hollow core fiber (HCF). A segment of the HCF is spliced between two segments of the single-mode fiber (SMF). The relative humidity (RH) sensing characteristics of the sensor are experimentally investigated by observing the intensity shift of resonant dips in the transmission spectrum, which shows the GO composite film-coated HCF has the good stability in the measurement of humidity. The maximum humidity sensitivity of 0.12 dB/%RH is obtained in the RH range of 30%–78%. The proposed sensor has the advantages of the simple structure, easy fabrication, good stability, and high performance, which can be applied to marine climate detection, tunnel air humidity detection, agricultural testing, and other fields.

A novel optical fiber hydrogen sensor based on the π-phase-shifted grating and partial coated Pd/Hf composite film is proposed and experimentally demonstrated in this paper. The hydrogen sensitive Pd/Hf film with the length of 4 mm is successfully deposited in the π-phase-shifted grating region by the magnetron sputtering process and rotating fixture technology. Since the hydrogen sensitivity between the notch and flank wavelengths of the π-phase-shifted grating is different due to the partial coating only on the π-phase-shifted grating region, the relative shift between the notch and flank wavelengths is employed to characterize the hydrogen concentration in this paper. The hydrogen calibration results show that the sensor shows the good response and repeatability. At the temperature of 20 °C and the hydrogen concentration of 2%, the wavelength distance shifts of 200 nm and 500 nm Pd/Hf coatings are 12.6 pm and 33.5 pm, respectively.

This paper investigates the use of artificial neural networks (ANNs) as a viable digital twin or alternative to the typical whispering gallery mode (WGM) optical sensors in engineering systems, especially in dynamic environments like robotics. Because of its fragility and limited endurance, the WGM sensor which is based on micro-optical resonators is inappropriate in these kinds of situations. In order to address these issues, the paper suggests an ANN that is specifically designed for the system and makes use of the WGM sensor’s high-quality factor (Q-factor). By extending the applicability and endurance to dynamic contexts and reducing fragility problems, the ANN seeks to give high-resolution measurement. In order to minimize post-processing requirements and maintain system robustness, the study goal is for the ANN to function as a representative predictor of the WGM sensor output. The GUCnoid 1.0 humanoid robot is used in the paper as an example to show how the WGM optical sensors may improve humanoid robot performance for a variety of applications. The results of the experiments demonstrate that the sensitivity, precision, and resolution of ANN outputs and actual WGM shifts are equivalent. As a consequence, current obstacles to the widespread use of high-precision sensing in the robotics industry are removed, and the potential of ANNs as virtual substitutes or the digital twin for genuine WGM sensors in robotics systems is validated. So, this paper can be very beneficial not only to the sensing technologies that are used in robotics, which are subjected to the dynamic environments, but also to the industrial automation and human-machine interface.

We proposed two schemes of generating and localizing dynamic gratings in optical fibers: one is based on the gain saturation in erbium-doped fiber; the other is based on Brillouin scattering in the fiber. By using these dynamic gratings, fully distributed strain/temperature sensors have been demonstrated. In this presentation, we review the principles, basic schemes, and experimental demonstrations of the novel dynamic grating techniques.

The fiber Bragg grating (FBG) sensing technology was used to monitor the situation of a crevice of the continuous beam joint and rails near rail expansion devices on a viaduct of the urban railway. The monitoring items consisted of the rail temperature, rail displacement, viaduct beam displacement, and strain of sliding rail in the rail expansion device section. The strain sensor was a prefabricate FBG strain gauge, the displacement sensor with different scales used an FBG stress ring, and the FBG of the temperature sensor was pre-drawn and fixed in a metal tube. Compensation sensors were used to balance environmental temperature changes. All FBGs were suspended adhered, therefore the chirped phenomenon of the FBG reflection peak was avoided, and the measurement accuracy was improved. The monitoring results matched to the manual test and theoretical estimation.

A simple and compact optical fiber directional bending vector sensor with simultaneous measurement of temperature based on the Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The device consists of a piece of photonic crystal fiber (PCF) sandwiched between two single mode fibers (SMFs) with a lateral offset splicing. It shows the capacity for recognizing positive and negative directions. Within a curvature range of -7.13 m^-1 to 7.13 m^-1, the bending sensitivities of two resonant dips with opposite fiber orientations are obtained to be 0.484 nm/m^-1 and 0.246 nm/m^-1, respectively. This simple MZI is formed by invoking interference between LP₀₁ and LP₂₁ core modes, which leads to that the sensor is not sensitive to ambient refractive index (ARI). The temperature sensitivity has also been investigated. Two dips have obviously different sensitivities on the temperature and bending, so two parameters of both curvature and temperature can be distinguished and measured simultaneously by constructing a matrix and using one simple model interferometer.

A magneto-optical sensor, using a dual quadrature polarimetric processing scheme, was evaluated for current metering and protection applications in high voltage lines. Sensor calibration and resolution were obtained in different operational conditions using illumination in the 1550-nm band. Results obtained indicated the feasibility of interrogating such sensor via the optical ground wire (OPGW) link installed in standard high power grids. The polarimetric bulk optical current sensor also was theoretically studied, and the effects of different sources of error considering practical deployment were evaluated. In particular, the interference from external magnetic fields in a tree-phase system was analyzed.

A spectral calibration technique, a data processing method and the importance of calibration and re-sampling methods for the spectral domain optical coherence tomography system were numerically studied, targeted to optical coherence tomography (OCT) signal processing implementation under graphics processing unit (GPU) architecture. Accurately, assigning the wavelength to each pixel of the detector is of paramount importance to obtain high quality images and increase signal to noise ratio (SNR). High quality imaging can be achieved by proper calibration methods, here performed by phase calibration and interpolation. SNR was assessed employing two approaches, single spectrum moving window averaging and consecutive spectra data averaging, to investigate the optimized method and factor for background noise reduction. It was demonstrated that the consecutive spectra averaging had better SNR performance.