Accurate real-time monitoring of moisture and wear debris in lubricating oil is critical for ensuring the stable operation and extended service life of industrial machinery. This paper proposes a dual-parameter optical fiber sensor based on a tapered photonic crystal fiber (PCF) structure cascaded with a fiber Bragg grating (FBG). The sensor exploits the strong evanescent field interaction between the tapered PCF region and the lubricating oil. By monitoring the wavelength shifts of envelope 1 and interference dip 2 in the transmission spectrum, simultaneous detection of moisture (0-1%) and iron-based wear debris (0-100 μg‧mL-1) is achieved. Experimental results demonstrate moisture sensitivity of 28.61 nm/% and wear debris sensitivity of 0.212 nm/(μg‧mL-1), with dual-parameter decoupling relative errors below 5%. A cascaded FBG enables temperature compensation over the range of 40-65 °C, enhancing measurement reliability under varying thermal conditions. The proposed sensor features straightforward fabrication and robust anti-interference capability, offering an effective solution for multiparameter online monitoring of industrial lubricating oil.

Aiming to address the limitations of existing fiber-optic respiratory sensors in acquiring real-time respiratory rate and amplitude measurements, this paper proposes a respiratory sensing scheme based on a transverse multi-fiber coupling structure. The proposed system converts respiration-induced stretching into variations in light intensity within an array of emergent optical fibers. An image sensor is applied to convert them into a multi-channel timesequence signal of light intensity. A stepper motor was employed to simulate breathing-induced stretching, and experimental results demonstrate that the respiratory rate can be accurately obtained using a peak-detection algorithm, with a deviation of less than 4%. Furthermore, a gray value integration method was developed to precisely measure respiratory amplitude, achieving a linearity of 0.993. A comparative analysis of respiratory waveforms between the proposed sensor and a reference sensor shows correlation coefficients of 0.9501 and 0.9302 for respiratory frequency and amplitude, respectively. Bland-Altman analysis indicated a mean difference of -0.03082 bpm in respiratory rate. In experiments where volunteers simulated abnormal respiratory patterns, the waveform output by the sensor could represent alterations in respiratory rate and amplitude, providing a feasible sensing scheme for abnormal respiration detection. Although the respiratory waveform during motion differs from that in static conditions, the proposed sensor can still successfully extract both respiratory rate and amplitude information. This demonstrates the sensor's strong adaptability and notable resistance to motion artifacts. The sensing scheme shows strong potential to provide reliable data support for home-based health management and early intervention in respiratory-related chronic diseases.

This paper proposes an anti-resonant hollow-core terahertz fiber with an eight-set, three-layer nested circular tube structure. The bending characteristics of the fiber are studied using the finite element method. By selectively removing certain nested tubes and the cladding on the side opposite to the bending direction, an open structure is constructed. By comparing the transmission characteristics of the open structure and the closed structure under bending conditions, the study demonstrates that the proposed open-structure design enables stable terahertz wave transmission under appropriate bending radius while exhibiting lower bending loss, with a minimum value of 0.031 dB/m. Moreover, the open-structure fiber shows improved single-mode transmission performance. Compared with conventional closed-structure terahertz fibers, the proposed open-structure fiber allows external substances to directly access the sensing region from the side, thereby facilitating analyte infiltration and improving practical applicability. Meanwhile, the proposed structure can still provide satisfactory sensing performance, with a maximum relative sensitivity of 99.98%. The practical feasibility of this hollow-core anti-resonant fiber is supported by its compatibility with existing fabrication techniques.

While quasi-bound states in the continuum (Q-BICs) enable vivid structural colors, their performance is often compromised by higher-order resonances at short wavelengths that limit spectral purity. This paper presents a dual-layer all-dielectric metasurface that overcomes this challenge through refractive index matching. The design not only suppresses short-wavelength resonances but also enhances the far-field coupling of the main resonance. It achieves continuous hue and brightness tuning across the visible spectrum, delivering colors with exceptional monochromaticity (<5 nm FWHM), high saturation (>90%), and a wide gamut covering 150.2% of sRGB. This study provides a robust solution for advanced display, data storage, and anti-counterfeiting applications.

This paper proposes a graded-index few-mode fiber design featuring a low-index central region within the core, aimed at achieving weakly coupled transmission of six spatial modes. The design addresses signal degradation in mode-division multiplexing systems caused by inter-mode crosstalk and differential mode delay (DMD). Through finite-element simulation, the radius and refractive index contrast of the low-index core region are optimized, resulting in an effective index difference greater than 0.001 between the first five adjacent mode groups, which ensures effective weak coupling. At 1550 nm, all supported modes exhibit a mode-field area exceeding 200µm2, effectively suppressing nonlinear effects. The dispersion characteristics across the C + L bands are analyzed, showing DMD values below 6 ps/km, indicating excellent dispersion performance. Furthermore, the introduction of a trench-assisted cladding structure significantly reduces bending loss, enhancing the fiber's suitability for long-haul transmission. The results demonstrate that the proposed fiber offers favorable transmission performance across the C + L bands, meeting the requirements of high-capacity MDM optical networks.

Diffractive waveguide architectures employing surface-relief gratings (SRGs) have been an active topic in the field of near-eye displays, where the design of SRGs offers significant research potential. This paper proposes a composite rectangular SRG structure formed by the spatial superposition of multiple first-order rectangular grating profiles, yielding a single composite relief structure. This configuration enables different wavelengths to experience more consistent energy-coupling behavior within the grating, thereby achieving uniform diffraction efficiency across the RGB bands and stable polarization response. The grating is systematically analyzed and optimized using rigorous coupled-wave analysis (RCWA) combined with a particle swarm optimization (PSO) algorithm. The results show that the polarization uniformity error is only 9.3%, and the uniformity error of the polarization-averaged diffraction efficiency is merely 4.3%. Over a wide field of view of 20 degrees, uniform diffraction efficiency is maintained for RGB wavelengths, demonstrating good wavelength multiplexing capability and suitability for uniform light propagation in a single-layer waveguide. Furthermore, even with grating height and width deviations within 20 nm, the efficiency fluctuation remains below 5%, indicating good fabrication tolerance and practical manufacturability.

Abstract A novel design of tunable hybrid plasmonic waveguide in the terahertz (THz) regime is present based on three-dimensional Dirac semimetals (3D DSM) with wedge-shaped structure. The propagation characteristics at a broad band from 0.5 THz to 3.3 THz are systematically investigated considering the structural parameters and Fermi energy of DSM, in terms of the normalized mode area, propagation length, figure of merit, and mode field distributions. By tuning the Fermi energy of DSM from 0.03 eV to 0.15 eV, the propagation length of the hybrid plasmonic waveguides could be effectively modulated with a modulation depth more than 95%. The crosstalk between adjacent waveguides is evaluated by analyzing the symmetric/anti-symmetric modes coupling process, which demonstrates the effective crosstalk suppression requires a minimum center-to-center separation of 60 μm. These results not only provide valuable insights for understanding the propagation mechanism of 3D DSM hybrid plasmonic waveguides, but also open new avenues for the applications of tunable and broadband THz waveguides devices.

This paper proposes a respiration monitoring mattress based on a plastic optical fiber(POF) end-face coupling structure. The pressure variations caused by respiration lead to deformations in an elastic transparent tube, which in turn alter the separation distance and angle between the end-faces of two POFs, consequently changing the coupling efficiency at the fiber end-faces. The output optical power is then converted into a voltage signal by a photodetector, providing real-time output of respiratory waveforms. Experiments demonstrate a strong linear relationship between output loss and applied pressure (R-2 > 0.99), enabling precise tracking of respiratory dynamics and quantification of their amplitudes. Dynamic testing within the frequency range of 0.2-0.33 Hz achieved frequency detection with an error of less than 8 %. Results indicate that peak detection algorithm can extract respiratory frequency cycle by cycle, realizing real-time acquisition of respiratory rate. After embedding the sensor into a mattress, comparative validation is conducted against a reference optical fiber respiratory belt, and Bland-Altman analysis revealed a mean difference of 0.0558 bpm, with mean absolute error (MAE) at 0.48 bpm. Finally, subjects simulated abnormal states such as apnea, sudden frequency changes, and amplitude variations. Results confirm that rapid and accurate detection can be achieved through information on respiratory rate and amplitude, thus providing comprehensive respiratory monitoring data.

As the two-dimensional counterpart of metamaterials, metasurfaces have demonstrated exceptional potential in optics due to their remarkable ability to flexibly manipulate electromagnetic waves, offering a platform for multidimensional wavefront manipulation. However, existing multiplexing techniques predominantly focus on a single physical dimension, and multi-parameter joint multiplexing still faces challenges such as limited design degrees of freedom and channel crosstalk. This paper proposes a wavelength-polarization multiplexed terahertz metadevice based on anisotropic meta-atoms, which enables independent wavefront manipulation of three-band responses under orthogonal linear polarizations within a single-layer structure, thereby creating six fully decoupled transmission channels. We designed and numerically validated a dual-polarization triple-frequency decoupled metalens, a dual-polarization achromatic vortex beam generator, and a six-channel holographic display device. This research provides a compact and efficient solution for multidimensional optical field manipulation, with promising applications in terahertz communications, high-capacity information encryption, and holographic displays.

With the increasing demand for precise polarization control, metasurfaces composed of subwavelength structures emerge as a core approach due to their remarkable advantages in compactness, multifunctionality, and integrability. They show strong potential in advanced photonic applications such as optical communications, vector beam shaping, and quantum imaging. This review systematically summarizes recent progress in metasurface‐based polarization manipulation across 2D real space, 3D real space, and momentum space. In 2D control, resonant phase, Pancharatnam–Berry (PB) phase and hybrid‐phase design enable modulation of polarization, amplitude, and phase of scalar and vector beams, laying the groundwork for high‐dimensional optical field modulation. In 3D control, both spin‐decoupling and non‐spin‐decoupling mechanisms realize continuous evolution of longitudinal polarization states, polarization trajectory engineering, and dynamic modulation—advancing metasurface devices from planar polarization elements toward volumetric optical field modulators. Momentum‐space control primarily focuses on bound states in the continuum (BICs) and quasi‐BICs (qBICs), where chiral qBICs exhibit unique advantages in circular polarization and narrowband high‐ Q responses. This review outlines design strategies and physical mechanisms of representative metasurface‐based polarization control methods, highlighting recent advances in functional integration, dimensional expansion, and reconfigurable responses.

Surface-enhanced Raman scattering (SERS) technology, leveraging its single-molecule-level detection sensitivity, molecular fingerprint recognition capability, and capacity for rapid, non-destructive analysis, has emerged as a pivotal analytical tool in food science, life sciences, and environmental monitoring. This review systematically summarizes recent advancements in SERS technology, encompassing its enhancement mechanisms (synergistic effects of electromagnetic and chemical enhancement), innovations in high-performance substrates (noble metal nanostructures, non-noble metal substrates based on semiconductors/graphene, and hybrid systems incorporating noble metals with functional materials), and its interdisciplinary applications. In the realm of food safety, SERS has enabled the ultratrace detection of pesticide residues, mycotoxins, and heavy metals, with flexible substrates and intelligent algorithms significantly enhancing on-site detection capabilities. Within biomedicine, the technique has been successfully applied to the rapid identification of pathogenic microorganisms, screening of tumor biomarkers, and viral diagnostics. For environmental monitoring, SERS platforms offer sensitive detection of heavy metals, microplastics, and organic pollutants. Despite challenges such as matrix interference and insufficient substrate reproducibility, future research directions aimed at developing multifunctional composite materials, integrating artificial intelligence algorithms, constructing portable devices, and exploring plasmon-catalysis synergy are poised to advance the practical implementation of SERS technology in precision diagnostics, intelligent regulation, and real-time monitoring.

Polymer optical fibers (POFs) are ideal for tactile sensing due to their flexibility, durability, and ease of integration. Optimizing POF-based sensors requires precise control of design parameters, such as polydimethylsiloxane (PDMS) mixing ratio, sensitive region length (SRL), sensitive region depth (SRD), Sanding Grit Size (SGS), and embedding depth. These parameters significantly affect linearity, sensitivity, and hysteresis, which are crucial for accurate tactile measurements. This study presents a comprehensive analysis of the interplay between these design parameters and their impact on sensor performance. A sensing pad was fabricated for each set of experiments by embedding POF in PDMS, with a cutout-sensitive region produced using the Targeted Precision Mold-Guided Abrasive Profiling fabrication method for accurate and reproducible sensitive region parameters. In the presence of applied force, surface scattering loss increases, significantly amplifying the sensor's output signal. Experimental results show strong correlations between design parameters and performance metrics modeled with first- or second-order polynomials. Using the NSGA-II multiobjective optimization algorithm, the exact optimal parameters were determined as a PDMS mixing ratio of 100:10, SRL of 15 mm, SRD of 0.66 mm, SGS of 320 Cw, and an embedding depth of 4.34 mm. This optimized configuration achieved high linearity (R 2 approximate to 0.9938), high sensitivity (78.25 mV/N), and low hysteresis (1%) across the force range (0-26 N). Performance remained stable within the optimal range (0-16 N), with slight degradation at higher forces due to strain saturation. While strain saturation limits performance beyond 16 N, this range suffices for most tactile sensing applications. This work provides valuable insights for optimizing tactile sensors with applications in robotics, healthcare, and industrial automation.

Respiratory sensing based on multi-fiber-optic end-face coupling is introduced, the proposed sensor is designed to capture variation of chest circumference due to breathing by measuring changes in fiber-optic end-face coupling efficiency, thereby enabling precise retrieval of respiratory waveforms. The proposed breathing sensors are fabricated utilizing 3D printing technology. Experimental results demonstrate that a tri-fiber end-face configuration exhibits high sensitivity and linearity for the displacement measurement, achieving a high sensitivity of up to 1.28 dB/mm. The linear response, hysteresis, and repeatability are 2.701 %, 3.353 %, and 3.131 %, respectively. A range of respiratory signal types are simulated via stepper motors, and the real-time capture of respiratory rate and respiratory amplitude data are demonstrated by utilizing an extreme value search algorithm. The results demonstrate a robust linear correlation between the measured amplitude of respiratory signals and chest circumference, enabling an accurate representation of the waveform characteristics of respiratory signals. This scheme can achieve full-waveform detection of respiratory signals, allowing effective assessment of respiratory status and early warning of abnormal breathing patterns based on the waveform information.

In the fields of biomedical engineering and intelligent sensing, accurate detection of joint flexion angles and directions holds significant application value. A bending angle detection scheme based on the coupling of single- fiber and multi-fiber within a tube is proposed to achieve simultaneous detection of bending angle and direction. The experimental results show that the output optical power of the optical fiber changes with the bending of the tube, and there is still optical output at the bend angle of 180 degrees. Additionally, the output optical power of the output fiber is correlated with the bending direction, with lower optical loss observed for the output fiber located at the-x direction of the fiber core center. Therefore, the bending direction of the tube can be determined by comparing the output optical loss between fibers. On the other hand, the bending angle can be obtained from the total loss of all output fibers. Experimental results indicate that the sensor possesses high sensitivity of up to 0.181dB/degrees for the angle range from 0 degrees to 180 degrees, and a linear regression coefficient R2 of 0.993. Additionally, experiments conducted with motors and finger joints have demonstrated the excellent reversibility and rapid response capabilities of the proposed angular sensor, thereby indicating its potential as an effective solution for precise monitoring of joint movements.

Flexible manipulation and efficient detection of terahertz wave polarization states remain critical challenges for advancing terahertz technologies in communications,biomedicine,and environmental monitoring.This study innovatively proposes an all-dielectric metadevice integrated platform that achieves coordinated polarization gen-eration and detection through multidimensional polarization-channel multiplexing.The proposed device inte-grates four polarization-conversion functional units into a single planar architecture,enabling synchronous generation of four independent polarization channels under unpolarized incidence while establishing direct cor-relations between their intensity profiles and the polarization state of incident waves.Leveraging the Stokes parameter framework,we develop a high-efficiency detection mechanism that precisely reconstructs incident polarization states utilizing only four-channel intensity information,overcoming the limitations of conventional multi-component cascaded polarization detection systems.Experimental results demonstrate the device's excep-tional polarization-resolving capability at 0.9 THz.Furthermore,the platform exhibits significant potential for multidimensional optical field manipulation,successfully generating four-channel polarization-vortex beams with independent topological charges.This compact multichannel polarization-control strategy opens new ave-nues,to our knowledge,for developing portable terahertz imaging systems,high-capacity communication devices,and multidimensional information encryption technologies,thereby accelerating the practical implementation of terahertz science and applications.

Terahertz (THz) technology represents a frontier at the intersection of photonics and electronics, with unique capabilities for next-generation communications, bio-imaging, and quality monitoring. Recently, THz waveguides have made significant advancements, which are pivotal for realizing compact and lightweight THz systems. However, most past review articles have demonstrated the THz waveguides in terms of their materials or applications. In this review, we focus on the development and application of two principal THz waveguide types: metal and polymer. Our methodology includes a thorough examination of their characteristics, working principles, and performance metrics. The novelty of this review lies in its integrative approach, bridging the gap between theoretical developments and practical implementations. Major findings highlight the superior low-loss performance of metal waveguides for high-power applications and the flexibility of polymer waveguides for integrated systems. Finally, we discuss the potential prospects of next generation THz waveguides, emphasizing the need for materials with lower absorption coefficients and the development of active THz devices.

Lubricating oil contaminants represent a major threat to mechanical equipment, and its sensitive and accurate detection is highly demanded in faults diagnosis and health condition monitoring. Here, a fiber-optical sensor consisting of Mach-Zehnder interferometer (MZI) and fiber Bragg grating (FBG) is proposed and experimentally demonstrated for simultaneous measurement of water content and wear debris in lubricating oil. A pair of MZIs constructed by misaligned welding single mode fibers is introduced to generate optical Vernier effect and enhance the detection sensitivity. The envelopes evolution of transmission spectra in response to varying concentrations of water and wear debris are measured, giving rise to the maximum sensitivities of 1.3043 dB/% and 0.0136 dB/mu gmL(-1), respectively. Most importantly, the temperature disturbance to the dual parameter sensing performance is analyzed, which could be compensated by cascading an FBG after the sensing MZI. Hence, our developed fiber-optical sensor based on cascaded MZIs with Vernier effect and FBG paves a new way toward developing novel lubricating oil contaminants sensors and related optofluidic devices.

A respiratory signal sensing scheme based on plastic optical fiber and smartphone is proposed. Respiratory signals are achieved by detecting the changes in abdominal circumference caused by the inhalation and exhalation of air from the lungs during breathing. A flexible clamp is prepared based on the 3D printing technology and the changes in abdominal circumference are leading to the lateral distance variation between the end face of the two plastic optical fibers in the clamp. The experiment demonstrates the scheme’s capability to detect a broad range of stable respiratory frequencies and weak signals. Finally, an automatic multi-scale peak detection algorithm is utilized to achieve real-time display of transient respiratory rate, which enables the timely feedback of apnea and abnormal signals. The proposed respiratory sensing system provides an efficient, accurate, convenient and low-cost solution for human physiological signal monitoring.

A compact fiber-optical curvature sensor based on the hollow-core fiber (HCF) assisted microbubble Mach–Zehnder interferometer (HMZI) is proposed and experimentally demonstrated. Transmission spectral characteristics of the proposed curvature sensor have been investigated with a maximum intensity-interrogated sensitivity of − 1.48 dB/m−1 in a curvature range from 1.22 m−1 to 3.46 m−1. The temperature-induced instability is measured below 0.30 dB in a large temperature range of 35 °C to 60 °C, suggesting a good temperature resistance performance. The overall sensing size is controlled to be around 600 μm. Hence, the HMZI curvature sensor has several desirable merits such as wide measurement range, temperature insensitivity, and ease of integration, which make it a promising candidate in curvature-related mechanical engineering and structural health monitoring applications.