Fiber optic sensors have been gradually used in aerospace, petrochemical, electronic power, civil engineering, and biomedical fields because of their many advantages such as the anti-electromagnetic interference, corrosion resistance, light weight, small size, high accuracy, and easy reuse. In recent years, sensing and demodulation technologies based on microwave photonics have attracted widespread attention. Optical fiber sensing combined with microwave photonics has higher sensitivity and flexibility, which is important for the demodulation of interferometric signals. This article introduces and analyzes the principle, structure, and performance of the demodulation technology of fiber optic interferometric signals based on microwave photonics from the perspective of system structures, such as filters, oscillators, and interferometers, and discusses the future research and development directions.
A large number of slopes appear along the line during railway construction, which will pose a threat to railway safety operation. Slope monitoring plays an important role in ensuring the safety of railway operation. Aiming at the difficulties of sensor multiplexing, low accuracy, and large disturbance by trains, this paper proposes a railway slope monitoring method based on integrated fusion detection of inclination and vibration. Instability and failure characteristics of the K3 slope in Shuohuang Railway and dynamic characteristics under the excitation of the train load are analyzed by the finite element method (FEM) analysis. Based on the above analysis, a slope monitoring system is established utilizing the self-developed dual-parameter fiber Bragg grating (FBG) sensor. The monitoring data of the past four years show that the slope is in a relatively stable state at present. The monitoring data are consistent with the results of the FEM. The feasibility of the damage identification method based on inclination and vibration characteristics is verified, which provides a new method for railway slope monitoring.
Glucose is an indispensable nutrient for metabolism in living organisms and is widely used in food, industry, and medical fields. Glucose is often added as a sweetener in food and often used in industry as a reducing agent for various products. In medical treatment, glucose is added to many drugs as a nutritional additive, and it is also an indicator that diabetics need to pay attention to at all time. Therefore, the market has a great demand for low-cost, high-sensitivity, fast, and convenient glucose sensors, and the industry has always attached great importance to the work of creating new glucose sensor devices. Therefore, we proposed a SnO2 nanofibers/Au structure multimode-single-mode-multimode (MSM) fiber surface plasmon resonance (SPR) glucose sensor. SnO2 nanofibers were fixed to a single-mode fiber core that had been plated with the Au film by electrospinning. When the glucose concentration increased at 5 vol% intervals, the corresponding resonance wavelengths had different degrees of redshifts. Comparing the two structures, as the glucose concentration range increased from 0 vol% to 60 vol%, the sensitivity increased from 228.7 nm/vol% in the Au structure to 337.3 nm/vol% in the SnO2 nanofiber/Au structure. At the same time, the linear correlation between the resonant wavelength and the refractive index of the two structures was greater than 0.98. Moreover, the SnO2 nanofibers/Au structure significantly improved the practical application performance of SPR sensors.
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.
We report the numerical and experimental studies of the two-dimensional Brillouin gain spectrum (BGS) distribution deformation induced by the self-phase modulation in the Brillouin optical time domain reflectometry (BOTDR) with a 20.6 km sensing distance. The BGS distribution deformation is investigated by analyzing the evolution of the point spread function along the fiber in the two-dimensional model of the BOTDR. In the simulation and experimental results, the specific deformation degree of the BGS distribution induced by the self-phase modulation is related to the pump pulse profile, pump pulse peak power, BGS demodulation method, and detected scattered light component. By comprehensively analyzing the evolution of the point spread function induced by the self-phase modulation and using the image deconvolution, a typical BOTDR sensor with a 25 ns pump pulse reaches the 20 cm spatial resolution over the 20.6 km sensing fiber.
In the paper, an optical fiber sensor based on a seven-core fiber composite structure is presented, which enables dual-parameter sensing of bending and temperature. The proposed structure is fabricated by combining the strongly-coupled seven-core fibers (SC-SCFs) and a weakly-coupled seven-core fiber (WC-SCF). The SC-SCF acts as a beam coupler and enhances the Mach-Zehnder interference, while the WC-SCF serves as the enhanced section of another Mach-Zehnder interference. Therefore, the spectrum response of the fiber structure mentioned above exhibits a superposition effect of two Mach-Zehnder interferometers (MZIs). Among them, two dips corresponding to different MZIs are used to measure bending and temperature. The experimental results show the bending sensitivity and temperature sensitivity of the two MZIs are −4.238 nm/m−1, −2.263 nm/m−1, 0.047 nm/°C, and 0.064 nm/°C, respectively. It proves that our sensor is very sensitive to bending. Through the dual-wavelength matrix method, the bending and temperature can be measured simultaneously. With the benefit of the composite structure, low cost, and ease of fabrication, the proposed sensor can be used in harsh environments.
This study presented a surface-functionalized sensor probe using 3-aminopropyltriethoxysilane (APTES) self-assembled monolayers on a Kretschmann-configured plasmonic platform. The probe featured stacked nanocomposites of gold (via sputtering) and graphene quantum dots (GQD, via spin-coating) for highly sensitive and accurate uric acid (UA) detection within the physiological ranges. Characterization encompassed the field emission scanning electron microscopy for detailed imaging, energy-dispersive X-ray spectroscopy for elemental analysis, and Fourier transform infrared spectroscopy for molecular identification. Surface functionalization increased sensor sensitivity by 60.64%, achieving 0.0221 °/(mg/dL) for the gold-GQD probe and 0.035 5 °/(mg/dL) for the gold-APTES-GQD probe, with linear correlation coefficients of 0.8249 and 0.8509, respectively. The highest sensitivity was 0.070 6 °/(mg/dL), with a linear correlation coefficient of 0.993 and a low limit of detection of 0.2 mg/dL. Furthermore, binding affinity increased dramatically, with the Langmuir constants of 14.29 µM−1 for the gold-GQD probe and 0.000 1 µM−1 for the gold-APTES-GQD probe, representing a 142 900-fold increase. The probe demonstrated notable reproducibility and repeatability with relative standard deviations of 0.166% and 0.013%, respectively, and exceptional temporal stability of 99.66%. These findings represented a transformative leap in plasmonic UA sensors, characterized by enhanced precision, reliability, sensitivity, and increased surface binding capacity, synergistically fostering unprecedented practicality.
In producing high-performance optical biosensors, the selected coupling agent and its fixation mode play an essential role as one of the decisive conditions for antibody incubation. In this work, we designed optical fiber biosensors by electrochemical polymerization to enable low detection limit (LOD) immunoassay. Based on the optical fiber lossy mode resonance (OF-LMR) achieved by In2O3-SnO2-90/10 wt% (ITO), we have simultaneously implemented the electropolymerized dopamine (ePDA) film on the ITO-coated fiber via the electrochemical method, utilizing the excellent electrical conductivity of ITO. After that, the immunoglobulin G (IgG) antibody layer was immobilized on the entire sensing region with the assistance of the ePDA film. The results of immunoassay were analyzed by recording the shift of the LMR resonance wavelength to verify the sensor performance. The LOD was evaluated as the lowest concentration of human IgG detected by the OF-LMR sensor, which was confirmed to be 4.20 ng·mL−1. Furthermore, the sensor achieved selective detection for specific antigens and exhibited a good recovery capability in chicken serum samples. The developed scheme provides a feasible opportunity to enhance the intersection of electrochemistry and optics subjects and also offers a new promising solution to achieve the immunoassay.
Optical fiber sensors have gained significant attention in recent years owing to their remarkable advantages of remote operation and rapid response. The integration of optical fiber sensing with the microfluidics technology has paved the way for the establishment of optical fiber optofluidic sensing. Optical fiber optofluidic systems possess the advantages of the low invasiveness, compact structure, excellent biocompatibility, and the ability to handle small analyte volumes, rendering them particularly suitable for serving as chemical sensors and biosensors. In this paper, we present an in-depth overview of optical fiber optofluidic chemical sensors and biosensors. Firstly, we provide a comprehensive summary of the types of optical fibers commonly employed in optofluidic chemical and biosensing, elucidating their distinct attributes and performance characteristics. Subsequently, we introduce and thoroughly analyze several representative sensing mechanisms employed in optical fiber optofluidic systems and main performance parameters. Furthermore, this review delves into the modification and functionalization of optical fibers. Additionally, we showcase typical biosensing and chemical sensing applications to demonstrate the practicality and versatility of optical fiber optofluidic sensing. Finally, the conclusion and outlook are given.
Acetone is a widely used volatile organic compound in various industries, and several gas sensors have been developed for its detection and real-time monitoring. This study reported a novel method for determining the acetone vapor concentration based on correlated laser speckles using polymer-dispersed liquid crystals (PDLCs). Here, PDLC films comprising a mixture of the thermotropic nematic liquid crystal (LC) and ultraviolet-curable polymers were fabricated using different LC mass ratios and ultraviolet curing conditions. The laser beam was transmitted through the PDLC film to generate scattered light and speckles. When the PDLC film was exposed to the acetone vapor, the acetone molecules diffused into the PDLC film and interacted with the LC molecules, modifying the orientation of the LC molecules and the equivalent refractive index of the LC droplets. This in turn decreased the correlation coefficient of the speckle images. The experimental results indicated that the PDLC gas sensor was selectively sensitive to different concentrations of the acetone vapor, ranging from 1 800 ppm to 3 200 ppm. In comparison with traditional LC gas sensors that use a polarizing microscope to detect the change in brightness of the modulated light field, the proposed method is simpler, less expensive, and more robust under external disturbances such as vibrations.