A thin-diameter photonic crystal fiber (TDPCF) magnetic field sensor based on hole collapse and infiltration is proposed and demonstrated. Due to the controlled hole collapse in TDPCF, the interaction between the mode field distribution and the magnetic fluid (MF) is effectively strengthened, causing the direct absorption and scattering of magnetic nanoparticles to transmitted light. Consequently, the proposed sensor exhibits a magnetic field sensitivity of up to 0.099 dB/Oe within the range of 0 to 60 Oe. The TDPCF-based magnetic field sensor has several desirable merits such as high sensitivity, convenient integration, and good stability, which provides a promising candidate in environmental and industrial-related magnetic field monitoring applications.

Staphylococcus aureus (S. aureus) represents a major threat to global health, and its sensitive and accurate detection is highly demanded in disease diagnosis and food safety. Here, a flexible terahertz (THz) metasurface based on bound states in the continuum (BIC) is proposed and experimentally demonstrated for the trace analysis of S. aureus. The unit cell structure incorporates a metallic square with two opening gaps, supporting a resonance transition from BIC to quasi-BIC states by introducing a symmetry perturbation to one opening gap location. Firstly, the refractive index (RI) spectra of the drip-dried S. aureus solution at different concentrations are characterized using THz-TDS. The frequency point possessing a maximum linearity between RI and S. aureus concentrations is selected as the resonance frequency of designed sensing quasi-BIC. Moreover, an experimental demonstration of the sensing performance is conducted by monitoring spectral evolution after target loading. The measured results indicate that the constructed biosensor exhibits a detection limit down to 200 cfu/mL and RI sensing sensitivity up to 267.1 GHz/RIU for S. aureus measurement. Hence, the flexible, easy manufacturing, and highly-sensitive quasi-BIC metasurface biosensor paves a new way toward developing novel bacteria sensors and BICs-related optoelectronic devices.

The design of a bending angle sensor consisting of plastic optical fibers embedded in transparent tubing is proposed for the detection of the angle of human joints. Highly sensitive sensing is achieved by the bending of the transparent tubing between the two fiber end faces which is functioning as a hollow-core optical fiber with the air inside the tubing as the core and the tubing as the cladding. Simulation results based on the ray tracing method show that when the transparent tubing was bent, both end-face coupling and internal reflection mechanisms coexisted in the beam transmission process, with reflection in the tubing being the main mechanism at a large bending angle. A motor-driven angle measurement system is constructed to achieve accurate control of the bending angle. Experimental results show that the sensor can achieve ultra-wide-angle detection ranging from 0 to 180° with a high sensitivity of 0.136 dB/° and a linear regression coefficient (R ² ) of 0.997. Finally, the dynamic response characteristics of the sensor are tested on both motor-controlled bending and joint bending, and the results show fast response, good repeatability, and stability of the proposed sensor.

Lubricating oil contaminants represents 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 consists 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/μg‧mL^-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.