Temperature-Compensated Bragg Grating Sensor for Curvature and Strain Measurement in a Configurable Rectangular Optical Fiber

This work presents a compact fiber Bragg grating (FBG)-based sensor that decouples curvature/strain and temperature effects using a configurable rectangular optical fiber design. The sensor structure integrates a side-hole rectangular fiber (SHRF) with dimension of 175 μm × 358 μm with a 97-μm diameter air hole, along with embedded reduced-cladding fibers (RCFs). It features two FBGs, one inscribed in the SHRF, which is sensitive to both curvature/strain and temperature, and another within an RCF embedded in the air hole, which serves as a temperature reference. The design accommodates both reduced-cladding silica optical fiber (RC-SOF) and reduced-cladding polymer optical fiber (RC-POF), with the RC-POF exhibiting significantly enhanced temperature sensitivity (−43.64 pm/°C in the range 20–80°C), four times greater than that of the SHRF (11.04 pm/°C), enabling precise thermal discrimination. Experimental results demonstrate a curvature sensitivity of −0.26 nm/m−1, an axial strain sensitivity of 0.89 pm/μϵ, and a bending-induced strain sensitivity of 1.09 pm/μϵ for the SHRF. The embedded RCFs show negligible curvature/strain response, with the RC-SOF exhibiting 10.70 pm/°C sensitivity (50–180°C). The SHRF's flat cladding enables clear orientation identification and enhances strain transfer/bonding. This all-fiber design provides thermal compensation without requiring complex packaging or separate reference elements.