Fundamental Characterization of Fresnel Reflection for Designing an Optical-Fiber Based Temperature Sensor

This study experimentally characterizes temperature-dependent Fresnel reflection at a silica optical fiber-water interface to provide foundational data for optical-fiber-based temperature sensors and fiber-optic bolometers (FOBs).Owing to their intrinsic immunity to electromagnetic interference (EMI), optical fiber sensors are well suited for diagnostics in extreme environments such as medical systems, nuclear facilities, and fusion reactors.Temperature-induced refractive-index variations of the optical fiber and water modify the Fresnel reflectance at the fiber interface through the thermo-optic effect.An experimental system employing a C-band light source was used to measure reflected-light intensity over a temperature range of 20 -80 ∘ C. The reflected-light intensity exhibits a linear increase with temperature (R 2 = 0.9772) and shows strong agreement with analytical predictions based on Fresnel theory (R 2 = 0.9928).These results validate Fresnel-reflection-based temperature sensing at a fiber-liquid interface and demonstrate a simple, compact, and EMI-resistant optical readout mechanism applicable to liquid temperature monitoring in medical and nuclear systems, as well as to plasma radiation detection in fusion plasma diagnostics using FOBs.