Temperature-independent refractive index measurement based on Fabry-Perot fiber tip sensor modulated by Fresnel reflection

A simple fiber tip sensor for refractive index (RI) measurement based on Fabry-Perot (FP) interference modulated by Fresnel reflection is proposed and demonstrated. The sensor head consists of an etching-induced micro air gap near the tip of a single-mode fiber. The microgap and the fiber tip function as two reflectors to form a FP cavity. The external RI can be unambiguously measured by monitoring the fringe contrast of the interference pattern from the reflection spectrum. The experimental results show that the proposed sensor achieves temperature-independent RI measurement with good linear response. The proposed sensor achieves a high RI resolution of up to 3.4 × 10 ^-5 and has advantages of low cost and easy fabrication.