Temperature-insensitive refractive index sensor based on a spindle-shaped few-mode fiber modal interferometer.

A designed temperature-insensitive modal interferometer for refractive index measurement based on spindle-shaped few-mode fiber (FMF) is proposed and demonstrated. The interferometer consisting of a specific length of FMF fused between two specific lengths of single-mode fibers is bent into a balloon shape and then burned by a flame into a spindle shape to sensitize. Due to the bending of the fiber, part of the light leaks from the core to the cladding and excites the higher-order modes, and the four modes in the core of FMF interfere with the higher-order modes in the cladding. Therefore, the sensor is more sensitive to the surrounding refractive index. The experimental results show that the highest sensitivity is 237.3 nm/RIU from 1.333 to 1.365. The sensor is insensitive to temperature, which solves the problem of temperature cross talk. In addition, with its advantages of a small mechanism, simple fabrication, low loss, and good mechanical strength, the proposed sensor has broad application prospects in chemical production, fuel storage, environmental monitoring, and other fields.