Fiber Bending Sensor With Turning Point in a Multimode Fiber Peanut-Like Structure

In this paper, we experimentally demonstrated a bending sensor based on Michelson interferometer (MI). The sensor was fabricated by fusion splicing single-mode fiber (SMF) and multimode fiber (MMF) spheres. The improved end-face packaging scheme was adopted to realize the measurement of bending. The advantage of MMF numerical aperture increases the mode field diameter and the number of modes. Fiber bending induces transition between modes, resulting in the generation and fade of fiber modes, achieving the emergence of bending turning points (BTPs). The BTP is defined as the point where the direction of interference dip shift changes due to fiber bending. By observing the direction of interference dip shift during bending, the degree of bending can be intuitively estimated. The simulation results verify the generation and fade of excited modes in the bending MMF peanut-like structure, which are in good agreement with the experimental results. When the sensor is installed in parallel and titled ways, due to the uneven force of the plate in the bending state, it shows the differences in sensitivity, BTP and initial bending response. Further, the BTP can be roughly adjusted by the MMF core diameter, and the deviation of the sensitivity is reflected in the length of the interference arm. Besides, compact size, low cost and low hysteresis make the sensor more competitive in harsh environment applications.