A High Sensitivity Refractive Index Sensor Based on Leaky Mode Coupler of MMI

A novel leaky mode coupler of multimode interference (MMI) fiber structure is proposed and experimentally demonstrated for surrounding refractive indices higher than that of silica fiber. The sensor is composed of a section of silica rod (SR) and multimode fiber (MMF) between two identical single-mode fibers (SMFs) to form a single-mode-silica rod-multimode-single-mode (SSRMS) fiber structure. In the proposed design, the SR acts as a mode coupler and a sensing element, while the MMF functions as the narrow spectral width generator. The output power at narrow spectral response of the SSRMS sensor was monitored corresponding to different surrounding high refractive indices (SHRI). The experimental results revealed that the SSRMS sensor exhibited high sensitivity up to −293.40 dB/RIU for SHRI ranging from 1.450 to 1.531. The sensor is highly suitable for SHRI applications, such as for monitoring insulation oil in a power transformer.