Refractive Index Measurement Based on Thin-Core Fiber In-Line Mach-Zehnder Interferometer and Differential Intensity Demodulation

A compact and reliable thin-core fiber (TCF) based in-line Mach-Zehnder interferometric (MZI) sensor for refractive index (RI) measurement is proposed and experimentally demonstrated. A section of TCF is sandwiched between a multi-mode fiber (MMF) and a single-mode fiber (SMF) to form the sensing head. In addition, the use of the differential intensity demodulation method not only greatly reduces the system cost, but also solves the problem of power fluctuation in the RI interrogation system. Three samples with different sensing lengths have been made, and the maximum sensitivity of 59.9 dB/RIU has been obtained in the range of 1.332–1.411 RIU, which means the measurement resolution is about 1.669 × 10−4 RIU. Besides, the proposed MZI-based RI sensor is less sensitive to temperature, thus the cross-sensitivity effect can be greatly weakened.