Theoretical study of composite cavity fiber MZI refractive index sensor based on transverse offset structure

In this paper, we present a Mach–Zehnder interferometer (MZI) sensor utilizing an optical fiber offset. The sensor core comprises multimode fiber (MMF), single-mode fiber (SMF), and offset no-core fiber (NCF), enabling it to detect the refractive index (RI) of liquids. The sensing mechanism of the sensor is theoretically analyzed, and various structural parameters are simulated, using the Beam Propagation Module (BPM). Simulation results indicate that the sensor efficiently operates across the entire wavelength range of 1.5–1.61 µm, providing continuous calibration curves for RI measurement. The sensor sensitivity to RI ranges of 1.3330–1.3340, 1.3334–1.3335, 1.3530–1.3540, and 1.3630–1.3640 are –14342, –14700, –17628, and –19600 nm/RIU, respectively. Additionally, the sensor exhibits a favorable linear response, laying a crucial foundation for developing the high-sensitivity liquid RI sensor design. The sensor boasts a simple, compact design, high sensitivity, and significant application potential in liquid RI measurements.