A Simple High-Sensitivity Sensor Based on Multimode Mach–Zehnder Interference for Refractive Index Measurement

The present study proposes and experimentally verifies a refractive index (RI) sensor with high sensitivity and simple structure based on multimode Mach-Zehnder interference, in which a suspend-core fiber (SCF) fused with single-mode fibers (SMFs) at both ends worked as the sensing unit. Femtosecond laser is used to process SCF, forming microfluidic channels for liquid in and out, and then, encapsulation is conducted to enhance the mechanical characteristics of the Mach-Zehnder interferometer (MZI) sensor. By analyzing simulation and experimental results, it has been confirmed that the multimode MZI arises from the effective RI difference between the fundamental mode and the higher order modes of the SCF core induced by RI variations of external environment. This study involves the preparation and testing of three sensors with varying SCF lengths. As the external RI varies in the range of 1.3333–1.3472, all sensors exhibit sensitivities at a comparable level. This observation aligns well with the theoretical analysis, indicating that fiber length does not have direct impact on the sensing performance. The maximum sensitivity was 1825.41 nm/RIU. The three sensors undergo tests to assess their repeatability and stability, revealing a maximum wavelength error of 0.1975 nm for repeatability and 0.18 nm for stability. In addition, the temperature sensitivity within the range of 20 ° C–100 ° C is found to be as low as 0.00667 nm/°C, indicating that the potential impact of temperature-induced wavelength shift can be disregarded. The sensor exhibits high sensitivity, excellent repeatability, and stability, promising extensive application in diverse fields, including biology, chemistry, medicine, and environmental monitoring.