Research on in-line MZI optical fiber salinity sensor based on few-mode fiber with core-offset structure

In this manuscript, a simple and innovative Mach-Zehnder interferometer (MZI) based on few-mode fiber (FMF) cascaded with a core-offset structure is presented to measure seawater salinity. The sensor is fused by the core-offset fusion method, which makes it easier for the sensor to excite higher-order modes and more sensitive to the changes in the external environment. Through the finite element analysis method, the interference spectrum is obtained by the interference of the fundamental mode LP01 and the higher-order modes LP21 and LP02, respectively. The experimental analysis reveals that the salinity sensitivity of the sensor is −0.27 nm/% from 0 % to 20 %. The temperature sensitivity is 0.15 nm/°C from 5 °C to 30 °C. The salinity sensor has a simple construction, good stability, and low cost, and has great application prospects in marine environment monitoring, aquaculture, and other fields.