Underwater Temperature and Salinity Fiber Sensor Based on Semi-Open Cavity Structure of Asymmetric MZI

A fiber optic Mach–Zehnder interferometer (MZI) sensor based on a semi-open cavity structure for simultaneous measurement of underwater temperature and salinity has been proposed and demonstrated. The optical fiber structure consists of five segments, including single-mode fiber (SMF), offset welded, and general welded no-core fibers (NCFs). We have performed simulations and experimental validations to verify its effectiveness. Furthermore, the detailed analysis on four sensors with varying design parameters has been completed. Among four sensors, sensor-4 exhibits the highest temperature sensitivity of 1.28 nm/°C ( ${R}$ is 0.99), while sensor-3 demonstrates the highest salinity sensitivity of −3.09 nm/wt% ( ${R}$ is −0.98). By calculating and analyzing the sensitivity of temperature and salinity for different resonance peaks, the simultaneous measurement of temperature and salinity has been achieved. During a 2-h stability test, all the sensors exhibited good stability, with a maximum wavelength drift of 0.24 nm. Thereinto, sensor-4 has demonstrated a smaller wavelength drift of 0.10 nm. The salinity response time of sensor-4, when the salinity changes from 0 to 2, 4, and 6 wt%, is less than 4 s. Furthermore, the experimental results indicate that the measurement sensitivity can be enhanced by increasing the contact area between the semi-open cavity Section and the external environment. In addition, by modifying the offset distance and sensing length, the sensitivity can be further improved. The proposed sensor exhibits excellent reversibility, stability, repeatability, and rapid response characteristics. It has promising potential applications in areas, such as aquaculture, biochemical detection, and marine environment exploration.