Simultaneous measurement of seawater flow velocity and salinity based on a microfiber coaxial Mach-Zehnder interferometer coupled with a T-shaped cantilever

Both seawater flow velocity and salinity are critical parameters in marine environmental observation. Accurately achieving simultaneous measurement of these two parameters remains a challenge in oceanographic monitoring. This study innovatively developed a sensing unit (MMZI-T) that integrates a microfiber coaxial Mach-Zehnder interferometer (MMZI) with both strain responsiveness and salinity sensitivity, coupled with a T-shaped cantilever beam, and proposes what we believe to be a novel method for simultaneous measurement of seawater flow velocity and salinity. By directly contacting seawater, the sensing unit converts the pressure effect of ocean current into stress on the cantilever beam, thereby stretching the microfiber and causing a shift in the interference spectrum, enabling flow velocity measurement. The interaction between its evanescent field and seawater enables salinity measurement. The theoretical expressions for seawater flow velocity sensitivity and salinity sensitivity were derived. Experimentally, the MMZI-T sensing unit was developed, and a measurement system based on a circulating water channel was designed, achieving simultaneous measurement of seawater flow velocity and salinity. The maximum sensitivity for seawater flow velocity measurement reached 152.6 nm/(m·s^-1), and the maximum salinity sensitivity reached 0.41 nm/‰. A second-order sensitivity matrix was proposed to demodulate the dual parameters. The mean relative error (MRE) and root mean square error (RMSE) for flow velocity measurement were determined to be 3.78% and 0.009 m/s, respectively, while those for salinity measurement were 1.85% and 0.576‰. The results demonstrate the high accuracy of the MMZI-T sensing unit. Due to its small size and high sensitivity, the MMZI-T sensing unit based on a microfiber coaxial Mach-Zehnder interferometer coupled with a T-shaped cantilever is expected to play an important role in the field of simultaneous ocean flow velocity and salinity measurement.