Absolute Salinity Measurement Based on Microfiber Coaxial Mach-Zehnder Interferometer

Seawater salinity is a key parameter in the study of ocean dynamics, accurate measurement of seawater salinity is crucial. However, current seawater salinity is a measure of practical salinity, which limits the accuracy of salinity measurements. The proposed absolute salinity compensates for the flaws in the practical salinity that does not accurately reflect the true salinity of seawater. Microfiber sensors enable highly sensitive measurements of absolute salinity in seawater. In this paper, a new method based on microfiber coaxial Mach-Zehnder interferometer (coaxial MZI) is proposed for seawater absolute salinity measurement. The absolute salinity sensitivity of microfiber coaxial MZI is calculated by finite element program, and the theoretical results show that the sensitivity increases with increasing wavelength and increases with the decrease of waist diameter. Thirteen microfiber coaxial MZIs with different diameter were fabricated and used for the measurement of absolute salinity. Experimental results show that the highest sensitivity is 2.38nm/parts per thousand, and the variations rules of sensor sensitivity are consistent with theoretical calculations. In addition, in order to promote the practical application of microfiber devices in the marine field, coaxial MZI is encapsulated. The salinity response time, time stability and vibration influence of the sensor were tested by experiments. Encapsulated coaxial MZI is expected to realize the in-site measurement of the absolute salinity of seawater.