Multi-Parameter Sensing and Demodulation of Temperature, Salinity and Pressure in Seawater Based on the Semi-Encapsulated Microfiber Mach-Zehnder Interferometer

Semi-encapsulated microfiber Mach-Zehnder interferometer is designed and developed for temperature, salinity and pressure (TSP) sensing in seawater. Based on the theoretical analysis, sensing experiment is performed with typical sensitivities of -2312 pm/℃, 631 pm/‰, and 3775 pm/MPa, respectively. Using sensitivity matrix method (SMM) , salinity and pressure measurements show large differences with those measured by conductivity-temperature-depth system. To improve it, machine learning method (MLM) is introduced and support vector regression model is used. By 25 tests under arbitrary TSP, errors relative to the full scale of about 19.14%, 4.01 % and 15.75 % are obtained. In addition, non-linear dependence of sensitivity on surroundings , stability of prediction, dependence of SMM accuracy on sensing dips and influence of the amount of data used in training are also investigated. Finally，SMM and MLM combined method shows relatively good performance for TSP measurement with errors of 10.67%, 5.25%, and 16.76%, respectively