Three-Wave Fiber Fabry–Pérot Interferometer for Simultaneous Measurement of Temperature and Water Salinity of Seawater

We report a fiber sensor for simultaneous measurement of temperature and water salinity. The proposed sensor structure is based on a three-wave Fabry-Pérot interferometer (FPI) fabricated in a single-mode optical fiber (SMF) by focused ion beam (FIB) milling. The open cavity of the three-wave FPI was filled with water under test and used as the main sensing element and the lengths of the open and silica cavity were comparable. These features of the proposed sensor lead to three groups of interference fringes with distinct sensitivities in the wavelength domain with respect to the changes in both the water salinity and ambient temperature are obtained. Consequently, simultaneous determination of the ambient temperature and water salinity variations is achieved using the sensitivity matrix method.