Hybrid Fiber-optic Sensor for Simultaneous Seawater Temperature and Salinity Measurements

In this letter, we propose a compact hybrid fiber-optic sensor for seawater temperature and salinity simultaneous measurements. The device consists of a hollow-core fiber (HCF)-based Fabry-Perot interferometer (FPI) and no-core fiber (NCF)-based anti-resonance (AR) structure. A U-shaped groove is inscribed in the HCF by femtosecond laser for liquid inflow and salinity sensing. A section of the polymer coating is retained on the NCF to excite the AR effect for temperature measurement. The incident light is partially reflected at the HCF fusion end-faces and forms the FPI reflection spectrum, while the transmission light in the NCF cladding is coupled to the polymer coating and forms the AR phenomenon. In theoretical modeling, the temperature and salinity responses of the hybrid sensor are analyzed separately, and the corresponding sensitivities are calculated theoretically. In the experiment, the results show that the temperature and salinity sensitivities are -4.948 nm/ and 0.235 nm/, respectively. In addition, the calibration test, time stability, and repeatability of the sensor are also evaluated by the experiments. The construction method of this hybrid sensing structure is flexible and accurate, so it is expected to be applied in the seawater parameter measurements.