Draw-tower-grating-based Fabry-Perot interferometers for simultaneous measurements of temperature and salinity

In this paper, a fiber optic sensor with draw-tower-grating (DTG)-based Fabry-Perot interferometers (FPIs) are proposed to measure the temperature and salinity of seawater simultaneously. The sensing structure utilizes DTG as the reflector, and two adjacent gratings can be cascaded to compose an FPI. By employing moisture-sensitive materials to the surface of the optical fiber, the variation in salinity can be measured by the axial strain applied by the coating on the sensing optical fiber. The salinity altering causes swelling or shrinking actions of the coating through absorbing or releasing water. Two different moisture-sensitive materials are chosen to eliminate the cross-sensitivity of temperature and salinity. The experimental results show that the sensor exhibits linear response to temperature and salinity changes with good repeatability and stability, and the temperature and salinity sensitivities are 12308.65 rad/°C and 95.02 rad/‰, respectively. The optical path configuration of the DTG sensing array matching compensation interferometer provides the possibility for efficient distributed solutions, which has promising potential for application in marine engineering.