High-sensitivity distributed temperature and salinity sensor based on frequency scanning Φ-OTDR and polyimide-coated polarization maintaining fiber

A highly sensitive distributed optical fiber sensor based on frequency scanning phase-sensitive optical time domain reflectometry (Φ-OTDR) for simultaneous salinity and temperature measurement was proposed and experimentally demonstrated. A bow-tie polarization-maintaining fiber (PMF) with a polyimide coating is utilized as the sensing fiber, as its coating acts as a hygroscopic material, which can convert ambient salinity change into strain on the fiber. Benefiting from the anisotropic structure of PMF, the frequency shift of Rayleigh scattering shows different sensitivities to temperature and salinity for the slow and fast axes, enabling the decoupling of these parameters using a single fiber. In our experiments, the frequency-scanning Φ -OTDR probes the two polarization axes of the polyimide-coated PMF with orthogonal pulse sequences to obtain the temperature and salinity sensitive coefficients of the two axes. The system achieves temperature sensitivities of −1407.8 MHz/K and −1348.9 MHz/K on the slow and fast polarization axes, respectively. Additionally, it demonstrates salinity sensitivities of 1028 MHz/(mol/L) and 1008.6 MHz/(mol/L) on the slow and fast polarization axes, respectively. Simultaneous measurement of temperature and salinity was carried out, and the systematic measurement uncertainties presented in the results were 0.0344 K and 0.0469 mol/L respectively, demonstrating the good discrimination ability of the sensor for temperature and salinity measurement.