Distributed Salinity Sensor With a Polyimide-Coated Photonic Crystal Fiber Based on Brillouin Dynamic Grating

Abstract

The distributed measurement of salinity is experimentally demonstrated for the first time, to the best of our knowledge, by Brillouin dynamic grating (BDG) with a polyimide-coated (PI-coated) polarization-maintaining photonic crystal fiber as the active sensing component. The position-related salinity change can be measured by mapping the birefringence change through the BDG, where the salinity altering can induce swelling or shrinking actions of the PI coating through absorbing or releasing water. To reveal the systematic characterizations of the proposed sensor, four PI-coated sensors with different coating thicknesses of 5 μm, 8 μm, 15 μm and 20 μm were fabricated as demos, achieving salinity sensitivities of 30.1 MHz/(mol/L) for 5 μm, 75.4 MHz/(mol/L) for 8 μm, 119.2 MHz/(mol/L) for 15 μm, and 139.6 MHz/(mol/L) for 20 μm, respectively. The proposed sensor shows a linear and reproducible response to the salinity change. And its measurement uncertainty is less than 10 MHz, yielding a salinity accuracy of 0.072 mol/L. The temperature crosstalk can be compensated through measuring the temperature-induced Brillouin frequency shift changes using Brillouin optical time-domain analysis.