Characterization of OTDR based fiber-optic humidity sensor

In this study, the optical time-domain reflectometer (OTDR) based fiber-optic humidity sensor (FOHS) was fabricated to measure humidity in inaccessible hazardous environments. The sensing probe of FOHS consists of a single mode optical fiber and a moisture-sensitive material (mixture of HEC and PVDF) which varies its refractive index according to the relative humidity [1, 2]; here, the end of fiber was coated with the moisture-sensitive material by using the dip coating method. By Fresnel reflection between the sensing material and the single mode fiber, the optical power of the OTDR varies with the relative humidity. To optimize the FOHS, we evaluated the variation of optical power according to the contents (0.23, 0.33, 0.43, 0.53, and 0.63 g) of PVDF in the moisture-sensitive material and wavelengths (1310 and 1550 nm) of the laser source. As the result, the moisture-sensitive material containing 0.43 g of PVDF had the highest sensitivity with 1310nm laser source.