Humidity Sensor Based on Hetero-Core Structured Fiber Optic Covered with Layer-by-Layer Thin Film

Abstract

Humidity sensors based on hetero-core structured fiber optic has been fabricated. In the hetero-core structured fiber optic, a short piece of singlemode fiber optic is inserted between multi-mode fibers. Layer-by-layer (LBL) technique is applied to prepare humidity sensitive film. The humidity sensitive LBL film, prepared using polyacrylate as polyanion and polyallylamine as polycation, was deposited on the singlemode fiber part of the hetero-core structured fiber optic. According to increasing of deposition cycle, the propagating loss increased because of increasing the refractive index at the sensing part by the formation of the film. After deposition procedure, the sensor part was immersed in calcium chloride solution to dope the deliquescence material in the thin film. The humidity sensors ware characterized as a function of relative humidity (RH). The sensor shows that the propagating light power increased according to increasing of RH. This is because the dry film having a higher refractive index absorbed moisture and swollen, and then the refractive index decreased resulting in the decreasing of the leakage of the propagating light at the singlemode fiber part. In addition, the increasing of calcium chloride concentration caused the enhancement of the sensitivity. This is because calcium chloride is deliquescence material and absorbed water from atmosphere, resulting in the swelling, decreasing of the refractive index at low humidity.