Optical Fiber Humidity Sensor With Porous TiO<sub>2</sub>/SiO<sub>2</sub>/TiO<sub>2</sub> Coatings on Fiber Tip

A multimode optical fiber humidity sensor composed of porous oxide films is demonstrated and optimized. Relative-humidity (RH) sensing is correlated with the shift of interference fringe due to the change of effective refractive index of porous coatings when exposed to different RH environments. An optical interference model that explains the sensing principle is presented, and the fabrication process, such as vacuum degree, baking temperature, is verified. Experimental results show that the average sensitivity is 0.7 nm/%RH (3.3%RH~63%RH), the proposed sensors present high repeatability, little hysteresis, and especially highly sensitive to low and middle moisture measure.