Optical fiber humidity sensor based on vernier effect of Fabry-Perot interferometers with microsphere

An optical fiber humidity sensor based on vernier effect of Fabry-Perot(F-P) interferometers constructed by microsphere and polyvinyl alcohol (PVA) film is proposed. A hollow capillary fiber (HCF) is spliced with the single-mode fiber (SMF), and the barium titanate ball is inserted into the HCF. A layer of PVA film is coated on the end face of the HCF to construct multiple F-P cavities, and two F-P cavities act as the reference cavity and sensing cavity. The lengths of the two cavities are relatively close to form the vernier effect. The experimental results show that the humidity sensitivity of the sensor is 0.316 nm/%RH in the range of 35 %RH-85 %RH. The fluctuation of the sensor in 60 min is less than 0.27 %RH. And the response time of the sensor is about 12 s. The sensor has the merits of high sensitivity, compact size and good stability.