In this paper, a relative humidity (RH) sensor based on vernier effect of two parallel Fabry-Perot interferometers is proposed and experimentally demonstrated. The polyvinyl alcohol (PVA) is chosen as the humidity sensitive material. The sensor consists of a reference cavity formed by single mode fiber - fiber capillary - single mode fiber (SMF-FC-SMF) structure and a sensing cavity formed by SMF-FC-coated structure. The sensor shows the high sensitivity of 1.78 nm/RH in the range of 55RH-85RH. We measured the stability of the sensor at 45 RH and 75RH, and the response time between 55RH and 75RH is 12 s. In addition, it also has the advantages of low temperature disturbance, simple principle, and easy manufacture.
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