Relative humidity sensor based on cascaded Fabry-Perot interferometers and Vernier effect

We study and experimentally demonstrate a highly sensitive humidity sensor, whose sensitivity has an amplification effect, through the use of the Vernier effect produced by a cascaded Fabry-Perot interferometer (FPI). A section of capillary was spliced with single-mode optical fiber, and then the capillary was immersed in polyimide (PI) solution to form PI film to construct the structure of the sensor. The proposed sensor comprises an air cavity, a PI cavity, and an air-PI mixing cavity. Owing to the approximate free spectral range of the air cavity and air-PI mixing cavity, the sensor produces a Vernier effect. Theoretical analysis shows that the relative humidity sensitivity of the sensor is only related to PI film. The experimental results show that in the range of 40~85% relative humidity, the sensitivity of the spectral envelope of the sensor can reach ~344.4 pm/% RH, which is 3.77 times higher than that measured by the sensor interference fringe. The experimental results are basically consistent with the theoretical analysis.