High sensitivity relative humidity sensor based on two parallel-connected Fabry–Perot interferometers and Vernier effect

• A novel high-sensitivity relative humidity sensor was first fabricated. • The sensing unit of the sensor was made by covering a multi-layer Polyimide wet-sensitive film on the capillary surface of the FPI. • The RH sensitivity of the sensor was up to ∼586.6 pm/% RH, which was 17.3 times that of a single FPI1 sensor without the Vernier effect. • The sensor had excellent performance including good stability, short response time, low cost, repeatable manufacturing. In this study, a highly sensitive relative humidity (RH) sensor based on two Fabry–Perot interferometers (FPIs) in parallel with the Vernier effect was designed and demonstrated. In the sensor, FPI 1 and FPI 2 were used as the detection and reference units, respectively. FPI 1 and FPI 2 had similar structures, both comprising an in-line sandwiched structure, i.e., single-mode fiber-capillary-single-mode fiber. However, the capillary surface of the sensing unit FPI 1 was coated with a multilayer polyimide (PI) wet-sensitive membrane used to measure the RH. When FPI 1 and FPI 2 had a small cavity-length difference, the Vernier effect was achieved by superimposing their output spectrum. The RH response, repeatability, and stability of the sensor structures were investigated. The RH sensitivity of the sensor was improved owing to the Vernier effect. Experimental results indicate that the RH sensitivity of the sensor is up to ∼586.6 pm/% RH, which is 17.3 times that of a single FPI 1 sensor without the Vernier effect. The measured temperature cross-sensitivity of the sensor is approximately 0.75 %RH/°C. In addition, the sensor has the advantages of simple manufacturing, robust structure, and stable operation.