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

Abstract

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, FPI1 and FPI2 were used as the detection and reference units, respectively. FPI1 and FPI2 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 FPI1 was coated with a multilayer polyimide (PI) wet-sensitive membrane used to measure the RH. When FPI1 and FPI2 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 FPI1 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.