Optical fiber Fabry–Perot humidity sensor based on polyimide membrane: Sensitivity and adsorption kinetics

An optical fiber Fabry–Perot humidity sensor was constructed by coating a foamed polyimide (PI) membrane on the end of a multimode fiber. This sensor had a sensitivity of 0.164 nm/%RH in the humidity range of 20–95% RH. Through building refractive index (RI) and volume expansion models of the PI humidity sensitive membrane (HSM), the relationship between the volume fraction of absorbed water in the PI HSM upon and ambient humidity was investigated and the quantitative relationship between the sensor’s sensitivity and adsorption kinetics of the PI HSM determined.