Optical relative humidity sensor based on a polyvinyl alcohol film and a phase-enhancement total-internal-reflection heterodyne interferometer

Abstract This study presents an optical relative humidity (RH) sensor based on the hydrophilic feature of a polyvinyl alcohol (PVA) film and a phase-enhancement total-internal-reflection (TIR) heterodyne interferometer. The sensor operation is based on the interaction of a TIR evanescent wave with the PVA film coated on the base of a phase-enhancement TIR apparatus. Relative humidity influences the PVA refractive index, thus bringing a significant variation in the phase difference between the s- and p- polarized states of a beam completely reflected from the TIR apparatus. The phase difference variation can be obtained using heterodyne interferometric technique, and the relative humidity measurement therefore is realized. In this study, the sensor produced measurement sensitivity and resolution superior to 0.49°/% RH and 0.5% RH in the range of 30%-75% RH.