Optical relative humidity sensor with high sensitivity based on a polyimide-coated symmetrical metal-cladding waveguide

An optical structure for relative humidity (RH) sensing based on a polyimide-coated symmetrical metal-cladding waveguide is proposed. The diffusion of water molecules into the polyimide film leads to a combined change of thickness and refractive index. Due to the high sensitivity of ultrahigh-order modes, the interaction between water molecules and polyimide will give rise to a dramatic variation in the reflected light intensity. Our optical RH sensor exhibits a good linearity and a high RH resolution of approximately 0.16% for RH values ranging from 36% to 76%. In addition, it provides a short response time, a favorable stability over a long period, and a small degree of hysteresis.