Highly Sensitive Optical Fiber Humidity Sensor Based on Polyvinyl-Alcohol Film-Induced Lossy Mode Resonance

A highly sensitive optical fiber relative humidity (RH) sensor based on lossy mode resonance (LMR) is proposed and demonstrated. The sensor consists of a no-core fiber (NCF) covered by a layer of transparent polyvinyl alcohol (PVA) with refractive index greater than that of silica. On the one hand, the transparency and high refractive index of PVA enable it to excite LMR in the NCF. On the other hand, PVA has good water absorption and swelling properties, making it highly sensitive to humidity. Therefore, both LMR excitation and humidity sensitive response can be achieved with only one material. When the external humidity changes, both the thickness and refractive index of the PVA film change accordingly, causing a large shift of the resonant wavelength of the LMR. Experimental results show that the sensitivity of the sensor can reach 7.34 nm/%RH within the RH range of 45%–76% and 1.36 nm/%RH within the RH range of 40%–85%, respectively.