Relative Humidity Sensor Based on Tilted Fiber Bragg Grating With Polyvinyl Alcohol Coating

A relative humidity (RH) sensor based on tilted fiber Bragg grating (TFBG) is proposed by utilizing polyvinyl alcohol (PVA) as the sensitive cladding film. RH increasing in the PVA coating will result in reduction of refractive index. Due to the TFBG's sensitivity to ambient refractive index, the spectral properties of PVA-coated TFBG are modified under exposure to different ambient humidity levels ranging from 20% to 98% RH. The transmission power of TFBG has different linear behaviors for two different humidity ranges (20%-74% RH and 74%-98% RH), and the sensitivity for each humidity range reaches as high as 2.52 and 14.947 dBm/%RH, respectively. Combining the advantages of optical fiber grating and PVA as a smart material, this design involves simple configuration, low cost, compactness, a small degree of hysteresis, stability, and wide dynamic sensing range as well. Therefore, the sensor could be applied in real-time RH monitoring for normal as well as extremely humid environments.