Long-Period Grating Relative Humidity Sensor With Hydrogel Coating

The long-period grating (LPG) coated with the hydrogel was investigated for monitoring the relative humidity (RH) levels-the ratio of the amount of water vapor in the air at a specific temperature to the maximum amount that the air could hold at that temperature. The experimental results showed that the resonant wavelength of the hydrogel-coated LPG was shifted when the RH varied from 38.9% to 100%, with a linearity of 99.5% and an accuracy of plusmn2.3% RH