Analysis of Polyimide-Coated Optical Fiber Long-Period Grating-Based Relative Humidity Sensor

A long-period grating (LPG)-based relative humidity (RH) sensor is developed, based on using a tailored layered polyimide coating on the grating region at the distal end of the fiber probe and allowing it to respond to humidity changes. The attenuation band of the coated LPG, on which the sensor is based, responds to the varying RH (in this case, over the region from 20% to 80% RH) through changes in the characteristic wavelength of the LPG. The experimental results confirm that the sensor created shows a linear response to the variation of RH levels. The sensitivity of the sensor is estimated to be 0.10 nm/%RH with negligible hysteresis (