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 (<1% RH). An advantage of the sensor design is that the probe is configured in reflection mode where the distal end of the fiber (containing the LPG) is coated with a silver mirror to monitor the wavelength shift at the same end of the fiber at which the light is launched (and to directly relate this to the RH change). The results obtained are compared with those from other probe designs (which often are in the less convenient transmission mode) and are seen to exhibit excellent overall performance and repeatability, showing the value of the probe for practical RH measurement, for example, in structural monitoring applications, a key area where effective monitoring of moisture and humidity change is important.