Polymer-coated fiber Bragg grating for relative humidity sensing

A fiber-optic-based humidity sensor has been fabricated using a fiber Bragg grating (FBG) coated with a moisture-sensitive polymer. The sensing concept exploits the inherent characteristics of the FBG and is based on the strain effect induced in the Bragg grating through the swelling of the polymer coating. A direct indication of the humidity level is given by the shift of the Bragg wavelength caused by the expansion of the sensing material. The FBG sensor used in this work has an approximate coating thickness of 33 μm and was exposed to different humidity levels at room temperature. The sensitivity of the sensor was estimated to be about 4.5 pm/%RH at a wavelength of 1535 nm, this being obtained through a process of linear regression. The resulting uncertainty in the measurement is ±4%RH and the response time of the sensor and the moisture expansion coefficient of polyimide were obtained from a series of experimental investigations and cross compared with the results of previous work.