Chitosan-Coated Polarization Maintaining Fiber-Based Sagnac Interferometer for Relative Humidity Measurement

A relative humidity fiber sensor based on polarization maintaining (PM) fiber Sagnac interferometery configuration is presented. The proposed sensor is functionalized with a thin layer of a moisture-sensitive natural polymer chitosan, whose degree of swelling varies as a function of relative humidity. The sensing scheme used in this study is based on the strain effect induced on the PM fiber to modulate its birefringence property through the swelling effect of chitosan. To optimize the sensor's response, the experiments were first conducted to evaluate the effect of chitosan concentration on the PM fiber, followed by investigating effects of chemically etched PM fiber and analyzing modified chitosan sensing film effect on the sensor's performance. As observed from the results, the optimized sensor exhibits a sensitivity of 81 pm/% RH for the relative humidity ranging from 20% RH to 95% RH with an uncertainty of +/- 2.04% RH.