Polymer based optical humidity and temperature sensor

Humidity control and moisture measurements have a wide range of applications like monitoring humidity in food storages, chemical plants, electronic instruments, building constructions, hospitals, museums and libraries. In this work we have developed a fully optical humidity sensor. The device is based on a hygroscopic polymer, polyimide, which undergoes reversible volume expansion on exposure to the humid environment. Multiphysics tool, COMSOL, is used to investigate the mechanism of humidity absorption by the polymer layers. Moisture absorption and diffusion into the polymer layers are modeled and simulated. The sensor is designed using fiber Bragg grating (FBG) developed along an optical fiber. Layers of the polyimide coated the FBG. The induced strain caused by polyimide expansion and deformed geometry of the fiber is modeled. To develop a high sensitive sensor, a π-phase shifted fiber Bragg grating (π-PSFBG) is selected as a sensing element because of its sharp spectrum signal. To further improve the spectral signal, an optimum apodization function is implemented in design of the device. The spectral signal of sensing element is modeled and simulated using mathematical analysis in MATLAB. The results of theoretical modeling are used to fabricate the sensor containing two 24-mm long π-PSFBGs separated by 12 mm on a SMF 28 fiber. A distributed feedback laser scanner is used to characterize the device precisely. The sensor response to the changes in the humidity and temperature of the environment is studied. The experimental results are relatively in good agreement with those obtained theoretically.