Low-cost relative humidity sensor based on thermoplastic polyimide-coated fiber Bragg grating

This paper presents the sensing model and scheme for utilizing thermoplastic polyimide-coated fiber Bragg grating (FBG) to measure the relative humidity (RH) on the basis of the application of FBG in chemical sensing. The volume expansion and shrinkage of the polyimide moisture sensitive material after water absorption and desorption have direct effects on the FBG and result in the shift of the Bragg wavelength. The low-cost coarse wavelength division multiplexing (CWDM) is used to interrogate the Bragg wavelength through converting the wavelength signals into the optical intensity ones. Experimental results show that the applicable low-cost relative humidity sensor has a linear, reproducible and reversible response over a wide RH range from 11% to 98%. The thickness and the length of the thermoplastic polyimide coating considered as a moisture sensitive polymer material are 8 μm and 2.5 cm, respectively. The sensitivity of the sensor is −0.000266V/%RH, and the response time is approximately 5 s.