Sensitive Humidity Sensor Based on a Special Dual-Mode Fiber

A novel humidity sensor based on a specially designed polyimide coated dual-mode fiber (DMF) is presented in this paper. The parameters and index profile of the DMF used in the experiments are specially designed to support only two modes, LP01 and LP02, propagating in the fiber at the operational wavelength, and the interference between these two modes produces a polarization-independent transmission spectrum. An in-line Mach-Zehnder interferometer is constructed by splicing a section of DMF between two pieces of standard single-mode fibers and employed in the detection of the surrounding relative humidity (SRH) of the environment. In the experiments, because the volume of the polyimide film coated on the DMF changes with SRH and thus produces the strain imposed on the DMF section, the interference dips in the transmission spectrum shift monotonically with the increasing SRH. According to the previous research, the strain sensitivities of the DMF used in this paper increase significantly when the peak wavelengths approach the critical wavelength. Therefore, in the experimental results observed, the experimental sensitivity is 153.5 pm/%RH in the SRH range of 30%RH-98%RH, which is over ten times larger than the largest sensitivity (15 pm/%RH) of polyimide-coated fiber Bragg grating reported in the literature. The proposed humidity sensor has advantages of robust, low cost, and ease of fabrication, which suits several practical applications in monitoring the SRH, especially when high sensitivity measurement is required.