Robust whispering gallery mode resonator for humidity measurement

A fiber-optic humidity sensor based on microsphere resonators with graphene oxide (GO) coated is proposed. A variable pitch off-axis spiral structure (VPOASS) excites multiple higher-order cladding modes in single-mode fiber (SMF) and introduces a strong evanescent field. Hollow glass microspheres are deposited on the surface of the VPOASS, and the GO film coated on the surface of the microspheres and the VPOASS. We used the VPOASS with similar to 125 mu m diameter to excite the whispering gallery modes (WGMs) by evanescent coupling. When water molecules enter the GO film, the refractive index (RI) and the size of the GO film will be changed, and then the size and effective RI of the microspheres, as well as the effective RI of the cladding modes propagating in SMF will be changed, resulting in the shift of transmission spectrum. The humidity sensitivity of the proposed sensor prototype is -37.3 pm/% RH over the range of 10% RH to 60% RH. The proposed sensing structure has potential application prospects in the field of humidity or volatile organic compounds sensing.