All-fiber-optic sensor for relative humidity measurement

A novel relative humidity (RH) sensor based on single-mode-multimode-single-mode (SMS) fiber structure is presented. The sensors are created through coating a thin layer of polyvinyl alcohol (PVA) on the multimode fiber deleted the cladding trough HF solution cauterization as the sensitive cladding film, whose refractive index varies as a function of humidity level. Due to the SMS fiber structure´s sensitivity to ambient refractive index, the spectral properties of SMS fiber structure coated PVA sensitive film are modified under exposure to different ambient humidity levels ranging from 50%~89%RH. The related theoretical analysis and the numerical simulation of spectral transmission of SMS fiber structure with different surrounding refractive index are also proposed. The sensitive of the RH measurement of 0.18nm/%RH in the range from 80%-89%RH is experimentally achieved. The experimental results obtained are consistent with the conclusion obtained by numerical simulating.