Optical fiber relative humidity sensor based on FBG incorporated thin-core fiber modal interferometer

A new fiber-optic relative humidity (RH) sensor based on a thin-core fiber modal interferometer (TCFMI) with a fiber Bragg grating (FBG) in between is presented. Poly (N-ethyl-4-vinylpyridinium chloride) (P4VP·HCl) and poly (vinylsulfonic acid, sodium salt) (PVS) are layer-by-layer deposited on the side surface of the sensor for RH sensing. The fabrication of the sensing nanocoating is characterized by using UV-vis absorption spectroscopy, quartz crystal microbalance (QCM) and scanning electron microscopy (SEM). The incorporation of FBG in the middle of TCFMI can compensate the cross sensitivity of the sensor to temperature. The proposed sensor can detect the RH with resolution of 0.78% in a large RH range at different temperatures. A linear, fast and reversible response has been experimentally demonstrated.