Humidity sensing by tailoring light absorption of SiO2/bromophenol blue (BPB) thin film on optical fiber

The fabrication of an evanescent wave fiber optic humidity sensor based on bromophenol blue (BPB) doped SiO2 thin film was demonstrated, modulating in light intensity. The sensing film was coated on a fiber core via a single-step dip coating method, followed by sol-gel processing of the precursor. A good exponential relationship was established between output light intensity and relative humidity. The sensor exhibited a high sensitivity and fast response and recovery, as well as low hysteresis, good stability and repeatability. Adsorption of ambient water triggered a ring-opening reaction of BPB, which enhanced light absorption of the sensing film significantly and affected the transmission of the evanescent wave.