Relative humidity sensor based on optical fiber gratings and polyvinyl alcohol

A relative humidity (RH) sensor operating in reflection mode is proposed and experimentally demonstrated by using a polyvinyl alcohol (PVA)-coated tilted-fiber Bragg grating (TFBG) cascaded by a reflection-band-matched chirped-fiber Bragg grating (CFBG). The sensing principle is based on the RH-dependent refractive index (RI) of the PVA coating which modulated the transmission of the TFBG. The CFBG is properly designed to reflect a broadband of light spectrally suited at the cladding mode resonance region of the TFBG, thus the reflected optical signal passes through and the intensity is modulated by the TFBG twice so that the sensitivity improves significantly. RH measurement with sensitivity of ~1.7402 μW/% RH in the range from 20% to 90% is realized.