Humidity sensor based on cascaded chirped long-period fiber gratings coated with TiO2/SnO2 composite films

We propose a novel scheme of high-sensitivity relative humidity (RH) sensor based on cascaded chirped long-period fiber gratings (CCLPFG) and sol–gel derived TiO2/SnO2 composite films. Two identical linearly chirped LPFGs without a separation were inscribed into a single-mode fiber core sequentially, which produces interference patterns in the transmission spectrum. The mole ratio of SnO2 is 4% in the sol–gel derived TiO2/SnO2 composite films that were deposited on the cladding surface. The improved adsorption ability of water molecules of the composite films, combined with fine fringe patterns of CCLPFG, enables this sensor to be very sensitive to humidity. In experiment, a wavelength shift of 12.37 nm was achieved for RH varying from 40% to 95% at 15 °C, which exhibits an approximately linear response. The RH sensitivity of this sensor will slightly decrease for increasing temperature. The sensitivities for 15 °C, 20 °C, 30 °C and 40 °C are 0.221 nm/%, 0.218 nm/%, 0.214 nm/% and 0.211 nm/%, respectively.