Simultaneous measurement of humidity and temperature based on an SiO ₂-nanospheres film deposited on a long-period grating in-line with a fiber Bragg grating

A novel configuration able to measure simultaneously relative humidity and temperature is proposed. The sensing head is based on a long-period fiber grating (LPG) coated with silica nanospheres in-line with a fiber Bragg grating. The polymeric overlay that changes its optical properties when exposed to different humidity levels is deposited onto the LPG using the electrostatic self-assembly technique (ESA), resulting into a humidity-induced shift of the resonance wavelength of the LPG. Considering the humidity range from 20% to 50% RH, a system resolution of 1.6% RH and 2.5 °C was achieved. At higher humidity, from 50% to 80% RH, the corresponding resolution values were 2.4% RH and 0.4 °C.