Relative Humidity Sensor Based on Tip of Multimode Optical Fiber Integrated with Photonic Crystal of Hydrogel Coated Polystyrene Nanoparticles

A relative humidity (RH) sensing device based on photonic crystal colloids auto-assembled directly on the tip of a multi-mode optical fiber is here described. The core-shell nanospheres are obtained by Polystyrene (PS) nanoporous structures (core) incorporated into a poly(N-isopropylacrylamide) (PNIPAM) hydrogel (shell) and are subsequently deposited via a vertical dip-coating technique on a multimode optical fiber tip. The sensing capability is due to the hydrogel shell that swells depending on the relative humidity that induce wavelength shift and amplitude changes in the photonic bandgap peak. Spectral characterization and sensing analysis are performed, and the obtained results show that spectral characteristics are highly non-linear and strongly manufacturing dependent. A resolution of 1% RH is measured at 30% RH that significantly decreases at higher RH. Hereby, numerical simulations based on the finite element method (FEM) have been performed in order to investigate the sensing mechanism.