Fiber Optic Probe Based on Self-Assembled Photonic Crystal for Relative Humidity Sensing

This work deals with optical fiber probes based on a photonic crystal deposited on the tip of a multi-mode optical fiber for relative humidity measurements. Easy manufacturing has been exploited through the self-assembly of core/shell nanoparticles in a colloidal crystal via a vertical deposition technique. Core/shell nanoparticles are synthetized having a polystyrene core and a hydrogel poly(N-isopropylacrylamide) shell. The swelling of the hydrogel shell due to water absorption makes the reflectance spectra of the photonic crystal dependent on the relative humidity (RH) in terms of wavelength shift and amplitude change of the reflected peak. Spectral characterization and sensing analysis of experimental probes are presented and discussed, showing that the absolute spectral characteristics of the proposed device are strongly manufacturing dependent. The sensing characteristic is highly nonlinear with relative humidity. A resolution of 1% RH is measured at 30% RH that significantly decreases at higher RH. Moreover, a manufacture-independent RH sensor characteristic has been retrieved by a proper normalization of the response. Finally, a deeper insight into the sensor behavior has been obtained by a theoretical model supported by transmission electron microscope and dynamic light scattering analysis. It suggests that the clustering of the polystyrene / poly(N-isopropylacrylamide) core/shell nanoparticles is mainly responsible for the fiber optic humidity response.