Optical fiber humidity sensors based on Localized Surface Plasmon Resonance (LSPR) and Lossy-mode resonance (LMR) in overlays loaded with silver nanoparticles

In this work, it is presented for the first time the fabrication and characterization of a novel optical fiber humidity sensor based on both Lossy-mode resonance (LMR) and Localized Surface Plasmon Resonance (LSPR). Those resonances were created using Layer-by-Layer (LbL) polymeric coating loaded with Ag nanoparticles (Ag NPs) and fabricated onto an optical fiber core. Firstly, it was observed a LSPR attenuation band, inherent to the presence of the Ag NPs in the coating. This LSPR band showed a slight intensity variation with Relative Humidity (RH) changes but no significant wavelength dependence was observed. Otherwise several LMR attenuation bands were observed in the 400-1100 nm spectral range as the thickness coating was increased. The LMR maxima depend strongly on the thickness and refractive index of the LbL overlay, and therefore, they show a strong wavelength response to Relative Humidity (RH) changes. A wavelength-based optical humidity sensor was achieved and the dynamic range (42.4 nm), transfer function, response time (476 ms and 447 ms for rise/fall respectively) were characterized. The results confirm that this sensor could be used even for monitoring human breathing.