Considerations for Lossy-Mode Resonance-Based Optical Fiber Sensor

This paper presents the fabrication of optical fiber relative humidity (RH) sensors by means of polymeric thin-films as lossy-mode resonance (LMR) supporting coatings. In particular, LMRs are observed because of the fabrication of polyallylamine hydrochloride (PAH) and polyacrylic acid (PAA) polymeric coatings onto cladding-removed multimode optical fibers on applying the layer-by-layer self-assembly method. More than one LMR can be generated as a function of the coating thickness. The generation of multiple LMRs produces notch band filter with a rejection band in the visible region. To compare the parameters of different LMRs, two devices with PAH/PAA coating thicknesses of 140 and 470 nm, respectively, are fabricated. The thickness and the refractive index of the PAH/PAA film can be modified as a function of the surrounding medium refractive index. Thus, surrounding medium relative humidity can be easily determined through the wavelength shift of the resonance (
${\sim}{\rm 34}~{\rm nm}$
between 20% and 80% RH).