Tapered single-mode optical fiber pH sensor based on lossy mode resonances generated by a polymeric thin-film

Lossy mode resonances can be generated with certain polymeric nanostructures, such as those obtained with a multilayered assembly of poly (allylamine hydrochloride) and poly(acrylic acid). This coating is adsorbed by the electrostatic self-assembly technique onto a tapered single-mode optical fiber, in order to evaluate its performance when detecting pH. According to the results reported in this paper, the high sensitivity given by a tapering process in a single-mode optical fiber is increased by the effect of this kind of electromagnetic resonances. Particularly, in a pH range from 4.0 to 6.0, the overall wavelength shift of this sensor reaches 200 nm, and the transmission at the resonance wavelengths can fall down to ${-}{\rm 50}~{\rm dB}$. These data provide results which can be taken into account to detect pH with high accuracy.