Lossy mode resonances toward the fabrication of optical fiber humidity sensors

In this work, the fabrication and characterization of novel humidity sensors based on lossy mode resonances (LMRs) are presented. Indium tin oxide (ITO) and In₂O₃ coatings fabricated onto an optical fiber are used as the LMR supporting coatings. Poly-acrylic acid and polyallylamin hydrochloride polymeric coatings were used as the sensitive layers. A comparative study between the ITO and the In₂O₃ devices is made. The In₂O₃-based humidity sensors show an improvement in the sensitivity if compared with the ITO-based ones within the same working range. Moreover, In₂O₃-based refractometers enable to carry out dual-reference measurements thanks to the resonances associated with TE and TM light polarizations instead of a single resonance as was the case for the ITO coated devices. In addition to this, the fabrication time has been reduced because the number of bilayers required for deposition of the In₂O₃ coating is only 13, whereas for deposition of the ITO coating 100 bilayers are necessary.