Evanescent-wave fiber-optic sensor: on power transfer from core-cladding interface to fiber end-face

Abstract

In this paper, the enhancement of collection efficiency in fiber-optic evanescent-wave (EW) sensors is studied. Both theory and experimental results are presented. The theory is based primarily on ray optics, but for the sake of simplicity and accuracy, wave optics is also considered. Fluorescent light is coupled into the core of a partly unclad multimode fiber via EW. Most power is carried to the unclad end-face by tunneling modes. Reflection from this rough end-face, which is modeled as a diffuse source, mixes the initial modes. Bound rays also play an important role, carrying the power to the other end-face. The amount of output power of the bound rays there is calculated. We also study the output power when the end-face is smooth. The comparison of these two cases of output power shows that the rough end-face enhances the collection of coupled evanescent waves.