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 key is to consider the roughness conditions at the end-face of large-core fibers. 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. 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 in relation to the surface condition of the far end-face, which may be smooth or rough. The comparison of these cases in terms of output power shows that a rough near end-face enhances the collection of coupled fluorescent light. In contrast, roughening of the far end-face while the near end-face is rough causes a transfer of the initial, mostly tunneling, modes to the radiation modes and decreases the collectable signal.
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