Evanescent-Wave Fiber-Optic Sensor: On Power Transfer From Core-Cladding Interface to Fiber End-Face

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.