Modeling and experimental investigation of the coupling efficient of a fiber-capillary fluorescent sensor

An evaluation model is proposed to estimate the fluorescence coupling efficiency of a small, capillary-shape fiber sensor, which takes advantage of the analysis approach for the coupling efficiency from a point lightsource to a fiber. To confirm the validity of the theoretical model, a group of 4 fiber-capillary units with increasing diameters and lengths are made and demonstrate with FITC solution. Both simulation and experiment results show that a triangle-arrangement of two excited fibers and one receiving fiber gives the best coupling efficiency of a capillary-shape fiber probe with a fixed diameter. The coupling efficiency is inversely proportion to the refractive index and attenuation coefficient of the fluorophore solution, besides being direct proportion to the core diameter R and the N.A., i.e., sinθ(θ is the receiving angle), of the receiving fiber within the distance from the fiber end to the limit decided by R/tanθ, and a fluorescence efficiency of 0.8% is demonstrated.