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.
A number of applications of fiber optic sensor for bioluminescence have been widely reported. The key component in
fiber optic fluorescent sensing system is sensor probe, whose efficiency will determine the overall sensor performance,
and haven't been thoroughly understood in previous studies. Questions about how the probe structural parameters such
as length, diameters and N.A. of its fiber components affect the collecting efficiency of the fluorescence signals. In this
paper, we present a pinpoint fiber-glass probe aiming at efficient fluorescence collection. According to the geometric
optics approach, we model a sensor probe based on the same principle of coupling ratio analysis between a light source
and a fiber, and demonstrate the quantitative relations of the fluorescent collecting efficiency and the structural
parameters of the probe by simulation and experiment.
Our theoretical analysis comes up with a optimal probe geometry which has the highest fluorescent efficiency of the
designed fiber optic sensor probe, and reveal detailed relations that the efficiency is direct proportion to the core diameter,
NA of receiving fiber, the length and the section radius of the sensing probe, where the ratio of the core diameter to
sectional area of the probe is the key factors for the fluorescent efficiency for certain detection accuracy.
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