Axial resolution limit of a fiber-optic fluorescence probe

We examine the limit of spatial resolution achievable when a single optical fiber is used for excitation and collection of fluorescence from a bulk specimen. We calculate the probability of detecting a fluorescent particle as a function of its position relative to the fiber face, using excitation wavelength X, radius a, numerical aperture N.A., and the particle's fluorescence and absorbance spectra. Treating Rhodamine B as a model fluorescent analyte and using appropriate fiber parameters, we show that the maximum axial resolution (defined as the axial distance in a homogenous solution within which 50% of the detected signal originates) achievable is similar to10 mum. We experimentally measured the axial resolution for a 500-muM aqueous solution of Rhodamine B with lambda = 543 nm, a = 1.31 mum, and a N.A. of 0.16 and found good qualitative agreement with the calculation. (C) 2003 Optical Society of America.