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 (lambda), 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 ~10 (mu)m. We experimentally measured the axial resolution for a 500- (mu)M aqueous solution of Rhodamine B with (lambda) = 543 nm, a = 1.31 (mu)m, and a N.A. of 0.16 and found good qualitative agreement with the calculation.