Axial resolution limit of a fiber-optic fluorescence probe

Abstract

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 λ, 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 μm. We experimentally measured the axial resolution for a 500-μM aqueous solution of Rhodamine B with λ = 543 nm, a = 1.31 μm, and a N.A. of 0.16 and found good qualitative agreement with the calculation.