Fluorescence correlation spectroscopy (FCS) is a sensitive technique used to probe size, concentration, flow velocity, and reaction kinetics in a dilute solution. Conventional FCS spectrometers achieve this sensitivity at the cost of using bulky optics. We demonstrate a technique that utilizes a single-mode optical fiber of 3.3 (mu)m mode field diameter to perform FCS measurements. We demonstrate that the technique has adequate sensitivity to perform FCS measurements on fluorescent beads of 13 nm radius, and that the results agree with theoretical predictions. Our method potentially allows FCS to be extended to remote and in vivo applications.
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