Optical fibers have been employed as waveguides to confine the excitation beam and fluorescence emission beam. With an excitation optical fiber, the excitation laser beam has been guided along the channel, leading to the so-called longitudinal excitation. As compared to the conventional transverse excitation, in which the excitation beam is perpendicular to the channel, longitudinal excitation has resulted in a twentyfold improvement in the signal-to-noise ration (S/N). This is because the longitudinal excitation beam is narrower than the channel, and so there is less background caused by the light scattered from the channel wall. The use of the optical fibers for longitudinal excitation and emission collection has led to an improved detection limit of fluorescein of 3 nM (or twenty thousand molecules).671