  We report the direct observation of periodic intensity modulation of a laser
pulse propagating in a hollow core waveguide. A series of equally spaced plasma
sparks along the gas-filled capillary is produced. This effect can be explained
by the beating of different fiber modes, which are excited by controlling the
size of the focal spot at the capillary entrance. As compared to an artificial
modulated waveguide structure, our presented approach represents an easier and
more flexible quasi-phase-matching scheme for nonlinear-optical frequency
conversion.
