Direct experimental observation of periodic intensity modulation along   straight hollow core optical waveguides
  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. 
T. Pfeifer and M. C. Downer