Generating optical supercontinuum and frequency comb in tenuous plasmas

There are several mechanisms by which the frequency spectrum of a laser broadens when it propagates at near-relativistic intensity in a tenuous plasma.Focusing on one-dimensional effects,we identify two strong optical nonlinearities,namely,four-wave mixing(FWM)and forward Raman scattering(FRS),for creating octave-wide spectra.FWM dominates the interaction when the laser pulse is short and intense,and its combination with phase modulation produces a symmetrically broadened supercontinuum.FRS dominates when the laser pulse is long and relatively weak,and it broadens the laser spectrum mainly toward lower frequencies and produces a frequency comb.The frequency chirping combined with group velocity dispersion compresses the laser pulse,causing higher peak intensity.