  There are several mechanisms by which the frequency spectrum of a laser
broadens when it propagates at near-relativistic-intensity in 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; Its combination with phase modulation
produces a symmetrically broadened supercontinuum. FRS dominates when the laser
pulse is long and relatively weak; It broadens the laser spectrum mainly
towards lower frequencies and produces a frequency comb. The creation of the
supercontinuum and frequency combs only frequency modulates, but does not
compress, the laser pulse.
