  In a recent one-dimensional numerical fluid simulation study [Saxena et al.,
Phys. Plasmas 13,032309 (2006)], it was found that an instability is associated
with a special class of one-dimensional nonlinear solutions for modulated light
pulses coupled to electron plasma waves in a relativistic cold plasma model. It
is shown here that the instability can be understood on the basis of the
stimulated Raman scattering phenomenon and the occurrence of density bursts in
the trailing edge of the modulated structures are a manifestation of an
explosive instability arising from a nonlinear phase mixing mechanism.
