  Absolute instability modes due to rescattering of SRS in a large nonuniform
plasma are studied theoretically and numerically. The backscattered light of
convective SRS can be considered as a pump light with a finite bandwidth. The
different frequency components of the backscattered light can be coupled to
develop absolute stimulated Raman scattering (SRS) and two plasmon decay (TPD)
instability near their quarter-critical densities via rescattering process. The
absolute SRS mode develops a Langmuir wave with a high phase velocity about
$c/\sqrt{3}$ with $c$ the light speed in vacuum. Given that most electrons are
at low velocities in the linear stage, the absolute SRS mode grows with much
weak Landau damping. When the interaction evolves into the nonlinear regime,
the Langmuir wave can heat abundant electrons up to a few hundred keV. Our
theoretical model is validated by particle-in-cell simulations. The absolute
instabilities may play a considerable role in the experiments of inertial
confined fusion.
