  It is proposed to replace the traditional counterpropagating laser seed in
backward Raman amplifiers with a plasma wave seed. In the linear regime,
namely, for a constant pump amplitude, a plasma wave seed may be found by
construction that strictly produces the same output pulse as does a
counterpropagating laser seed. In the nonlinear regime, or pump-depletion
regime, the plasma-wave-initiated output pulse can be shown numerically to
approach the same self-similar attractor solution for the corresponding laser
seed. In addition, chirping the plasma wave wavelength can produce the same
beneficial effects as chirping the seed wave frequency. This methodology is
attractive because it avoids issues in preparing and synchronizing a
frequency-shifted laser seed.
