Beyond nonlinear saturation of backward Raman amplifiers
  Backward Raman amplification is limited by relativistic nonlinear dephasing resulting in saturation of the leading spike of the amplified pulse. Pump detuning is employed to mitigate the relativistic phase mismatch and to overcome the associated saturation. The amplified pulse can then be reshaped into a mono-spike pulse with little precursory power ahead of it, with the maximum intensity increasing by a factor of two. This detuning can be employed advantageously both in regimes where the group velocity dispersion is unimportant and where the dispersion is important but small. 
Ido Barth, Zeev Toroker, Alexey A. Balakin, and Nathaniel J. Fisch
physics.plasm-ph