[en] Amplification of laser pulses based on the backscattering process in plasmas can be performed using either the response of an electron plasma wave or an ion-acoustic wave. However, if the pulse durations become very short and the natural spread in frequency a substantial amount of the frequency itself, the Raman and Brillouin processes start to mix. Kinetic simulations show the transition from a pure amplification regime, in this case strong-coupling Brillouin, to a regime where a considerable downshift of the frequency of the amplified pulse takes place. It is conjectured that in the case of very short pulses, multi-modes are excited which contribute to the amplification process