One of the most promising methods for obtaining ultraintense ultrashort laser pulses is currently based on the use of Raman backscattering in a plasma. It can provide an output power higher by a factor of 104?105 than that with the usual technique for amplifying frequency modulated pulses in the plasma [1]. The compression regime based on Raman backscattering in the plasma was experimentally implemented [2, 3]. In particular, the implementation of a nonlinear regime with depletion of the pump pulse was demon strated. However, the nonlinear regime achieved in experiments was not transferred to the stage of a significant amplification of an output pulse. The reason was various parasitic effects responsible either for the enhancement of noise (thermal fluctuations of the plasma and the prepulse of the amplified pulse) or for the violation of the conditions of three wave matching for Raman backscattering because of the inhomogeneity of the plasma density. Here, another method of the use of Raman backscattering in the plasma is proposed to obtain intense ultrashort laser pulses with a given phase front. In this case, requirements on the parameters of the plasma and on the duration of its existence in this regime are softer than those in the usual scheme of Raman compression. The method is based on the fact that phase perturbations of the pump pulse that are smooth in the longitudinal direction are concentrated in the weakest wave (plasma wave). As a result, the amplified pulse will have a weakly perturbed initial phase front and, correspondingly, good focusability.

1. V. M. Malkin, G. Shvets, and N. J. Fisch, Phys. Plasmas 7, 2232 (2000).
2. A. A. Balakin, D. V. Kartashov, A. M. Kiselev, S. A. Skobelev, A. N. Stepanov, and G. M. Fraiman, JETP Lett. 80, 12 (2004).
3. A. A. Balakin, G. M. Fraiman, N. J. Fisch, and S. Suckewer, Phys. Rev. E 72, 036401 (2005).