I. Introduction
Direct regenerative amplification of ultrashort pulses to mJ level may lead to a broadening of the optical spectrum caused by nonlinear effect of self-phase modulation. To avoid this, chirp compression techniques are commonly used, especially in femtosecond laser systems. Implementation of these techniques in picosecond lasers, however, is limited because of the narrow spectral width of the pulses. In order to overcome the limitations, various techniques based on the use of additional optical elements with a negative nonlinear refractive index, such as GaAs plates [1] or phase-mismatched second harmonic generation (SHG) crystals [2] are available for the spectral broadening compensation in picosecond lasers.