The results of numerical simulation of spatial noise filtering prior to the stage of nonlinear temporal compression of PW laser pulses are presented. It is shown that small-scale self-focusing (filamentation instability) can be substantially suppressed in a diffraction-grating optical compressor, since the spatial frequencies in the instability region lag behind or overtake the main pulse. This effect in an asymmetric compressor is weaker than in a symmetric one, despite a more efficient smoothing of spatial fluence fluctuations.
