Shaping of femtosecond laser pulses of petawatt level with plasma layers is analyzed. It is shown that, for electron density exceeding the critical density by several times, it is possible to generate asymmetrical pulse with the amplitude of the first halfcycle, which is practically equal to the maximal amplitude of the pulse. These pulses are necessary for effective generation of relativistic electron mirrors. Spatial structure of the pulse transmitted through a plasma layer is corresponding qualitatively to the field structure after diffraction of the same initial pulse on a slit.
