In this paper, we report the experimental and theorical results of self-transmission of pi-cosecond light pulses of 1 μm in undoped semi-insulating GaAs at Brewster angle incidence. The best fitting density of the deep level EL2 is 1.2×l016cm-3, the two-photon absorption coefficient is 29 cm/GW and the free-carrier cross section is 2.7×10-17 cm2.