Experiments on deep drilling of steel by 300 ps, 1 ps and 125 fs laser pulses are reported. The ablation rate dependence on the channel depth was studied and energy losses in through channels for different radiation parameters were measured. The low-threshold cluster-assisted air breakdown was revealed to play an important role in ablation by 300 ps pulses. The ablated particles remaining inside the channel between laser shots provide substantial reduction of the air breakdown threshold. Laser-induced spark produces noticeable shielding effect and, presumably, is main reason of observed deep channel widening. Pronounced strengthening of light shielding by laser-induced spark was observed under steel target ablation comparing with pure air without target for ultrashort (125 fs, 1 ps) laser pulses. The dramatic reduction of the drilling rate in deep channels was observed for all examined pulsewidths. In the case of 300 ps pulses, the drilling rate falls down sharply by two order of magnitude at a certain critical channel depth increasing with the incident laser fluence. It was found that the integral plasma transmittance (breakdown plus ablation) remains unchanged when the drilling rate decreases.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.