Role of gas environment in the process of deep-hole drilling by ultrashort laser pulses

The detailed study of the role of air pressure in deep hole drilling by femtosecond and picosecond intense laser pulses (Ti:Al 2 O 3 and Nd:YAP lasers) was performed in the range 1÷1000 mBar. Steel sample plates were mostly tested, experimental data obtained for ceramic materials is also presented. The following ablation parameters were measured and analyzed: ablation rates and their dependence on the channel depth, ablated crater morphology, optical transmission in channels after through hole formation. Both percussion and helical drilling regimes were used. Special attention was paid to two strong gas assisted effects typical of sub-picosecond and sub-nanosecond material ablation, which are low threshold gas breakdown in deep channels and nonlinear interaction of ultra-short intense pulses with air resulting in conical emission. Unwanted aspects of both phenomena were shown to disappear in a moderate vacuum of ~100 mBar. A new approach to formation of such a vacuum in drilled channels was also proposed and experimentally modeled using ultra-high repetition rate nanosecond laser pulses.