Increasing the Specific Removal Rate for Ultra Short Pulsed Laser-Micromachining by Using Pulse Bursts

The influence of the burst mode is investigated for steel, copper, silver and gold. It is shown that the gain in the removal rate, reported in literature, is generally caused by the lower fluence of the single pulses in the burst which is nearer the optimum value showing higher efficiency. Therefore, identical or even better results are obtained with single pulses of the same fluence but at higher repetition rate. However, for copper, silver and gold and a three pulse burst the second pulse hinders the ablation of the first pulse and it is supposed that it even re-deposits material. But compared to the first pulse the energy of the third pulse is much better converted to removed material leading to a real gain of 16% in efficiency for copper. An explanation of this typical behavior and its absence in the case of steel can be given by models of the ablation process dealing with a bulging and/or spalled layer and plasma as well.