Micromachining of invar foils with GHz, MHz and kHz femtosecond burst modes

In this work, a burst mode laser is used for micromachining of 20 m-250 m thick Invar (Fe64/Ni36) foils. Holes were drilled by firing multiple pulses transversely onto the sample without moving the beam (percussion drilling). The utilized laser system generates a burst of a controllable number of pulses (at 1030 nm) with tunable pulse-to-pulse time spacing ranging from 200 ps to 16 ns. The sub-pulses within the burst have equal amplitudes and a constant duration of 300 fs that do not change regardless of the spacing in time between them. In such a way, the laser generates GHz to MHz repetition rate pulse bursts with a burst repetition rate ranging from 100 kHz to a single shot. Drilling of the material is compared with the non-burst mode of kHz repetition rate. In addition, we analyze the drilling speed and the resulting dependence of the quality of the holes on the number of pulses per burst as well as the average laser power to find the optimal micromachining parameters for percussion drilling. We demonstrate that the micromachining throughput can be of an order of magnitude higher when using the burst mode as compared to the best results of the conventional kHz case; however, excess thermal damage was also evident in some cases.