Advances on Percussion Drilling with Femtosecond Laser in GHz-burst Mode

. Micromachining of various materials with femtosecond lasers operating in the GHz-burst regime has recently attracted great attention. In this contribution, we show our latest results on top-down percussion drilling in different dielectrics in this new operating regime. The dependence on the burst parameters such as burst repetition rate, number of pulses per burst, and burst energy are discussed. Moreover, we will focus on the influence of the burst shape on the drilling process. The quality of the drilled holes and their reachable dimensions are presented.


Introduction
Femtosecond laser micromachining of dielectrics with GHz-bursts has recently been demonstrated.Interesting results showing an enhancement of the ablation efficiency have been published [1][2][3].Moreover, top-down percussion drilling has been reported where almost cylindrically shaped holes with glossy inner walls were obtained featuring high aspect ratios and avoiding any crack formation [4,5] In this contribution, we show our latest results on percussion drilling in dielectrics with a femtosecond laser operating in GHz-burst mode.We investigate the influence of the different burst parameters, especially the burst shape, on the drilling performance.

Experimental setup
Our experiments were performed using an industrial femtosecond laser (Tangor 100 from Amplitude) operating at 1030 nm and emitting femtosecond pulses of about 500 fs pulse duration.The laser allows for selecting trains of femtosecond pulses (bursts) at a pulse repetition rate of 1.28 GHz.The number of pulses within the burst, the burst energy as well as the number of bursts can be chosen.Moreover, naturally, the shape of an amplified burst is gain depleted.In this experiment, we can choose the burst shape between different forms as schematically illustrated in Fig. 1.The drilling experiments are carried out on a setup which is entirely described in Ref. 5. The investigated samples are sodalime, fused silica, AF 32, and crystalline Sapphire.

Results and discussion
The depth and diameters of the drilled holes were measured with an optical measuring microscope (MF-B1010D, Mitutoyo) using a 10x objective.Figure 2 depicts a microscope image of a series of drilled holes in sodalime for the three burst shape configurations.We observe an excellent drilling quality of the holes which are almost cylindrical and have smooth inner walls as formerly observed for GHz-burst drilling [4,5].Furthermore, we investigated the evolution of the hole depth as a function of the number of bursts applied at the same point on the sample.Figure 3 shows the results for the three different burst shapes (flat, decreasing, and increasing).The burst energy was fixed to 200 μJ and the bursts contained 100 pulses in all three cases.With a decreasing burst shape, the depth increases faster (1.2 µm/burst) than in the other two cases (0.6 µm/burst).
a flat burst allows for achieving deeper holes with the same burst energy.

Conclusion
In conclusion, we investigated the burst shape on the drilling performance of holes drilled by a femtosecond laser operating in GHz-burst mode.We found out that a flat burst shape allows for attaining deeper holes than a burst with decreasing or increasing burst shape when the same energy is distributed within the burst.However, a decreasing burst shape features a higher drilling rate for small numbers of bursts.
The authors are grateful to CNRS Innovation for the funding support.

Fig. 2 .
Fig. 2. Microscope image of a series of drilled holes in sodalime for flat (a), decreasing (b), and increasing (c) burst shape.

Fig. 3 .
Fig. 3. Drilling depth as a function of number of bursts for three different burst shapes in sodalime, 100 pulses per burst, burst energy 200 μJ.