Enhanced ablation efficiency using GHz bursts in micromachining fused silica

We report on micromilling cavities into fused silica by a 1030 nm femtosecond laser using 2.17 GHz bursts. The milled cavities show an increased depth per layer for a higher number of pulses per burst while the ablation efficiency is also increased. The maximum ablation efficiency for the optimum fluence achieved in our experiments is 3.05mm³/min/W for a burst number of 10, which is 7.4 times higher than for the non-burst condition (0.41mm³/min/W). Furthermore, the ablation threshold for each sub-pulse is significantly reduced from 0.64J/cm² for the non-burst condition to 0.15J/cm² for 10 bursts. Beside the ablation efficiency, the surface roughness is also increased with the increasing burst number, while two ablation behaviors can be distinguished, namely, a gentle ablation regime for lower burst numbers and a coarse ablation regime, dominated by breaking out the surface rather than ablating it.