Influence of MHz bursts on the ablation efficiency of fused silica

We report on micromilling of fused silica using a 1030 nm femtosecond laser with an intraburst repetition frequency of 40 MHz. Milling of cavities reveals a monotonously increasing ablation efficiency with increasing subpulse number, while the ablation threshold of each subpulse decreases. These results prove that MHz bursts benefit the milling of fused silica. In comparison to the non-burst mode, the maximum efficiency increases about 1.6 times by using a 10 subpulse burst. In addition, bursts cause coarse ablated surfaces, whereas low subpulse numbers in conjunction with increasing fluences lead to a reduction of the surface roughness. A subpulse number of 2 keeps the roughness at the same level as for non-burst at high fluences while enhancing the ablation efficiency about 0.08 mm(3)/min/W.