Burst mode ablation of stainless steel with tunable ultrashort laser pulses

In this study, an ultrashort pulse laser is used to investigate the removal efficiency and the ablation quality of stainless steel. The employed solid state laser is capable of varying the pulse duration from 0.27 to 10 ps and generates bursts with an intraburst pulse repetition frequency of 65 MHz with up to nine pulses per burst. Depending on the fluence per pulse, the pulse duration, and the number of pulses per burst, the removal efficiency and the ablation quality are presented and discussed based on the depth of the ablation structures and the surface roughness of the structured bottoms. The results prove that compared to pulse durations in the picosecond regime, the ablation efficiency in the femtosecond regime is significantly higher. The removal efficiency per burst is not affected by an increase in the number of pulses in the burst, but a smoothing effect can be identified for a certain number of pulses in the burst depending on the fluence and the pulse duration, which has a positive effect on the ablation quality. The temperature distribution and the heat accumulation induced by the high intraburst pulse repetition frequency are calculated with a semiempirical two-temperature model. The simulation results demonstrate that the melting film depth has a major influence on the smoothing effect caused by the burst mode.