An effective focusing setting in femtosecond laser multiple pulse ablation

Most of reported works on laser micro-machining have been carried out under 'tight-focusing' arrangement. However, defocusing has to be considered when dealing with depth evolution in multiple pulse ablation. In this paper, we show that laser intensity distribution is very sensitive to defocusing and, in multiple pulse ablation, defocusing is unavoidable even under the 'tight focusing'. Both theoretic analysis and experimental result indicate that convergent defocusing is a better option for some micro-machining purposes. The optimal defocusing arrangement not only results in a maximum material removal rate (MRR) and maximum depth evolution, but also prevents harmful heat damage caused by extreme high intensity at the focal center.