Numerical simulation of multi-pulsed femtosecond laser ablation: effect of a moving laser focus

In femtosecond laser helical drilling, the laser focus moves downward into the workpiece as material is removed, which is one of the main features used to improve the processing efficiency. Numerical modeling is the primary method used to study this process. To reduce the spatial scale of the numerical model, the influence of the helical trajectory was not considered in this study. Instead, the influences of the pulse interval and the downward velocity of the laser focus on the ablation process during deep-hole processing with multi-pulse femtosecond laser ablation were explored, and the existing two-temperature model was adapted. We used the critical-point phase separation model to describe the material ablation process with a femtosecond laser. Using a copper workpiece, we simulated an ablation process in which multiple pulses from a femtosecond laser were focused onto the workpiece and the focus moved into the workpiece at a constant speed. We used the finite element method to determine the variation in the electron and lattice temperatures, as well as the ablation depth under different pulse intervals with the focus moving downward at different velocities. The results demonstrate that the pulse interval is an important factor affecting the ablation depth during multi-pulse femtosecond laser ablation. As the pulse interval increased, the ablation depth first increased and then decreased. The laser ablation efficiency was highest when the pulse interval was 200 ps. The downward velocity of the laser focus determined the defocusing distance during laser processing. We obtained a high processing efficiency when a reasonable downward velocity was adopted to maintain the defocusing distance within approximately 50 nm.