Simulation and experiment on femtosecond and nanosecond laser processing

We performed 1.5-dimensional simulation of the Fokker-Planck equation using the CIP method to investigate femtosecond-laser heating processes. We found that the heat flux in the solid part approaches the classical thermal conduction theory like Spitzer-Harm theory in quite a short time scale omega(p)t<100, while the heat flux on the vacuum side becomes free streams which don't depend on temperature gradient. On the basis of this result, we performed hydrodynamic simulation using the CIP method with classical thermal conduction and the experimental ablation depth was replicated very well showing that even fs pulse laser processing can be satisfactorily described by the classical heat conduction. As an application to nanosecond laser, we tried to drill a crack-free high-aspect-ratio hole through glass by YAG laser of 1mum wavelength, 5ns pulse width and 30-630mJ/pulse. The hole is 25.5mm long with 500mum radius and no crack is observed even in atmospheric condition. This success is due to translucent adhesive tape pasted on glass surface in which the temperature is controlled and thermal stress is reduced.