Surface profile of material ablated with high-power lasers in ambient air medium

In general, material processing with high-power ultra-short-pulsed lasers yields cleaner surfaces, as long as the intensity profile of the laser beam is well shaped. However, the beam suffers distortions during propagation through ambient atmospheric media such as air. Passage through such media causes the beam to self-focus, increasing the intensity further and causing the breakdown of the gas. The resulting plasma distorts the beam's original profile and the ablated surface conforms to the beam profile. A numerical scheme is developed here to calculate the intensity profile of an optical beam propagating through a medium. Intensity distribution of the beam is then used to determine the profile of the processed surface by a geometrical method developed recently. The calculated profile is compared with the experimentally obtained surface with good agreement. For medium spot sizes, the self-focusing and plasma effects tend to cancel each other, maintaining the intensity profile of the beam similar to the original Gaussian distribution. For small spot sizes when the intensity is high, the plasma effects are found to distort the beam profile. This indicates that the experimental parameters can be adjusted to improve the quality of the machined surface. (C) 2005 American Institute of Physics.