Effect of plasma on ultrashort pulse laser material processing

Abstract

Machining with high power ultrashort-pulsed lasers is becoming a preferred technique in material processing. However, the laser beam passing through a medium, e.g., air, experiences the self-focusing Kerr effect. High intensities increased further by self-focusing cause optical breakdown of the air, generating plasma. The associated diffusion compensates for the Kerr effect but it also deforms the laser beam. In the present article, properties of the plasma columns so induced by the femtosecond laser pulses are studied, which are similar to the long filaments induced with collimated ultrashort pulses. It is found that the two effects balance each other very closely for the part of the beam. Thus, placing the focal position at an appropriate position results in improved drilling and cutting, i.e., with flat bottom, parallel wall, and less dross. Theoretical calculations of the intensity profile of the optical beam propagating through air are found to be commensurate with the experimental observations.