Intensity profile distortion at the processing image plane of a focused femtosecond laser below the critical power: Analysis and counteraction

• Experimental demonstration of profile distortion at high fluences of cavities machined at the image plane using a femtosecond laser.
• Theoretical and numerical evidences of the distortion of the machining beam considering the phase modulation induced by the second order non-linearity induced by the focus of the laser.
• Experimental solution to counteract this problem and thus to obtain the true cavity profile even at higher fluencies.

Femtosecond laser surface processing of materials allows for precise micro or sub-micrometer machining with restricted detrimental side effects. Thus, fine control of the laser intensity distribution (repeatability) in the processing plane is of outmost importance for industrial implementation. In this paper, we study the influence on non-linear effects on the machining quality. We experimentally study the profiles of cavities machined at the image plane of a focused femtosecond laser for a large set of fluences on stainless steel below the critical power. A strong distortion of the cavities is observed for high energetic levels. A beam analysis performed in the machining conditions reveals that the cavity profile follows the laser beam profile even at high fluences where the beam undergoes an increasing distortion. Numerical calculations of the laser beam distribution taking a Kerr effect into account are presented showing a good agreement with the experimental laser profile. To counteract the beam distortion at high fluences, we propose and successfully employ a robust solution consisting in geometrically forming the image processing plane before the laser focusing point. This ensures a beam profile free of distortion even at high fluences. Experimental evidence is made, showing a significant quality increase of the cavity profiles with an image plane placed before the focus point.