Beam Shaping for Uniform and Energy-efficient Surface Structuring of Metals with Ultrashort Laser Pulses in the mJ Range

Scaling the average power of ultrafast lasers into the kW range for high-throughput surface structuring and micromachining by the pulse energy requires an adapted processing strategy in order to minimize surface defects due to high fluences and maintain a high surface quality. One promising approach to distribute the pulse energy uniformly on the sample surface is spatial beam shaping using a spatial light modulator. The local phase was modulated by computer-generated holograms for the generation of uniform intensity distributions in the focal plane. The corresponding intensity distributions resulting from the different holograms were monitored with an on-axis camera to ensure accurate beam shapes during surface structuring and micromachining of the two materials stainless steel and cemented tungsten carbide. Uniform surface structures and flexible, dimensionally accurate cavities were machined with a shaped ultrafast laser beam and high pulse energies up to over 1 mJ on the sample surface. Various beam shapes with a cross-sectional area up to 0.42 [mm.sup.2] were generated on the sample surface for energy-efficient micromachining and homogenous surface structuring. The capability of high-energy pulse beam shaping was demonstrated by the fabrication of a large-area checkerboard pattern with small transition zones