Scaling the productivity of laser structuring processes using picosecond laser pulses at average powers of up to 420 W to produce superhydrophobic surfaces on stainless steel AISI 316L.

We investigate the approach to scale up the productivity of the laser-based generation of superhydrophobic surfaces by means of increased average laser powers to enhance the surface structuring rates. Polished surfaces (mean roughness depth SRz = 0.076 µm) of stainless steel AISI 316L were processed with a laser delivering 8 ps long pulses with a constant pulse energy of 1.4 mJ at pulse repetition rates of 100 kHz or 300 kHz corresponding to average laser powers of 140 W or 420 W, respectively. When the feed rate for the corresponding pulse repetition rate is adjusted in a way to result in a similar temperature increase due to heat accumulation effects and the re-deposition of nanoparticles formed during processing is avoided, comparable surface structures with similar wetting behavior are obtained.

Determination of the thermally induced focal shift of processing optics for ultrafast lasers with average powers of up to 525 W.

The continuous increase of the average laser power of ultrafast lasers is a challenge with respect to the thermal load of the processing optics. The power which is absorbed in an optical element leads to a temperature increase, temperature gradients, changing refractive index and shape, and finally causes distortions of the transmitted beam. In a first-order approximation this results in a change of the focal position, which may lead to an uncon-trolled change of the laser machining process. The present study reports on investigations on the focal shift induced in thin plano-convex lenses by a high-power ultra-short pulsed laser with an average laser power of up to 525 W. The focal shift was determined for lenses made of different materials (N-BK7, fused silica) and with different coatings (un-coated, broadband coating, specific wavelength coating).