High Rate Laser Processing of Metals Using High Average Power Ultrashort Pulse Lasers
J. Schille, L. Schneider, L. Hartwig and U. Loeschner
High rate laser processing of technical grade stainless steel by using high pulse repetition frequency (PRF) ultrashort pulse lasers is studied in order to gain deeper insights into material removal for micro fabrication. For this, high average power picosecond and femtosecond lasers are utilized, providing pulses of tens of megahertz repetition rate and output powers of 76.0 and 32.0 W, respectively. For (ultra)fast raster scanning of the laser beam, the laser systems are synchronized with a high precision galvanometer scanner or rather an in-house developed polygon scan system. The influence of the processing parameters (laser power, pulse energy, pulse repetition rate, pulse duration, and fluence) on the ablation rate, removal efficiency, and throughput are studied. In picosecond laser irradiation of stainless steel, for example, the maximum material removal rate is 5.40 mm³/min, obtained with 76.0 W average laser power. This is lower than the removal rate of 6.80 mm³/min by using the femtosecond laser beam and 32.0 W average output power. The varying removal rate is the result of the different removal efficiency of picosecond and femtosecond laser pulses, which was determined in the experimental study of 0.07 and 0.22 mm³/W/min, respectively, using identical processing parameters except the pulse duration. Finally, the machining quality of laser processed micro cavities is evaluated by means of surface roughness measurements and microscopic analysis. As a result, the mid-fluence range between 0.40 and 1.50 J/cm² is identified for ultrashort pulses as most suitable processing regime for microfabrication with regard to removal efficiency, throughput, and machining quality.
Keywords: Femtosecond laser, picosecond laser, stainless steel, microprocessing, average power, high pulse repetition frequency (PRF), ultrashort pulse, high rate, high throughput, polygon