Laser drilling was one of the first applications of solid-state lasers. Nevertheless, it is presently a field of growing interest. To produce a large number of holes at high speed, it is necessary to use lasers with high beam quality. One approach was the unstable resonator, where excellent beam quality was expected but never obtained. This is due to stress-induced birefringence, resulting in bi-focusing and unsatisfactory laser applications.
With a newly developed birefringence compensation scheme, it is possible to avoid this drawback. The laser consists of one cavity with segmented rods, a quartz rotator, and a gradient reflectivity mirror as outcoupler. At 220 W average power, the beam parameter product of the birefringence-compensated laser was 6 mm*mrad with a resonator length of 0.5 m. Pulse energy was attenuated to 1/10 to drill stainless steel sheets.