Active mirrors for kW-class fundamental-mode thin-disk lasers

The development of the thin-disk laser [1] marks a major improvement in terms of power scaling at good beam quality when compared to classical solid-state laser concepts. The highly efficient cooling scheme leads to a significant reduction of thermally induced wave front distortions. The reduced spherical component of the distortions, often referred to as “thermal lensing” [2], enables the realization of brilliant lasers with a broad dynamic stability range. When it comes to fundamental-mode operation, however, the aspherical components of the wave-front distortions introduced by the strong temperature gradient at the edge of the pump spot in the laser crystal lead to diffraction losses and therefore severely limit the power scalability. As stable fundamental-mode operation is an important prerequisite for the efficient generation of ultra-short pulses by mode-locking, these losses are also limiting the scalability of the average power of thin-disk based femtosecond oscillators. This limitation can be overcome by means of intra-cavity deformable mirrors which are used to compensate for the aspherical components of the wavefront distortions [3].