A significant reduction of the influence of the thermal lens in thin-disk lasers in high power laser operation mode could
be achieved, using dynamically stable resonators. For designing the resonator, investigations of thermally induced phase
distortions of thin-disks as well as numerical simulations of the field distribution in the resonator were performed. This
characterization was combined with thermo-mechanical computations.
On the basis of these studies, about 500 W output power with an averaged <i>M<sup>2</sup></i> = 1.55 could be demonstrated, using one
disk. Almost 1 kW output power with good beam quality could be extracted, using two disks. For the purpose of further
power scaling in nearly fundamental mode operation, experiments using more than two disks are in preparation.
In principle, the thin-disk laser concept opens the possibility to demonstrate high power, high efficiency and good beam
quality, simultaneously. For this purpose, a very homogeneous pump power distribution on the disk is necessary as well
as very low phase distortions of the disk itself.
Spatial mode structure and thermal lens effects in an Yb:YAG thin-disk laser have been investigated as function of the
pump power in linear and folded resonators. Whereas thermal lens is shown to be very weak due to the thin disk
geometry, a strong correlation of the laser mode with respect to the power density distribution of the pump radiation is
exhibited. The experimental results are compared with numerical simulations of the field distribution within the
resonator as well as in the far field demonstrating the excellent homogeneity of the disk as laser active medium.