5. Conclusion

In conclusion, further average power scaling of Kerr-lens mode-locked thin-disk oscillators will have mainly technical limitations related to the thermal lensing in the dispersive mirrors. This can be circumvented by implementing large beam sizes on the already available dispersive optics and substrates with high thermal conductivity. In principle, the TEM00 CW performance of thin-disk lasers can serve as an upper limit for the average power scaling which currently lies well above 1 kW [70].

Peak power scaling of this technology is already facing the complications that are related to a reduced air pressure environment, however, these can be to a large extend circumvented by the implementation of the active multi-pass scheme and increased output coupling ratios. However, this limitation is rather technical and does not set a fundamental limit towards output peak powers in the GW range. The geometrical energy scaling concept described in combination with the intrinsic advantages of the KLM technique provides this peak power scalability nonetheless, at the expense of reduced ambient pressure. A limitation that is more fundamental will be due to the difficulties to initiate mode-locking. In other words, one single mode-locking element has to support starting from nearly zero peak power inside of the oscillator while also needing to provide stable mode-locked operation at intra-cavity peak powers exceeding 10 GW, thus, covering a huge peak power range. These demands on starting and running stably are rather contradictive, rendering this limitation intrinsic to all types of mode-locked oscillators.
