## 2 Average Power Scaling of Modelocked Thin-Disk Lasers: Challenges and Milestones
### Harnessing Intracavity Nonlinearity
Furthermore, a larger gain per roundtrip results in longer minimum achievable pulse duration [28, 29]. This is not an issue in industrial applications such as high-speed micromachining, where obtaining short pulse duration is not of critical importance. In the case of scientific applications, shorter pulse durations are more critical, in particular since it simplifies further pulse compression schemes. - Multiple gain passes through different disks The combination of several laser heads in one cavity has already been demonstrated [12], but not in the context of modelocked TDLs. In this case the main difficulty is to achieve fundamental mode operation, as each disk can show a different thermal lensing behavior. - Vacuum environment Operating the oscillator in vacuum by placing it in a vacuum chamber allows for a reduction of the nonlinearity of the ambient environment by several orders of magnitude, since the nonlinear refractive index \(n_{2}\) parameter varies linearly with air pressure for medium vacuum levels [39]. In addition to a minimal nonlinear phase shift at very high intracavity peak power levels, the advantages of operating the oscillator in vacuum are many-fold: - Fine adjustment of the air pressure inside the cavity enables one to tune the nonlinear phase shift and minimize the pulse duration at a given output power. This eliminates the need for a moving Brewster plate in the cavity that can introduce aberrations and small losses. - The low amount of nonlinearities allows one to operate in simple oscillator geometries with low gain per cavity round-trip (one or two passes on the disk used as a folding mirror in the cavity). - Only small amounts on negative GDD are required to compensate for the small nonlinear phase shifts even at high intracavity peak powers. This results in lower parasitic losses and thermal effects that can occur in the dispersive mirrors. - Turbulences of air and related pointing instabilities are minimized. - Operating in a moderate vacuum environment allows for keeping the oscillator optics clean, which is critical at high intracavity powers. - In addition to the above-mentioned points, developing robust vacuum oscillator technology will facilitate future intracavity HHG experiments, for which operation in vacuum is essential to avoid UV-light absorption by air. Furthermore, for other nonlinear experiments, one could consider flooding the chamber with other gases. On the other hand, heat convection due to air is nearly completely eliminated by operating the oscillator in vacuum. In our experiment, we observed increased thermal effects in critical optics such as the dispersive mirrors when the oscillator was operated in vacuum. Nevertheless, using thermally improved mirrors and/or actively cooling these critical elements in the cavity (in the same way the disk or the SESAM are cooled) can solve these issues. The advantages of this promising new approach were recently confirmed for the first time with the demonstration of a thin disk oscillator with 275 W [17]. In the next paragraph, we will summarize this recently achieved power scaling result which paves the way to even higher average output powers and pulse energies from such ultrafast oscillators.