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However, this implies high peak-intensities inside the oscillator cavity and therefore strong nonlinear effects even in air. Other limitations might arise due to damage thresholds of the intra-cavity optics because of the intra-cavity peak intensities approaching several 100 GW=cm2 and peak fluences up to several 10 mJ=cm2 or even higher during the pulse buildup phase. Up to now, these high intensities have not posed the major limitation to thin-disk KLM oscillators; however, this situation might change in the future when even higher peak and average powers will be targeted, especially in combination with a compact resonator design. Favorably low intensities can be provided by the pulse formation in the normal dispersion regime (chirped-pulse regime) which was first investigated in Ti:Sa oscillators [64] and is nowadays commonly employed to increase the pulse energies obtainable from fiber oscillators [65]. The pulses that form inside such an oscillator are strongly chirped, resulting in lower peak-powers at the same pulse-energy as compared to the solitons under anomalous dispersion. In contrast to Eq. (1), these pulses theoretically scale better in pulse energy with

> 

method of mode-locking allowed a major improvement in pulse energy [44, 67], the output from Yb-based mode-locked thin-disk oscillators did not improve over the anomalous dispersion regime [32, 68]. One of the reasons is the relatively narrow emission bandwidth of Yb: YAG and the necessity to introduce an additional spectral filter into the oscillator cavity. This spectral filtering was not performed in the work [68] due to additional complexity, losses and the high intra-cavity average power usually associated with thin-disk lasers. Moreover, no practical demand for the realization of a stable chirped-pulse regime has arisen till today since the limits of the anomalous dispersion regime in mode-locked thin-disk oscillators are not yet explored. However, this situation was different for the Ti:Sa bulk oscillators. Although this chirped-pulse regime appears attractive for power scaling and energy scaling [69], the downside seems an increased demand on the self-amplitude modulation to keep these pulses stable and provide reliable pulse-build-up. Due to the difficulties associated with this reliable pulsebuild-up, the positive dispersion regime was not further investigated in thin-disk oscillators. To date the highest peak-powers are obtained from solitonic oscillators working in the anomalous dispersion regime and this situation is unlikely to change until some technical limitations associated with high intra-cavity average power and extremely low repetition rate (very long

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

<sup>2</sup> [66]. While in Ti:Sa oscillators, this

respect to the dispersion-compensation such that E∝ β

resonator length) will approached.

currently lies well above 1 kW [70].

5. Conclusion

104 High Power Laser Systems

Oleg Pronin<sup>1</sup> \* and Jonathan Brons<sup>2</sup>

