4.1. Further power scaling opportunities

Even though the geometrical scaling-procedure enables near 100 MW pulses directly from KLM oscillators, it relies on a substantial intra-cavity peak- and average power increase. This raises the demands on the coating damage thresholds during stressful events such as the transition to mode locking. The elevated intra-cavity average powers of more than 1 kW also require careful selection of the utilized materials to prevent thermal lensing instabilities. Complementary to the intra-cavity power-scaling, it is possible to boost the oscillator output by enlarging the output-coupling ratio. This concept has been realized with a mode-locked thin-disk oscillator in [62] for the first time using an imaging multi-pass configuration. Up to 72% of the circulating power were extracted from the SESAM mode-locked oscillator in [30] resulting in 145 W average power output while the intra-cavity value was only 200 W. The short interaction length in the thin disk cannot replenish such high power-loss during a single roundtrip which needs to be overcome by an increased number of beam-passes through the disk, e.g., with an imaging multi-pass cell (20 passes realized in [30]).

The number of disk passes cannot be made arbitrarily high, however, since any thermal lens in the disk is accumulated, giving rise to a narrowing of the cavity-stability zones with respect to the pump power. While this effect has obviously not hindered comprehensive implementation in an oscillator-cavity working at the center of the stability-zone [30], it is not as obvious that the same can be done for KLM-cavities that are more sensitive to the presence of thermal lenses. However, recently a first demonstration of this multi-pass concept in a thin-disk KLM oscillator was realized with six double passes through the thin disk per round-trip resulting in 130 W average and 20 MW peak output power [63]. With respect to a reference oscillator, the peak power did not drop when increasing the output-coupling ratio to 30%, rendering it an encouraging result towards scaling the output coupling ratio to 50%.

### 4.2. Positive dispersion regime

All oscillators presented (see Table 1) were mode locked in the anomalous dispersion regime providing bandwidth limited, unchirped pulses with a well behaved temporal phase. 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 respect to the dispersion-compensation such that E∝ β <sup>2</sup> [66]. While in Ti:Sa oscillators, this 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 resonator length) will approached.

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

Kerr-Lens Mode-Locked High-Power Thin-Disk Oscillators

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contradictive, rendering this limitation intrinsic to all types of mode-locked oscillators.

[1] Russbueldt P, Hoffmann D, Hofer M, Lohring J, Luttmann J, Meissner A, et al. Innoslab amplifiers. IEEE Journal of Selected Topics in Quantum Electronics. 2015;21(1):447-463.

[2] Eidam T, Hanf S, Seise E, Andersen TV, Gabler T, Wirth C, et al. Femtosecond fiber CPA system emitting 830 W average output power. Optics Letters. 2010;35(2):94-96. DOI:

[3] Fattahi H, Barros HG, Gorjan M, Nubbemeyer T, Alsaif B, Teisset CY, et al. Third-generation femtosecond technology. Optica. 2014;1(1):45-63. DOI: 10.1364/OPTICA.1.000045

[4] Zhang J, Brons J, Lilienfein N, Fedulova E, Pervak V, Bauer D, et al. 260-megahertz, megawatt-level thin-disk oscillator. Optics Letters. 2015;40(8):1627-1630. DOI: 10.1364/

[5] Pronin O, Seidel M, Lucking F, Brons J, Fedulova E, Trubetskov M, et al. High-power multi-megahertz source of waveform-stabilized few-cycle light. Nature Communications.

Author details

\* and Jonathan Brons<sup>2</sup>

DOI: 10.1109/JSTQE.2014.2333234

2015;6:6988. DOI: 10.1038/ncomms7988

10.1364/OL.35.000094

OL.40.001627

\*Address all correspondence to: oleg.pronin@mpq.mpg.de

1 Max Planck Institute of Quantum Optics (MPQ), Germany 2 Ludwig Maximilian University of Munich (LMU), Germany

Oleg Pronin<sup>1</sup>

References
