Abstract

Ultrashort laser pulses in the microjoule regime are interesting for many applications, including micromachining, pumping of parametric devices, as well as basic research, e.g. in high-field physics [1]. Since these applications rely upon nonlinear effects, high pulse energies and peak powers are desired. To some extend the pulse energies from an oscillator can be scaled by increasing the resonator length, e.g. using passive multipass Herriott cells [2–3], which, however, may compromise the efficiency of loss-sensitive resonators. Pulse energies of up to 11 μJ were obtained previously, however being forced to purge the laser system in helium [1,3]. Here, we especially emphasis on a thin-disk laser oscillator with selfimaging active multipass cell (AMC) (see Fig. 1) and large output coupling rates for a suppression of nonlinear optical effects [4, 5]. The experimentally achieved pulse energies of more than 25 μJ at sub-picosecond pulse length of 928 fs are believed to be the highest ever obtained directly from an ultrafast laser oscillator without further amplification stages. With this system we have obtained stable single pulse operation in ambient atmosphere with average output powers above 76 W at a repetition rate of 2.93 MHz, corresponding to 13 passes through the AMC. A semiconductor saturable absorber mirror was used to start and stabilize passive soliton mode locking. The experimental results are in good agreement with numerical calculations including the appearance of Kelly sidebands. We present a modification to the soliton area theorem that is applicable for such a laser oscillator with active multiple pass cell and large output coupling rate. The resulting reciprocal dependence of the pulse width on the pulse energy is shown in Fig. 1 (left). In this case the laser was operated with 11 passes through the AMC and pulses of up to 811 fs in duration. While numerically simulating the laser, we also investigated the intracavity pulse dynamics within one round-trip and limitations for scaling of the attainable energies to more than 80 μJ. Highly efficient material processing without significant heat affects is demonstrated. The simulations show that at the moment the pulse energies are limited by double pulses caused by gain filtering effects. In Fig. 1 (right) the pulse duration over pulse energy as anticipated for a constant OC rate, GDD, and SPM-coefficient γ_SPM according to the modified soliton area theorem are shown for various amounts of GDD/γ_SPM. As can be infered from the figure, even larger pulse energies can be obtained by increasing the total GDD inside the cavity or by decreasing the SPM. However, an increase in GDD will eventually result in strong Kelly sidebands which also destabilize single pulse behaviour.

© 2009 IEEE