Drs Jakub, Fischer Julian, Modsching Norbert, Labaye François, Wittwer Valentin J, Südmeyer Thomas
Opt Express. 2021 Oct 25;29(22):35929-35937. doi: 10.1364/OE.440196.
We experimentally investigate the limits of pulse duration in a Kerr-lens mode-locked Yb:YAG thin-disk laser (TDL) oscillator. Thanks to its excellent mechanical and optical properties, Yb:YAG is one of the most used gain materials for continuous-wave and pulsed TDLs. In mode-locked operation, its 8-nm wide gain bandwidth only directly supports pulses with a minimum duration of approximately 140 fs. For achieving shorter pulses, a Kerr-lens mode-locked TDL oscillator can be operated in the strongly self-phase modulation (SPM) broadened regime. Here, the spectral bandwidth of the oscillating pulse exceeds the available gain bandwidth by generating additional frequencies via SPM inside the Kerr medium. In this work, we study and compare different laser configurations in the strongly SPM-broadened regime. Starting with a configuration providing 84-fs pulses at 69 W average power at 17 MHz repetition rate, we reduce the pulse duration by optimizing various mode-locking parameters. One crucial parameter is the dispersion control which was provided by in-house-developed dispersive mirrors produced by ion-beam sputtering (IBS). We discuss trade-offs in average power, pulse duration, efficiency, and intra-cavity peak power. For the configuration operating at the highest SPM-broadening, we achieve a minimum pulse duration of 27 fs, which represents the shortest pulse duration directly generated by any ultrafast TDL oscillator. The corresponding full width at half maximum (FWHM) spectral bandwidth exceeds more than five times the FWHM gain bandwidth. The average output power of 3.3 W is moderate for ultrafast TDL oscillators, but higher than other Yb-based laser oscillators operating at this pulse duration. Additionally, the corresponding intra-cavity peak power of 0.8 GW is highly attractive for implementing intra-cavity extreme nonlinear optical interactions such as high harmonic generation.
我们通过实验研究了克尔透镜锁模Yb:YAG薄片激光器（TDL）振荡器中脉冲持续时间的极限。由于其优异的机械和光学性能，Yb:YAG是连续波和脉冲TDL中最常用的增益材料之一。在锁模运行中，其8纳米宽的增益带宽仅直接支持最短持续时间约为140飞秒的脉冲。为了获得更短的脉冲，克尔透镜锁模TDL振荡器可以在强自相位调制（SPM）展宽 regime下运行。在此，振荡脉冲的光谱带宽通过克尔介质内部的SPM产生额外频率而超过可用增益带宽。在这项工作中，我们研究并比较了强SPM展宽 regime下的不同激光配置。从一种在17兆赫兹重复频率下以69瓦平均功率提供84飞秒脉冲的配置开始，我们通过优化各种锁模参数来缩短脉冲持续时间。一个关键参数是色散控制，它由内部开发的通过离子束溅射（IBS）制造的色散镜提供。我们讨论了平均功率、脉冲持续时间、效率和腔内峰值功率之间的权衡。对于在最高SPM展宽下运行的配置，我们实现了27飞秒的最短脉冲持续时间，这是任何超快TDL振荡器直接产生的最短脉冲持续时间。相应的半高全宽（FWHM）光谱带宽超过FWHM增益带宽的五倍以上。3.3瓦的平均输出功率对于超快TDL振荡器来说适中，但高于在此脉冲持续时间下运行的其他基于Yb的激光振荡器。此外，相应的0.8吉瓦腔内峰值功率对于实现腔内极端非线性光学相互作用（如高次谐波产生）极具吸引力。
