Efficient Yb-doped laser oscillator delivering 729 mW in 22-fs pulses

Thanks to their excellent thermal and spectral properties [1] and the possibility to pump them by high-power diodes, Yb-doped gain materials have revolutionized many application areas in ultrafast science [2] . However, laser oscillators based on those gain materials have so far been severely limited in power and efficiency for operation in the sub-30-fs regime [3] , [4] . Standard collinearly-pumped Yb-doped bulk laser oscillators use dichroic mirrors with a high transmission for the pump at around 980 nm and a high reflectivity at the lasing wavelengths. This leads to a strongly varying group delay dispersion (GDD) and limits the expansion of the optical spectrum towards short wavelengths [3] . Furthermore, to reach higher power levels, increased output coupling is required, resulting in a stronger population inversion. For common broadband Yb-doped gain materials such as Yb:CALGO, this shifts the gain maximum towards shorter wavelengths until reaching the spectral edge of the dichroic mirrors. Both reasons set a severe limitation to fully exploit the available gain bandwidth for the generation of even shorter pulses and hinder further power scaling of Yb-doped bulk oscillators in the few-cycles regime. Here, we show that we can overcome these spectral and power limitations by using a novel pump mirror design relying on cross-polarized optical pumping.