Over the past two decades, oscillator development has been focused on power and energy scalable mode-locking techniques and ultrashort pulse generation directly from mode-locked fibre and crystal oscillators. While Yb-doped materials have been the most successful in energy and power scaling experiments [1,2], their relatively narrow spectral bandwidths pose a challenge for generating sub-100 fs pulses . Pulses with a duration of about 200 fs can be generated with Yb:YAG – the material of choice for power-scalablity – when combined with Kerr-lens mode-locking . However, this is still far from the fluorescence emission bandwidth limit of 120 fs. Other potential candidates such as Yb: LuScO3, Yb:KYW and Yb:CALGO show poor crystal quality or exhibit properties which prohibit further power scaling, especially in the thin-disk configuration. Theoretically there is no limitation for solitons circulating inside the cavity to reach or even exceed the emission bandwidth of the gain medium. Several experiments have already demonstrated this effect with crystalline bulk oscillators and even with Yb:YAG . However, no explanation has been provided for the intra-cavity spectral broadening. Moreover, bulk oscillators have limited potential for average and peak power scaling. In this contribution, we report a Kerr-lens mode-locked (KLM) Yb:YAG thin-disk oscillator with output spectral bandwidth (FWHM) of 33 nm directly from the oscillator. This is more than 3 times broader than the fluorescence spectrum of Yb:YAG (9 nm).
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