Femtosecond and attosecond spectroscopy benefit directly from the availability of ultrashort, high-power lasers at megahertz (MHz) repetition rates. High repetition rates allow decreasing the measurement's acquisition time and enhancing signal to noise ratio. Moreover, efficient extension of the spectral band-width of high repetition rate sources to beyond one octave paves the way for novel broadband spectroscopy techniques, such as femtosecond fieldoscopy. Nowadays, ytterbium thin-disk oscillators and fiber lasers deliver 100-W-scale average power, at MHz repetition rates [1, 2]. However, their pulse duration is limited to hundreds of femtoseconds due to the narrowband emission cross-section of ytterbium, which can be overcomed by employing efficient, external spectral broadening schemes based on solid state materials  or gas filled hollow-core fibers [4,5]. In what follows we report on the external spectral broadening of 265 fs, 6 μJ pulses from a Yb:YAG thin-disk oscillator to 7fs (Fourier transform limit-FTL) in three nonlinear stages.
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