Klas, R.R.KlasEschen, W.W.EschenKirsche, A.A.KirscheRothhardt, J.J.RothhardtLimpert, J.J.Limpert2022-03-062022-03-062020https://publica.fraunhofer.de/handle/publica/26221210.1364/OE.28.006188High harmonic sources can provide ultrashort pulses of coherent radiation in the XUV and X-ray spectral region. In this paper we utilize a sub-two-cycle femtosecond fiber laser to efficiently generate a broadband continuum of high-order harmonics between 70 eV and 120 eV. The average power delivered by this source ranges from > 0.2 µW/eV at 80 eV to >0.03 µW/eV at 120 eV. At 92 eV (13.5 nm wavelength), we measured a coherent record-high average power of 0.1 µW/eV, which corresponds to 7 · 109 ph/s/eV, with a long-term stability of 0.8% rms deviation over a 20 min time period. The presented approach is average power scalable and promises up to 1011 ph/s/eV in the near future. With additional carrier-envelop phase control even isolated attosecond pulses can be expected from such sources. The combination of high flux, high photon energy and ultrashort (sub-) fs duration will enable photon-hungry time-resolved and multidimensional studies.enfiber laserlaser pulsecoherent radiationfemtosecond fiber lasershigh average power620621journal article