Cascaded harmonic generation from a fiber laser: a milliwatt XUV source

Recent progresses in femtosecond ytterbium-doped fiber laser technology are opening new perspectives in strong field physics and attosecond science. High-order harmonic generation from these systems is particularly interesting because it provides high flux beams of ultrashort extreme ultraviolet radiation. A great deal of effort has been devoted to optimize the macroscopic generation parameters. Here we investigate the possibility of enhancing the single-atom response by producing high-order harmonics from the second, third and fourth harmonics of a turnkey 50 W, 166 kHz femtosecond Yb-fiber laser providing 135 fs pulses at 1030 nm. We show that the harmonic efficiency is optimal when the process is driven by the third harmonic, producing 6.6 ± 1.3 × 10¹⁴ photons/s at 18 eV in argon, which corresponds to 1.9 ± 0.4 mW average power.