Optical frequency comb with submillihertz linewidth and more than 10 W average power

Growing demands for high average and peak powers in extreme nonlinear optics(1), attosecond-pulse(2,3) and extreme ultraviolet comb generation experiments(4,5) can find a powerful solution in fibre-based mode-locked lasers. Using passive enhancement cavities(6), fibre lasers have produced high-repetition-rate femtosecond pulse trains with multikilowatt average powers and peak powers reaching hundreds of megawatts(7). One major challenge for novel high-resolution spectroscopy and precision measurement in suboptical wavelength regions(8-10) is to transfer the state-of-the-art optical phase coherence into the extreme ultraviolet domain through the extreme nonlinear optics enabled by these high-power systems. We demonstrate here that optical frequency combs produced by high-power fibre lasers reach unprecedented levels of performance in both precision and average power. We achieve a record low relative linewidth of < 1 mHz between a traditional Ti:sapphire frequency comb and a novel 10 W average power fibre comb, at the same time demonstrating all the necessary elements for power scaling precision comb technology to beyond 10 kW.