High-repetition-rate XUV frequency comb via femtosecond enhancement cavity at 200 MHz

Extreme ultraviolet (XUV) frequency combs serve as a powerful tool for advancing nuclear atomic clocks and high-resolution spectroscopy in the ultraviolet domain. Nevertheless, key challenges remain in realizing XUV frequency comb sources that simultaneously offer high repetition rates, excellent frequency stability, and compact configurations suitable for real-world applications. In this paper, we report on the generation of a high-repetition-rate XUV frequency comb at 200 MHz, enabled by intracavity high-order harmonic generation inside a femtosecond enhancement cavity (fsEC). The system is driven by a home-built, high-power and high-stability Yb-fiber frequency comb delivering 20 W of average power at 1037 nm with a repetition rate of 200 MHz. Through careful design and active optimization of the fsEC, we achieved an intracavity power enhancement ranging from ∼ 350 to 450 times, yielding a peak intensity of 3.58 × 10 13 W/cm 2 . This intense driving field facilitates high-order harmonic generation (HHG) up to 21st order (25 eV), covering the 49–148 nm spectral range (8–25 eV) with a photon flux sufficient for precision metrology. This source not only establishes a critical platform for next-generation nuclear clock transitions and attosecond metrology but also opens up broad opportunities in fundamental spectroscopy and high-throughput XUV science.