Extreme ultraviolet (XUV) optical frequency combs have significant applications in precision measurement physics and ultrafast science. They are essential tools for high-resolution XUV spectroscopy, such as precise spectroscopic measurements of few-electron atomic or ionic systems (e.g., He⁺ and Li⁺), which probe the limits of quantum electrodynamics (QED) theory. Combining XUV optical frequency comb spectroscopy with thorium-229 nuclear transition could advance the development of a novel optical clock—the nuclear optical clock. In ultrafast science, utilizing the high-order harmonic generation (HHG) process inherent in XUV optical frequency comb generation reveals ultrafast phenomena under conditions of extremely high repetition rates (>50 MHz). In addition, XUV optical frequency combs provide high-repetition-rate and low-flux XUV light sources, which facilitate time- and angle-resolved photoelectron spectroscopy studies. The spectral coverage and output power are critical parameters for assessing the performance of an XUV optical frequency comb and realizing its broad applications. Extending its spectral coverage and increasing the output power are core objectives in developing XUV optical frequency combs and in the present study.

To optimize the key parameters of an XUV optical frequency comb, we enhance the peak power within the femtosecond enhancement cavity (fsEC) by compressing the pulse duration of the driving laser source. Firstly, we utilize a solid-core photonic crystal fiber for spectral broadening. The driving laser, assisted by our self-constructed beam stabilization system, is coupled into the solid-core photonic crystal fiber. The fiber’s photonic bandgap structure enables high-efficiency transmission of specific wavelengths through the solid fiber core, resulting in spectral broadening via the self-phase modulation (SPM) effect. We then use multiple chirped mirrors for compressing pulse duration. The multilayer structure of the chirped mirrors ensures precise dispersion compensation by providing different group delays for different frequency components. Finally, in the high-order harmonic generation and output coupling stage, we employ a bow-tie-shaped fsEC with six mirrors and in-cavity focusing to increase the peak power of the driving laser. The amplified driving laser pulses interact with the gaseous medium at the focal point, generating XUV light, which is then coupled out by either an Al₂O₃ Brewster plate or a micro-nano grating mirror (GM).

Our driving laser has a repetition rate of approximately 80 MHz, a maximum output power of 100 W, a central wavelength of 1035 nm, and a pulse duration of 490 fs. Fig. 2(a) shows the spectral broadening achieved with the solid-core photonic crystal fiber, and Fig. 2(b) shows the pulse shapes before and after compression. We successfully compress the pulse duration from 490 fs to 56 fs using chirped mirrors. With the aid of the fsEC and an amplification factor of over 100, we achieve a peak intensity of 9.3×10¹³ W/cm² at the focal point. Subsequently, we use an Al₂O₃ Brewster plate to couple out the XUV radiation and image the harmonic profile on a sodium salicylate plate. Figs. 5(a)-(c) show the fluorescence images for Xe, Kr, and Ar as the gaseous medium, respectively. The discrete spots on the fluorescence screen correspond to different harmonic orders. From the fluorescence images, we determine that the highest harmonic order generated is the 35th, corresponding to a photonic energy of about 42 eV (approximately 30 nm in wavelength). By replacing the output coupling device with a GM, we measure the power of the 17th harmonic (61 nm) and obtain a maximum average power of 9.3 μW when using Xe as the working gas, as shown in Fig. 6.

XUV optical frequency combs are high-quality, narrow linewidth, table-top coherent XUV light sources. In this study, we aim to expand the spectrum and enhance the power of the XUV optical frequency comb by compressing the pulse duration of the driving laser and increasing the peak power within the femtosecond enhancement cavity. Using a solid-core photonic crystal fiber for spectral broadening and chirped mirrors for precise dispersion compensation, we achieve a pulse duration of about 56 fs and a peak intensity of 9.3×10¹³ W/cm² within the fsEC. Using high-order harmonic generation processes with noble gases such as Xe, Kr, and Ar, we achieve an XUV optical frequency comb with the shortest wavelength of 30 nm (35th harmonic) and an output power of 9.3 μW at 61 nm (17th harmonic). This study lays a solid foundation for the succeeding precision spectroscopic measurements of few-electron atoms and molecules using the XUV optical frequency comb.