Spatial and Temporal Study of the Post-Compressed High-Power Laser Pulses for Coherent Extreme Ultraviolet Source Development

Post-compression based on self-phase modulation (SPM) is widely used for femtosecond pulse shortening. However, the influence of the driving beam wavefront on different compression schemes remains unexplored. Using a Yb-doped fiber laser (230 fs, 200 kHz), we experimentally compare pulse compression in a hollow-core fiber (HCF) and a multi-pass cell (MPC). The HCF compresses pulses to 27 fs with an efficiency of approximately 55% and improves beam quality via modal filtering. The MPC achieves 34 fs pulses with an efficiency of approximately 90% and exhibits a quasi-waveguide mode-filtering effect, substantially enhancing output wavefront quality even when the input wavefront is poor. High-harmonic generation (HHG) experiments show that the HCF-driven source yields a higher photon flux (1.25 × 10¹¹ photons/s) compared to the MPC-driven source (4.95 × 10¹⁰ photons/s). Using a second Yb-doped fiber laser (223 fs, 100 kHz), a cascaded MPC–HCF scheme generates 7.5 fs pulses with an overall efficiency of approximately 70%, enabled by employing a larger-core HCF in the second compression stage. HHG experiments performed with the compressed pulses demonstrate that spatial phase evolution is a critical parameter in post-compression design for such applications.