Double-pass multipass cell compressor for peak power enhancement of µJ level pulses

Multipass cells (MPCs) have proven to be a robust and versatile solution for spectral broadening and subsequent compression of high average power Yb-based ultrafast lasers with multi-100 fs pulse durations. However, generating high compression ratios and peak power enhancement at low pulse energies (few-µJ) with compact setups remains challenging because of the moderate peak intensity available for broadening. In this regime, bulk solid-state plates are typically required, and their discrete nature in the nonlinear propagation reduces the B-integral tolerable per pass without beam degradation. In this paper, we present a simple approach for nonlinear pulse compression at µJ pulse energy with large compression ratio. We suggest a bulk-based double-pass MPC scheme, where the pulses propagate once through a traditional MPC, are partially compressed at the output and then are sent back for a second broadening step through the same MPC. We numerically optimize and apply this concept to compress a modelocked thin-disk laser with 112 W average power and 8.8 µJ pulses from 600 fs to 40 fs, achieving a compression ratio of 15, and peak power enhancement of 9.3, while maintaining a high overall optical efficiency of 89% and excellent spatial beam quality. The final peak power of our system reaches 122 MW, and the high reachable peak intensity of our system is confirmed by generating a microplasma in ambient air that will be used in future THz generation experiments.