  Multi-pass cell (MPC) compressors have proven to be the method of choice for
compression of high average power long-pulse Yb lasers. Yet, generating sub-30
fs pulses at high pulse energy with compact and simple components remains a
challenge. This work demonstrates an efficient and cost-effective approach for
nonlinear pulse compression at high pulse energy using a hybrid air-bulk MPC.
By carefully balancing the relative nonlinear contributions of ambient air and
fused silica, we achieve strong spectral broadening without dispersion
engineering or pressure-control inside the cell at 400 uJ pulse energy. In this
way, we compress pulses from 220 fs to 27 fs at 40.3 W of average power (100
kHz repetition rate), enhancing the peak power from 1.6 GW to 10.2 GW while
maintaining 78% of the energy within the main pulse. Our approach combines the
strengths of gas-filled and bulk compression schemes and exhibits excellent
overall optical transmission (91%) and spectral uniformity. Moreover, we
utilize the INSIGHT technique to investigate spatio-temporal couplings and
geometrical aberrations of the compressed pulse. Our results demonstrate
remarkable temporal homogeneity, with an average Strehl ratio of 0.97
consistently observed throughout the entire spectral profile. Additionally, all
spectrally-integrated Zernike coefficients for geometrical aberrations maintain
values below 0.02{\lambda}.

Optics