  Multi-pass cell (MPC) based temporal pulse compressors have emerged in the
last years as a powerful and versatile solution to the intrinsic issue of long
pulses from Yb-based high-power ultrafast lasers. However, the spectral
broadening of high-energy (typically more than 100 uJ) pulses has only been
realized in complex setups, i.e., in large and costly, pressure-controlled
vacuum chambers to avoid strong focusing, ionization, and damage on the
mirrors. Here, we present spectral broadening of 2 mJ pulses in a simple and
compact (60 cm long) multi-pass cell operated in ambient air. Instead of the
traditional Herriott cell with concave-concave (CC/CC) mirrors, we use a
convex-concave (CX/CC) design, where the beam stays large at all times allowing
both to minimize damage and operate in ambient air. We demonstrate spectral
broadening of 2.1 mJ pulses at 100 kHz repetition rate (200 W of average power)
from 2.1 nm (pulse duration of 670 fs) to a spectral bandwidth of 24.5 nm,
supporting 133 fs pulses with 96% transmission efficiency. We show the
compressibility of these pulses down to 134 fs, and verify that the spectral
homogeneity of the beam is similar to previously reported CC/CC designs. To the
best of our knowledge, this is the first report of a CX/CC MPC compressor,
operated at high pulse energies in air. Because of its simplicity, small
footprint and low cost, we believe this demonstration will have significant
impact in the ultrafast laser community.
