Nonlinear compression of mJ-level pulses via double-pass loose focusing in air

Developing high-intensity lasers with ultrashort pulses is crucial for strong field physics and its applications [1]. Simultaneously amplifying ultrashort pulses and shortening pulse duration represent a great challenge. The most prevailing ultrashort high-power lasers are titanium:sapphire (Ti:Sa) and ytterbium (Yb) ion-doped lasers. Compared to extremely broadband Ti:Sa lasers, lasers using Yb-doped gain media are found to have better average power scalability but a narrower emission bandwidth. There are commonly two methods employed to achieve high peak and average power of femtosecond sources: optical parametric amplification (OPA) and post-compression. OPA offers a method to amplify a supercontinuum pulse over a large bandwidth by pumping it with narrow-band but high-energy laser pulses. Post-compression methods directly broaden the spectral width of the high-power pulses through nonlinear interactions such as self-phase-modulation [2].