High-power femtosecond molecular broadening and the effects of ro-vibrational coupling

Scaling spectral broadening to higher pulse energies and average powers is a critical step in ultrafast science, especially for narrowband Yb-based solid state lasers, which have become the new state-of-the-art. Despite their high nonlinearity, molecular gases as a broadening medium inside hollow-core fibers have been limited to 25 W, at best. We demonstrate spectral broadening in nitrogen at ten-fold average powers up to 250 W with repetition rates from 25 to 200 kHz. The observed ten-fold spectral broadening is stronger compared to the more expensive krypton gas and enables pulse compression from 1.3 ps to 120 fs. We identified an intuitive explanation for the observed average power scaling based on the density of molecular ro-vibrational states of Raman active molecules. To verify this ansatz, the spectral broadening limitations in O 2 and N 2 O are experimentally measured and agree well. On these grounds, we propose a perspective on the role, suitability, and limits of stimulated Raman scattering at high average and peak powers. Finally, high-harmonic generation is demonstrated at 200 kHz. These findings can have strong implications for intense, high-repetition-rate, pulsed ps laser propagation in the atmosphere where the dominant species are N 2 and O 2 .