Raman-enhanced spectral compression of high-energy femtosecond laser pulses in molecular gases
Nonlinear pulse propagation in gas-filled waveguides has attracted substantial attention over the past decade, and a variety of capabilities have been reported. However, there is no prior report of spectral compression in gas-filled waveguides or cavities, which would offer a natural route for scaling to much higher pulse energies than have been reached in solid structures. Here we report a high-energy spectral-compression technique based on nonlinear propagation in gas-filled capillaries. With 0.1- to 1-mJ pulses, compression of the spectral width by a factor up to 12 (from 60 nm to 5 nm) is demonstrated. Key to this advance is recognition that the process plays out differently in gases than in solids. In a noble gas (Ar), we find that even small structure in the spectrum, which is mapped to the time profile, of the input pulse can degrade the compression process. We identify the delayed Raman response of molecular gases (N2O and N2) as a mechanism that smooths and symmetrizes the nonlinear index modulation, which reduces the impact of spectral asymmetry and fine structure and enhances the fidelity of the compressed peak. The technique can be implemented with a capillary filled with ambient air, for sub-millijoule operation without a dedicated gas system. These results initiate a new direction in the optics of gas-filled waveguides and establish Raman-enhanced spectral compression as a robust route to high-energy narrowband optical sources, with potential impact in a broad range of applications.