Femtosecond Laser Pulse Spectrum Transformation during Propagation in the Atmosphere

Transformation of spectral parameters of a femtosecond laser pulse propagating in the atmosphere and in pure molecular nitrogen in the filamentation mode is experimentally studied; its causes are analyzed. The spectral broadening of pumping radiation in a wide range, from 350 to 1100 nm, is shown to be due to the combined action of Stokes stimulated Raman scattering at rotational transitions of nitrogen molecules in air, coherent anti-Stokes scattering, and cascade parametric four-wave mixing. The study of these processes is to make it possible to create highly efficient white light sources for atmospheric spectroscopy and remote sensing.