Studies of the spatial distribution and beam quality of Stokes output from a Raman generator pumped by a broad-band KrF laser

One-dimensional stimulated Raman scattering (SRS) theory is extended to three dimensions by the concept of transverse modes associated with the Fresnel number. Two simplified models are introduced to evaluate the beam parameters and waist position under low gain. The dependence of waist size and position of the Stokes beam on the focus position and Rayleigh length of the focused pump laser is analyzed and verified by experiment. The Fresnel number approach is utilized to evaluate the energy density distribution and the mode structure of the pump laser in a Raman cell under high gain. The different configurations are classified and relevant mode formulation and spatial distribution are discussed. The experimental result shows that the beam quality of the Stokes output from a Raman generator depends essentially on the pump configuration and the geometric parameters of the pump laser and is insensitive to the practical pump power, which agrees with the theoretical analysis. The results obtained are adopted to improve the beam quality of the first-order Stokes output from the Raman generator with different configurations.