Stimulated Raman Scattering and Nonlinear Focusing of High-Power Laser Beams Propagating in Water

摘要

The physical processes associated with propagation of a high-power (power >critical power for self-focusing) laser beam in water include nonlinearfocusing, stimulated Raman scattering (SRS), optical breakdown and plasmaformation. The interplay between nonlinear focusing and SRS is analyzed forcases where a significant portion of the pump power is channeled into theStokes wave. Propagation simulations and an analytical model demonstrate thatthe Stokes wave can re-focus the pump wave after the power in the latter fallsbelow the critical power. It is shown that this novel focusing mechanism isdistinct from cross-phase focusing. While discussed here in the context ofpropagation in water, the gain-focusing phenomenon is general to any mediumsupporting nonlinear focusing and stimulated forward Raman scattering.