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

TL;DR

This paper investigates how high-power laser beams in water undergo nonlinear focusing and stimulated Raman scattering, revealing a new gain-focusing mechanism where the Stokes wave can re-focus the pump beam, applicable to various media.

Contribution

It introduces a novel gain-focusing mechanism driven by stimulated Raman scattering, distinct from traditional cross-phase focusing, supported by simulations and analytical modeling.

Findings

Stokes wave can re-focus the pump after it falls below critical power

The gain-focusing mechanism is general to media with nonlinear focusing and SRS

Simulation and analytical models confirm the new focusing process

Abstract

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