Nonlinear underwater propagation of picosecond ultraviolet laser beams

Meter-scale nonlinear propagation of a picosecond ultraviolet laser beam in water, sufficiently intense to cause stimulated Raman scattering (SRS), nonlinear focusing, pump-Stokes nonlinear coupling, and photoexcitation, was characterized in experiments and simulations. Pump and SRS Stokes pulse energies were measured, and pump beam profiles were imaged at propagation distances up to 100 cm for a range of laser power below and above self-focusing critical power. Simulations with conduction band excitation energy U=9.5eV, effective electron mass m=0.2m_e, Kerr nonlinear refractive index n₂=5×10^-16cm²/W, and index contribution due to SRS susceptibility n2r=1.7×10^-16cm²/W produced the best agreement with experimental data.