Intense Underwater Laser Propagation and Ionization at Visible and Ultraviolet Wavelengths

Intense underwater laser propagation, filamentation, and ionization are under investigation at NRL for applications including remote laser acoustic generation for low-frequency sonar. Time-resolved absorption spectroscopy of fs underwater laser ionization revealed hydrated electron density of 5.4 x 10$^{\mathrm{18}}$ cm$^{\mathrm{-3}}$ and lifetime of 350 ps. In addition, high-resolution fluorescence imaging of ns underwater laser propagation using two-photon absorbing dye, independently confirmed previous measurements of 100 micron diameter filament structures [Helle, et al., Appl. Phys. Lett. \textbf{103}, 121101]. A patented scheme for generating an elongated, meter-scale, high energy density underwater plasma [USP 9,088,123] is under study, in which such a filament structure could serve as a target for a second energetic ``heater'' laser pulse. Early experiments suggested improved ionization efficiency using the current configuration, with a 266 nm filament pulse, and a 532 nm heater pulse. 1- and 2-D simulations using a nonlinear laser propagation code are underway to predict beam envelope propagation, filamentation, and stimulated Raman and Brillouin scattering behavior. Results from recent experiments and simulations will be presented.