High-resolution femtosecond measurements of underwater laser ionization and filamentation for electrical discharge guiding

Abstract

Summary form only given. Laser triggering and guiding of underwater electrical discharges is being investigated at the Naval Research Laboratory. Laser-guided underwater discharges have potential applications in advanced micromachining and pulsed power switching1. Key elements of this technology are underwater laser ionization and the generation of extended underwater optical filaments. We report several new measurements of underwater laser ionization, including high resolution imaging using a 2-laser pump-probe technique with femtosecond time resolution. We used 532 nm, 4 ns, 20 mJ lens-focused pump pulses to ionize a water sample, and independently-timed 400 nm, 50 fs, submillijoule perpendicular-propagating probe pulses to generate shadowgraphs and interferograms. Shadowgraph images appear to show gas bubbles approximately 5 μm in diameter throughout the pump beam path. The number and density of these bubbles was observed to increase with time during the pump pulse. A sufficiently dense trail of residual gas bubbles can merge to form a vapor channel, which has been shown to guide underwater discharges1. Time-resolved spectra of nanosecond laser-ionized water reveal black-body radiation lasting more than 10 ns after the ionizing pump pulse. Interferograms of underwater volumes ionized using 800 nm, 50 fs, 20 mJ laser pulses revealed plasma lifetimes of order 10 ps or less. Recently, using 532 nm, 4 ns pulses with up to 60 mJ, our group demonstrated the generation of underwater optical filaments over 55 cm in length, corresponding to more than 35 Rayleigh lengths. To our knowledge, this is a record length for an underwater optical filament, and could enable techniques for guiding underwater discharges over distances of order of a meter or longer2. Underwater beam profiles were imaged at 4 cm intervals, revealing a characteristic filament diameter of ~100 μm. Simulations using the HELCAP 4D nonlinear laser propagation code also predict reproducible optical filaments of the same diameter. Plans and recent measurements for underwater optical filament characterization, as well as laser guiding of electrical discharges, will be discussed.