LAUSR

NRL is developing underwater laser acoustic generation techniques using nonlinear optics to form shaped plasmas. One or more laser pulses are focused to ionization intensity in water, where enhanced absorption causes plasma generation and intense localized heating. The size and shape of the resulting super-heated steam “shaped charge” determines the acoustic waveform, energy spectral density (ESD), and directivity. Using a single laser pulse we have demonstrated the ability to vary the ESD centroid frequency from 15 kHz to a few MHz, with lower frequencies generated by more elongated plasmas. Recent research has focused on a proposed “igniter-heater” technique utilizing two laser pulses to generate meter-scale elongated plasma (T.G. Jones, et al., “Two Laser Generation of Extended Underwater Plasma,” U.S. patent 9,088,123), with ESD centered near 1 kHz, which is useful for long-range sonar and communications. Key to this technique is generation of an underwater plasma filament serving as an ionized “ignit...