Characterization of underwater laser acoustic source for navy applications

An intense remote underwater laser acoustic source, utilizing laser-induced breakdown (LIB), is under development at the Naval Research Lab. In a novel configuration, a tailored intense laser pulse can be designed to propagate many meters underwater and compress at a predetermined remote location. Controlled compression of these optical pulses is governed by a combination of optical group velocity dispersion (GVD) and nonlinear Kerr self-focusing (NSF). Optical compression can result in underwater LIB, localized heating, and acoustic shock generation. Recent experiments include near-field acoustic source characterization using lens-focused 400 and 800 nm pulses of a femtosecond Ti:sapphire laser, as well as 532 and 1064 nm pulses of a Q-switched YAG laser. Sound pressure levels over 210 dB were achieved using a compact laser. Acoustic source characterization included measurements of: the acoustic power spectrum from a few Hz to 15 MHz, photoacoustic energy conversion efficiency, and directivity. Governing optical parameters of water have all been directly measured as part of this work, including the GVD of water at 400 nm, and Kerr index of water at 400 and 800 nm. Optical compression was demonstrated separately using GVD and NSF. Nonlinear optical studies included measurement of conditions for optical filament generation, and the filament´s effects on acoustic signals. Ongoing and planned experiments include simultaneous optical compression using GVD and NSF, and optical and acoustic propagation studies in a bubbly salt water tank. Experimental results will be presented, and laser sources and techniques for underwater acoustic generation will be compared.