Abstract:

An intense remote underwater laser acoustic source is under development at the Naval Research Laboratory. 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 and nonlinear Kerr self‐focusing. Optical compression can result in laser‐induced breakdown, localized heating, and acoustic shock generation. Recent experiments include near‐field acoustic source characterization using lens‐focused 400 and 800 nm pulses of a Ti:sapphire laser, as well as 532 and 1064 nm pulses of a YAG laser. Sound pressure levels over 210 dB were achieved using a compact laser. Acoustic source characterization includes measurements of photoacoustic energy conversion efficiency, acoustic power spectrum, and directivity. Nonlinear optical studies included the precise measurement of the Kerr index of water at 400 and 800 nm, as well as conditions for optical filament generation, and their effects on acoustic signals. Planned experiments include tests in a bubbly salt water tank. Experimental results will be presented, and laser sources and techniques for underwater acoustic generation will be compared. [This work is supported by the U.S. Office of Naval Research.]