Remote underwater intense laser acoustic source

A novel underwater acoustic source is under development in which a tailored intense broadband laser pulse propagates many meters underwater and compresses at a predetermined remote location. Controlled compression of these optical pulses is governed by a combination of optical group velocity dispersion (GVD) and nonlinear self-focusing, resulting in photoionization, localized heating, and shock generation. Configurations involving both atmospheric and underwater laser propagation are also under investigation for applications where airborne lasers generate underwater acoustic signals. The ability to project an acoustic source far from a laser driver, and the increased photoacoustic energy conversion efficiency from the photoionization-shock generation mechanism, are significant improvements on previous laser acoustic source development. Recent experiments included the first demonstration of underwater acoustic generation using an intense lens-focused broadband laser pulse. Initial measurements of sound pressure levels yielded 170 dB for an optical pulse energy of 3 mJ, and are in the range of useful levels for some Navy applications. Optical GVD was precisely measured. Further acoustic measurements of this source, including photoacoustic energy conversion efficiency, acoustic power spectrum, and radiation pattern are underway. Experimental results will be presented.