Atmospheric laser propagation

An underwater photo-ionization laser acoustic source developed by researchers at the US Naval Research Laboratory could enable remote acoustic generation from compact air- and shipborne lasers, allowing aircraft to communicate without the need for source hardware in the water.

The investigators successfully completed long-distance acoustic propagation and shock-generation field testing of the device at the Lake Glendora Test Facility of Naval Surface Warfare Center located in Crane, Ind.

Housed in a floating structure, the Nd:YAG laser (bottom left) generates underwater acoustic pulses that travel to a distant hydrophone-equipped boat (bottom right). Courtesy of US Naval Research Laboratory.

Using a pulsed Nd:YAG 532-nm laser housed on a floating platform, they directed pulses by steering mirrors through a focusing lens and into the water surface. Each laser pulse produced acoustic pulses with sound pressure levels measuring ~190 dB. These were detected by boat-mounted hydrophones at distances of up to 140 m. Previous lab acoustic propagation distances were limited to ~3 m.

Because the laser pulses can travel through both air and water, a compact laser on either an underwater or airborne platform can be used for remote acoustic generation. Properly tailored pulses can travel many hundreds of meters through the air, remaining relatively unchanged, then quickly compress upon entry into water.

With the potential to expand and improve both naval and commercial underwater acoustic applications, the sources could be efficient tools for undersea communications from aircraft.