15 September 2009

Lasers can generate underwater sound

A new technology that uses flashes of laser light to remotely create underwater sound has the potential to improve Naval and commercial underwater acoustic applications, including undersea communications, navigation, and acoustic imaging.

Scattered light from a 532 nm laser pulse can be seen as it enters the water in the Salt Water Tank Facility and ionizes a small volume of water for acoustic generation.

Efficient conversion of light into sound can occur by concentrating the light sufficiently to ionize a small amount of water, which then absorbs laser energy and superheats, said Dr. Ted Jones, a physicist in the Plasma Physics Division at the Naval Research Laboratory, who is leading a team of researchers from the Plasma Physics, Acoustics, and Marine Geosciences Divisions.

The result is a small explosion of steam, which can generate a 220 decibel pulse of sound. Optical properties of water can undergo manipulation via very intense laser light to act like a focusing lens, allowing nonlinear self-focusing (NSF) to take place. In addition, the slightly different colors of the laser, which travel at different speeds in water due to group velocity dispersion (GVD), can occur so the pulse also compresses in time as it travels through water, further concentrating the light. By using a combination of GVD and NSF, it is possible to obtain controlled underwater compression of optical pulses.

The driving laser pulse has the ability to travel through air and water, so a compact laser on either an underwater or airborne platform can work for remote acoustic generation. Since GVD and NSF effects are much stronger in water than air, a properly tailored laser has the ability to travel many hundreds of meters through air, remaining relatively unchanged, then quickly compress upon entry into the water. Atmospheric laser propagation is useful for applications where airborne lasers produce underwater acoustic signals without any required hardware in the water, such as undersea communications from aircraft.

Also, commercially available, high-repetition-rate pulsed lasers, steered by a rapidly movable mirror, can generate arbitrary arrays of phased acoustic sources. On a compact underwater platform with an acoustic receiver, such a setup can rapidly generate oblique-angle acoustic scattering data, for imaging and identifying underwater objects. This would be a significant addition to traditional direct backscattering acoustic data.

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