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Lasers Generate Sound in H2O

Photonics.com
Sep 2009
WASHINGTON, DC, Sept. 8, 2009 – A new technology that uses flashes of laser light to remotely create underwater acoustics is being developed by scientists at the Naval Research Laboratory. The new acoustic source has the potential to expand and improve both Naval and commercial underwater acoustic applications, including undersea communications, navigation and acoustic imaging.

Dr. Ted Jones, a physicist in the plasma physics division, is leading a team of researchers from the plasma physics, acoustics, and marine geosciences divisions in developing this acoustic source.

H2O-Laser.jpg
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. Air bubblers and controlled water and air temperatures can create oceanlike conditions in the laboratory. (Image: Naval Research Laboratory)

Efficient conversion of light into sound can be achieved by concentrating the light sufficiently to ionize a small amount of water, which then absorbs laser energy and superheats. The result is a small explosion of steam, which can generate a 220-dB pulse of sound. Optical properties of water can be manipulated with very intense laser light to function as 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 be arranged so that the pulse also compresses in time as it travels through water, further concentrating the light. By using a combination of GVD and NSF, controlled underwater compression of optical pulses can be attained.

The driving laser pulse has the ability to travel through both air and water, so that a compact laser on either an underwater or airborne platform can be used 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.

For more information, visit: www.nrl.navy.mil


GLOSSARY
doppler effect
The effect produced on a wave frequency because of the relative motion of a source or an observer. The radiation emitted from a source that moves away from an observer appears to be of lower frequency than the radiation emitted from a stationary source. The radiation emitted from a source moving toward the observer appears to be of a higher frequency than that from a stationary source.
photonics
The technology of generating and harnessing light and other forms of radiant energy whose quantum unit is the photon. The science includes light emission, transmission, deflection, amplification and detection by optical components and instruments, lasers and other light sources, fiber optics, electro-optical instrumentation, related hardware and electronics, and sophisticated systems. The range of applications of photonics extends from energy generation to detection to communications and...
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