Lasers generate underwater sound
September 4, 2009
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 ocean-like conditions in the laboratory. Credit: Naval Research Laboratory photo
Scientists at the Naval Research Laboratory (NRL) are developing a new technology for use in underwater acoustics. The new technology uses flashes of laser light to remotely create underwater sound. 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.
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 decibel pulse of sound. Optical properties of water can be manipulated with 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 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.
6 hours ago
No longer would we have to drop sonar probes and explosives from airplanes, nor reveal the position of our own subs by use of active sonar. Light beams from aircraft can generate pulses of sound anywhere in range, instantly. Subs could generate pulses of sound appearing to be emitted hundreds of meters away from the sub itself.
Most interesting would be the phased-array possibility.... laser beams sent to create sound pulses at an array of locations around the sub, but timed to focus their combined sound energy at a target some distance from the sub.
In a future movie, Somali pirates attacking a target ship will find their boat haunted by sounds just below the boat, as they close with the ship, the sounds become intollerable, ultimately cracking the hull. ...send my royalty check to charity.
First of all, I think we'd all rather see the military use modern sonar rather than the old technique of just throwing explosives overboard to generate the sound waves (or the even older reason, just to destroy the target!)
Further, it makes little sense for marine life to run ashore to reach quiet, since bridges, boats, construction, dredging and even garbage disposal creates noises closer to shore than navy sonar.... and there are millions of such sources compared to no more than a few hundred active sonars world wide, if you include all the world's platforms.
(I'm not counting the low power sonars used to detect fish.... I assume you don't think fishermen go out and install sonars on their boats in order to chase fish away?)