A U.S. Naval Research Laboratory team recently tested an underwater acoustic laser capability that might one day provide a source of voice or data communications for submarines; navigational data for submarines or underwater robots; and sonar to locate mines or other objects in shallow water—all from an aircraft and without the need for hardware in the water.
Communicating with or from a submerged submarine remains a challenge in the 21st century and often requires the submarine to surface, exposing crewmembers to potential danger. Using trailing wires or buoys for submarine communications limits maneuverability and hinders stealth. In addition, underwater robotic vehicles often rely on inertial navigation technologies that tend to become more error-prone the farther the robots move. And searching for underwater mines is difficult, dangerous and time-consuming under any conditions. Underwater laser acoustic technologies, however, could potentially perform all those functions and more from an aircraft, explains Ted Jones, a research physicist with the Naval Research Laboratory Plasma Physics Division, who led the team of scientists.
“Right now, when you want to make sound in water, you have to have an acoustic source, like a hydrophone, in the water, and that puts your equipment at risk or requires equipment on location. We’re developing a laser acoustic source so that you don’t need anything in the water,” says Jones. “We’re not the first ones to use lasers to generate underwater acoustics, but we’ve done a lot of innovations and improvements on techniques that I think have really brought the laser acoustic source forward quite a bit and made it more practical for the Navy and for commercial purposes.”
Those innovations include the use of high-intensity lasers with very short pulses that ionize the water, superheating a small volume that creates a tiny piston, which generates an intense acoustic pulse. The team also uses a wavelength that propagates well underwater, so that they can control the shape of the piston and the intensity of the acoustic pulse. In addition, they use a combination of nonlinear optical focusing techniques that increase the standoff distance between the laser and the surface of the water. By using different colored lasers, they precisely control the acoustic pulse. “The technique is called group velocity dispersion. You’re taking advantage of the fact that different colors of light travel at different speeds. You put the slow colors at the front of the pulse, and the fast colors in the back, and you have this stretched-out pulse. You can program exactly where that compresses longitudinally in the water,” says Jones.
The tests were conducted at the Glendora Lake Hydro-acoustic Test Facility in Crane, Indiana, which marks the first time the capability has been tested outside a laboratory. The nanometer wavelength laser, which is housed in a floating structure, generates underwater acoustic pulses, which travel to a distant hydrophone-equipped boat. Steering mirrors directed the pulses down through a focusing lens and into the water surface. Each laser pulse produced an acoustic pulse with a sound pressure level of approximately 190 decibels, which traveled up to 190 meters. Previous laboratory tests resulted in propagation distances of only three meters.
The team is planning for more tests in the spring and summer, which could look at propagation below the surface. They also hope to improve propagation distances. Initial results show they should be able to generate 230 decibels with a pulse that is less than one joule of energy.
Researchers in other parts of the country focus more on the communications and signal processing technologies of underwater acoustic lasers, but the work being done by the Naval Research Laboratory will likely benefit researchers in those and other areas. “We have some interest from other parts of the Navy. I’ve even been contacted by people in the oil industry who are interested in measuring the water depth in wells,” says Jones. “We’re interested primarily in generating a very intense acoustic pulse. We trying to make the loudest acoustic source possible with the most compact laser possible.”