U.S. Navy Tests Lasers as a Source of Sonar
The U.S. Navy has begun field tests on a new way to generate underwater sonar by pulsing laser beams into the open sea. The research could modernize the way that aircraft communicate with submarines, replacing a clunky technique of tethering buoyed sonar equipment to helicopters and skimming it along the water's surface.
With a laser-based approach, no hardware would ever have to touch the water, making it stealthier and more durable.
Researchers have known for some time that laser beams can initiate acoustic waves in the water. But it wasn't until last year, when physicists at the Naval Research Laboratory's Plasma Physics Division tested the phenomenon at Lake Glendora in Indiana, that its potential for changing naval communication was validated. They are now improving the focus and intensity of their lasers before heading back out for more tests in April.
We have made a lot of important innovations that have made it more useful, said Ted Jones, a physicist at NRL who is leading the experiments.
When a laser beam penetrates the surface of the water, it behaves like a tiny steam piston. The molecules around it immediately turn into superhot plasma. The sudden injection of energy forces some of the electrons in the molecules to break free, ionizing the water and causing it to expand in a wave. These waves can then be detected by acoustic sensors in the water.
The more pressure is generated, the farther the wave will travel. And the farther it travels, the more useful it is. Even if it propagated a few meters, it would be useful, Jones told InnovationNewsDaily.
In tests last November, Jones shot a green laser into the lake for fractions of a second. He measured acoustic pulses that propagated for more than 450 feet (140 meters) and reached 190 decibels.
By Jones' calculations, this already makes the technology useful. But he is keen on improving the results. One way to do this is to get more of the energy from the laser into the acoustic pulse. In the past, only 1 percent of that energy was converted into acoustic pressure. Based on recent lab r results, Jones thinks he will get the conversion rate up to 10 percent in field tests this spring.
For now, they are beaming the lasers from a pontoon in the middle of the lake. But, if this technology becomes viable, the lasers will travel all the way down from airplanes to communicate with submarines, or to scout the ocean floor for mines.
In these scenarios, weather could pose an obstacle.
If it's raining or cloudy, that would be a problem for this technology, Jones said. Still, not having hardware in the water to make sounds would be a big advantage.
And for those worried about the possibility of skewering marine animals with laser beams , the chances of that are minimal. The energy from the laser disperses very quickly in water.
As long as any creatures aren't within a few meters of where the light goes in, it should be all right, Jones said.