National Geographic News
Lasers that can create loud bangs under the sea might someday replace sonar for sending messages to submarines, Navy physicists have announced.
Conventional sonar mapping uses pulses of sound, which require towed arrays of speakers and receivers.
"You have to pull [the array] with a vessel," said Ted Jones, a plasma physicist with the U.S. Naval Research Laboratory in Washington, D.C.
"It's slow and expensive. It might take hours or even days to search a large area."
The new technique—a 21st-century form of Morse code—uses self-focusing laser beams to superheat BB-size quantities of water up to 70 feet (20 meters) beneath the waves.
The result is "a little piston of steam" that expands at supersonic speeds, creating an underwater bang loud enough to be heard miles away, Jones said.
The laser pings could also be used for any of the other things normally done with sonar, such as searching for underwater objects or mapping the seabed.
To conduct a search, users could disperse passive-sonar buoys, which are designed to listen but not transmit.
Then an airplane could fly overhead, beaming laser-generated pings across the search region.
"You could quickly do a sonar search over a large area," Jones said.
Coming Into Focus
In designing the new technique, the biggest challenge was getting the laser beam to focus its energy on a small enough area to generate the pop.
One trick, Jones said, is to make the beam more intense at its core, so the middle section of the beam heats the water fastest.
That heat causes the light around the core to bend inward until the beam focuses into a tiny point.
At the same time, the laser is made up of different wavelengths of light, which travel through water at slightly different speeds.
If "you put the slowest first and fastest at the end," Jones said, it will quickly build up energy at the focal point until the superheated water goes bang.
Make a Beeline for ETs
Robert J. Sawyer, a Canadian science fiction writer, said that one advantage of using a laser beam to communicate with submarines is that the beam would be difficult to intercept above ground.
Still, anyone with nearby sonar receivers could hear the bangs once the beam entered the water.
More interesting than secrecy, Sawyer said, is the fact that similar laser pings might be an even better means of communication than radio in the search for intelligent extraterrestrials—minus, of course, the water.
"Lasers are more efficient" than radio waves—the medium currently used by the SETI (Search for Extraterrestrial Intelligence) Institute—he said. "There's no point in broadcasting [everywhere] when you can make a beeline.
"So we're talking to our subs the way we're hoping to talk to aliens someday."
Findings presented today in Portland, Oregon, at a meeting of the Acoustical Society of America.
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