Short bursts turn columns of air into energy conduits
Copyright U.S. Navy
Laser pulses lasting tiny fractions of a second have created superhighways in the air that are potentially capable of transporting megawatts of laser power. The advance should help scientists detect pollution in the atmosphere. It could also enable more exotic applications such as redirecting lightning and building practical laser weapons.
Lost in the hype surrounding President Reagan’s Strategic Defense Initiative and other laser-based weapon systems was the fact that it’s difficult to deliver large amounts of energy through the atmosphere via laser. Air absorbs laser energy, heats up and expands. That low-density air acts like a defocusing lens, causing the beam to spread apart and weaken.
To traverse meters or kilometers through the atmosphere intact, laser beams have to be released in short, intense pulses. But at about 50 quadrillionths of a second in duration, such pulses can’t deliver enough sustained energy to remotely power an aircraft or burn a hole through an incoming intercontinental ballistic missile.
Howard Milchberg, who leads the intense laser-matter interactions group at the University of Maryland in College Park, wondered if he could use those rapid, low-energy pulses to clear the way for a longer-duration, higher-energy laser beam. A single pulse wouldn’t do the trick, his team found, but multiple adjacent pulses fired simultaneously just might.
In a recent experiment, Milchberg and his team fired four quick laser pulses in a square configuration. The quartet of pulses cut through the air, heating and disturbing molecules in its wake. The result was a single high-density region surrounded by a shell of lower-density air. Essentially, the pulses carved out a conducting wire for light in the air: a laser-friendly core enclosed by an insulating layer.
The researchers followed up the air-preparation pulses with a laser beam released over the course of seven billionths of a second. The beam’s energy barely diminished over the course of 70 centimeters, the researchers report February 26 in Physical Review X.
“It’s a really intriguing experiment,” says Alexander Gaeta, a Cornell University physicist. He’s most intrigued by the finding that the thoroughfare in the air remained stable for a few milliseconds. That’s analogous to discovering that a baseball thrown by a major league pitcher leaves an imprint in the air for nearly 500 years. “It’s kind of astonishing,” Gaeta says.
The millisecond gap provides plenty of time for a high-energy laser beam to travel. “In the laser world,” Milchberg says, “milliseconds is infinity.” He says that his team’s technique could eventually allow lasers to deliver megawatts of power over kilometers through the atmosphere. For the time being, he plans to test his apparatus over tens of meters.
The new technique could improve efforts to remotely detect polluting aerosols and other particles in the atmosphere, Gaeta says. Currently scientists use quick-pulse lasers that cause certain airborne molecules to fluoresce. Soon scientists may be able to achieve a more complete survey by probing for longer periods of time.
The setup could also protect population centers from lightning, Milchberg says. Just as the airborne thoroughfare provides a path of least resistance for lasers, it could also coax lightning to take a desired path from cloud to ground during a thunderstorm.
Then there’s the possibility of death rays – or directed-energy weapons, the more formal term for lasers designed to burn or destroy a target. Milchberg isn’t shy about saying the new study brings such technology closer to reality; Gaeta agrees. And while the Cold War is over, interest in laser weapons is going strong: The U.S. Navy reportedly will deploy a drone-killing laser weapon system on one of its ships.
Milchberg receives funding from the Navy and Air Force, but it is for basic research with no specific application in mind.
N. Jhajj et al. Demonstration of long-lived high-power optical waveguides in air. Physical Review X. Vol. 4, February 26, 2014, 011027. doi: 10.1103/PhysRevX.4.011027.
R. Cowen. Laser pioneer reflects on making Einstein’s idea real. Science News. Vol. 177, May 8, 2010, p. 36.