Physicists have just set a new record by confining a self-focusing laser pulse to a cage of air, along a 45-meter (148-foot) long university corridor.
With previous results well below a metre, this latest experiment led by physicist Howard Melchberg of the University of Maryland (UMD) breaks new ground for trapping light in channels known as atmospheric waveguides.
A paper describing the research has been accepted into the journal X physical review, aThey can be found in the meantime On the arXiv prepress server . The results could inspire new ways to achieve laser-based long-range communications or even advanced laser-based weapons technology.
“If we had a longer entrance, our results show that we could have modified the laser to have a longer waveguide,” he says. says UMD physicist Andrew Tartaro.
“But we got our clue right in our lobby.”
Lasers can be useful for a range of applications, but coherent rays of light must be precisely arranged Twisted and focused Somehow. Left to its own devices, the laser will scatter, losing its power and effectiveness.
One of these focusing techniques is waveguidewhich is exactly what it sounds like: it directs electromagnetic waves into a specific path, preventing them from scattering.
Optical fiber is one example. This consists of a glass tube along which electromagnetic waves are directed. Because the cladding around the outside of the tube has a lower refractive index than the center of the tube, light trying to scatter is instead bent by the tube, keeping the beam along its length.
In 2014, Milchberg and his colleagues successfully demonstrated what they call atmospheric waveguides. Instead of using a physical structure like a tube, they used laser pulses to synthesize their laser light. They found that the pulsed laser produces plasma that heats the air in its wake, leaving a trail of low-density air behind. It’s like Lightning And thunder in miniature: Expanding low-density air sounds like tiny thunderclaps that follow the laser, creating what’s known as a filament.
Less dense air has a lower index of refraction than the air around it – such is the cladding around an optical fiber tube. So shooting these filaments in a specific configuration that “traps” the laser beam at its center effectively creates a waveguide out of air.
Preliminary experiments Described in 2014 Create an antenna waveguide about 70 cm (2.3 ft) long using four filaments. To scale the experiment, they needed more leads—and a much longer tunnel down to get their lights on, preferably without having to move their heavy equipment around. Hence, a long corridor at UMD’s energy research facility was modified to allow safe diffusion of the emitted laser through a hole in the laboratory wall.
Corridor entry points are blocked, shiny surfaces are covered, and laser absorbent curtains are deployed.
“It was a really unique experience.” says electrical engineer Andrew Goffin UMDfirst author on the team paper.
“There’s a lot of laser imaging work that goes on outside of the lab that you don’t have to deal with when you’re in the lab — like putting up blinds for eye safety. It was definitely stressful.”
Finally, the team was able to create a waveguide capable of traversing a 45-meter corridor — accompanied by crackling and popping noises, and the tiny thunderclap produced by the laser’s “lightning” filaments. At the end of the air waveguide, the laser pulse in the center retained about 20 percent of the light that would otherwise have been lost without the waveguide.
Back at the lab, the team also studied a shorter 8-meter pneumatic waveguide, to take measurements of processes that occurred in the atrium, as they did not have the equipment to do so. These shorter tests were able to retain 60 percent of the light that would otherwise be lost. Small thunderclaps were also useful: the more active the waveguide was, the louder the pop.
Their experiments revealed that waveguides are extremely ephemeral, lasting only hundreds of milliseconds. To channel something that travels at the speed of light, though, that time is plentiful.
The research suggests where improvements can be made; For example, higher pointing efficiency and length should result in less light loss. The team also wants to experiment with different colors of laser light, and a faster filament pulse rate, to see if they can direct a continuous laser beam.
“50 Meter Gauge Reach for Airwave Guides Literally Opens Path for Longer Waveguides and Many Applications,” says Milchberg.
“Based on the new lasers we’ll have soon, we have the recipe for extending our guides to a kilometer and beyond.”
Search accepted X physical reviewand is available at arXiv.
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