Physicists have just set a new record by confining a self-focused laser pulse to an air cage stretching the length of a 45-meter (148-foot) university corridor.
With previous results well under a meter, this latest experiment, led by University of Maryland (UMD) physicist Howard Milchberg, breaks new ground to confine light to channels known as air waveguides.
An article describing the research was included in the journal physical check X, and can now be found on the preprint server arXiv . The results could inspire new ways to achieve long-range laser-based communications or even advanced laser-based weapons technology.
“If we had a longer corridor, our results show that we could have adjusted the laser for a longer waveguide.” says UMD physicist Andrew Tartaro.
“But we got our guide spot on for the hallway we have.”
Lasers can be useful for a number of applications, but the coherent beams of neatly arranged light have to be cooped up and focused somehow. Left to its own devices, a laser will scatter and lose power and effectiveness.
One such focusing technique is the waveguideand it’s exactly what it sounds like: it directs electromagnetic waves down a specific path and prevents them from scattering.
glass fiber is an example. This consists of a glass tube along which electromagnetic waves are guided. Since the cladding around the outside of the tube has a lower index of refraction than the center of the tube, light attempting to scatter will be bent back into the tube instead, maintaining the beam along its length.
In 2014, Milchberg and his colleagues successfully demonstrated a so-called air waveguide. Instead of using a physical construct like a tube, they used laser pulses to focus their laser light. They found that a pulsed laser creates a plasma that heats the air in its wake, leaving a path of lower-density air. It’s like lightning and Thunder in Miniature: The expanding, lower-density air creates a sound like a tiny thunderclap that follows the laser, creating what is known as a filament.
The less dense air has a lower refractive index than the air around it – like the cladding around a fiberglass tube. Firing these filaments in a specific configuration that “locks” a laser beam in their center effectively creates an airborne waveguide.
The first experiments described in 2014 made an air waveguide about 70 centimeters (2.3 feet) long using four filaments. To scale up the experiment, they needed more filament — and a much longer tunnel through which to shine their lights, preferably without having to move their heavy equipment. Therefore, a long corridor in UMD’s energy research facility was altered to allow for the safe propagation of lasers beamed through a hole in the lab wall.
Corridor entrances have been blocked, shiny surfaces covered, laser absorbing curtains installed.
“It was a truly unique experience” says UMD electrical engineer Andrew Goffinthe first author on the team’s paper.
“Shooting lasers outside of the lab involves a lot of work that you don’t have to bother with when you’re in the lab — like putting up curtains to protect your eyes. It was definitely exhausting.”
Eventually, the team was able to create a waveguide that could traverse a 45-meter corridor—accompanied by crackling, popping sounds, the tiny thunderclaps generated by their laser filament, “Lightning.” At the end of the air waveguide, the laser pulse in the center had blocked about 20 percent of the light that would otherwise have been lost without a waveguide.
Back at the lab, the team also examined a shorter, 8-meter-long air waveguide to take measurements of the processes taking place in the hallway, where they didn’t have the appropriate equipment. These shorter tests were able to hold back 60 percent of the light that would have been lost. The tiny thunderclaps were useful too: the more energetic the waveguide, the louder the bang.
Their experiments showed that the waveguide is extremely transient, lasting only hundredths of a second. However, to guide something moving at the speed of light, this time is sufficient.
Research indicates where improvements can be made; For example, higher guide efficiency and length should result in even less light leakage. The team also wants to try different colors of laser light and a faster filament pulse rate to see if they can run a continuous laser beam.
“Reaching the 50 meter scale for airwaveguide literally paves the way for even longer waveguides and many applications,” says Milchberg.
“Based on new lasers that we will be getting soon, we have the recipe to extend our guidance to a kilometer and beyond.”
The research was accepted Physical Check Xand is available at arXiv.
