Scientists set a world record by firing a 50-meter air-guided laser beam

A laser is guided down the UMD corridor in the experiment to delay light as it makes its 45-meter journey. (CREDIT: Laboratory for Intense Laser-Matter Interactions, UMD)

Not every university has laser pulses powerful enough to burn paper and skin down the hallway. But that’s what happened at the UMD Energy Research Center, an unremarkable building on the northeast corner of campus. If you visit the utilitarian white-and-grey hall now, it will seem like an ordinary university hall to you – unless you look behind the cork board and notice the metal plate covering the hole in the wall.

But for a few nights in 2021, UMD physics professor Howard Milchberg and colleagues turned the hallway into a laboratory: the shiny door surfaces and water fountain were closed to avoid potentially blinding reflections; adjacent corridors were blocked off with signs, warning tape and special black curtains that absorb laser radiation; and scientific equipment and cables were located in a normally open promenade.

As the team members got to work, a clicking sound warned them of a dangerously powerful path being lasered down the hallway. Sometimes the path of the beam ended on a white ceramic block, filling the air with louder pops and a metallic taste. Every night, the researcher sat alone at a computer in a nearby laboratory with a walkie-talkie and performed the required laser settings.

Their efforts were to temporarily convert the rarefied air into a fiber optic cable, or more precisely, an air waveguide that would guide light tens of meters. Like one of the fiber optic internet cables that provide efficient backbones for optical data streams, an airborne waveguide charts a path for light. These airborne waveguides have many potential light collection or transmission applications, such as detecting light emitted by atmospheric pollution, long-range laser communications, or even laser weapons.

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With air waveguide, there is no need to unwind a solid cable and take into account the limitations of gravity; instead, the cable quickly loses its footing in the air. In a paper accepted for publication in the journal Physical Review X, the team described how they set the record by shining light into 45-meter airborne waveguides and explained the physics behind their method.

The researchers conducted their record-breaking atmospheric alchemy at night so as not to disturb (or disable) colleagues or unsuspecting students during the work day. They needed to approve safety rules before they could repurpose the corridor.

“It was a truly unique experience,” says Andrew Goffin, a UMD graduate student in electrical and computer engineering who worked on the project and is the lead author of the journal’s final paper. “There is a lot of work involved with firing lasers outside of the lab that you don’t have to deal with when you’re in the lab, like putting up curtains to protect your eyes. It was definitely exhausting.”

Left to right: Eric Rosenthal, physicist, US Naval Research Laboratory; Anthony Valenzuela, US Army Research Laboratory physicist; and Andrew Goffin, a UMD graduate student in electrical and computer engineering, are aligning optics at a porthole in the wall to direct a laser beam from the lab down a corridor. (CREDIT: Laboratory for Intense Laser-Matter Interactions, UMD)

The whole job was to see how far they could push the technique. Milchberg’s lab has previously demonstrated that a similar method works at distances of less than a meter. But the researchers ran into a hurdle in extending their experiments to tens of meters: their lab is too small to move the laser around. Thus, the hole in the wall and the corridor become a laboratory space.

“There were serious problems: the huge scale of up to 50 meters forced us to reconsider the fundamental physics of the generation of air waveguides, and the desire to direct a powerful laser into a public corridor 50 meters long naturally raises serious safety problems. Milchberg says. “Fortunately, we got great cooperation from both physicists and the Maryland Environmental Safety Authority!”

Distributions of laser radiation after passing through the corridor without a waveguide (left) and with a waveguide (right). (CREDIT: Laboratory for Intense Laser-Matter Interactions, UMD)

Without fiber optic cables or waveguides, a beam of light—be it a laser or a flashlight—would constantly expand as it travels. If allowed to spread uncontrollably, the intensity of the beam may drop to a useless level. Whether you’re trying to recreate a sci-fi laser blaster or determine the levels of pollutants in the atmosphere by energizing them with a laser and capturing the light emitted, it’s important to ensure efficient and concentrated light delivery.

Milchberg’s potential solution to this light-limiting problem is supplemental light, in the form of ultrashort laser pulses. This project builds on previous work from 2014 in which his lab demonstrated that they could use such laser pulses to create waveguides in the air.

50 averaged images of unguided (top row) and steered (bottom row) modes depending on the propagation distance. (CREDIT: Phys. Rev. X)

The short pulse technique exploits the laser’s ability to deliver such high intensity along a path called a filament that it creates a plasma, a phase of matter in which electrons are torn off from their atoms. This energy path heats the air, so it expands and leaves a low-density air path after the laser. This process resembles a tiny version of lightning and thunder, where the lightning’s energy turns air into plasma, which explosively expands the air, creating thunderclaps; the pops the researchers heard along the path of the beam were tiny relatives of thunder.

But these low-density filaments alone were not what the team needed to control the laser. The researchers needed a high-density core (the same as fiber optic cables for the Internet). Thus, they created many low density tunnels that naturally dissipate and merge into a moat surrounding a denser core of undisturbed air.

The 2014 experiments used a setup of just four laser filaments, but the new experiment used a new laser setup that automatically increases the number of filaments based on laser energy; the threads are naturally distributed around the ring.

Researchers have shown that this technique can increase the length of the air waveguide, increasing the power they can transmit to targets at the end of the corridor. At the end of the laser path, the waveguide retained about 20% of the light that would otherwise be lost from the target area. The distance was about 60 times greater than their record for previous experiments.

The team’s calculations suggest that they are not yet close to the theoretical limit of the method, and they say that much higher management efficiency can easily be achieved with this method in the future.

“If we had a longer corridor, our results show that we could adjust the laser for a longer waveguide,” says Andrew Tartaro, a UMD physics graduate student who worked on the project and is the author of the paper. “But we found our guide just right for the corridor we have.”

The researchers also ran shorter eight-meter tests in the lab, where they explored the physics of the process in more detail. In a shorter test, they managed to deliver about 60% of the potentially lost light to the target.

The popping sound of plasma formation was used in their practice tests. In addition to pointing out where the beam was located, it also provided the researchers with data. They used a line of 64 microphones to measure the length of the waveguide and how strong the waveguide was along its length (more energy expended in making the waveguide results in a louder pop).

The team found that the waveguide lasted only hundredths of a second before vanishing into thin air. But that’s eons for the laser flashes the researchers sent through it: in that time, light can travel more than 3,000 km.

Based on what the researchers learned from their experiments and simulations, the team is planning experiments to further increase the length and efficiency of their airborne waveguides. They also plan to direct different colors of light and investigate whether a higher filament repetition rate can create a waveguide to guide a continuous, powerful beam.

“Achieving the 50-meter scale for airborne waveguides literally paves the way for even longer waveguides and many applications,” says Milchberg. “Based on the new lasers we’ll be getting soon, we have a recipe for extending our guides to one kilometer or more.”


For more science news, visit our New Innovations section at The bright side of the news.

Note. Materials provided above by the University of Maryland. Content can be edited for style and length.

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