Laser experiment of almost 50 meters sets record in university corridor
It’s not at every university that laser pulses powerful enough to burn paper and skin are sent down a hallway. But that’s what happened at UMD’s Energy Research Facility, a nondescript-looking building in the northeast corner of campus. Now, when you visit the utilitarian white-and-grey hall, it looks just like any other university hall – as long as you don’t peek behind a cork board and see the metal plate covering a hole in the wall.
But for a few nights in 2021, UMD physics professor Howard Milchberg and his colleagues transformed the hallway into a lab: The glossy surfaces of the doors and a water fountain were covered to avoid potentially blinding reflections; connecting corridors were cordoned off with signs, warning tape and special laser-absorbent black curtains; and scientific equipment and cables normally occupied open walking space.
As members of the team worked, a popping sound warned of the dangerously powerful path the laser was shooting down the hallway. Sometimes the jet’s journey ended at a white ceramic block, filling the air with louder pops and a metallic tinge. Each night, a researcher sat alone at a computer in the adjacent lab with a walkie-talkie and made the requested adjustments to the laser.
Their efforts were to temporarily convert thin air into a fiber optical cable– or, more specifically, a sky waveguide– that would guide the light for tens of meters. Like one of the fiber optic internet cables that provide efficient highways for streams of optical data, an air waveguide prescribes a path for light.
These air waveguides have many potential applications related to collecting or transmitting light, such as detecting light emitted from atmospheric pollution, long-range laser communications, or even laser weapons. With an air waveguide, you don’t have to uncoil a solid cable or worry about the limitations of gravity; instead, the cable quickly forms unsupported in the air.
In a paper accepted for publication in the journal Physical assessment X the team described how they set a record by guiding light in 150-foot air waveguides and explained the physics behind their method.
The researchers performed their record-breaking atmospheric alchemy at night to avoid disturbing (or zapping) colleagues or unsuspecting students during the workday. They had to get their safety procedures approved before they could repurpose the corridor.
“It was a really unique experience,” said Andrew Goffin, a UMD electrical and computer engineering student who worked on the project and is lead author of the resulting journal article. “There’s a lot of work that goes into shooting lasers outside the lab that you don’t have to deal with when you’re in the lab, like hanging curtains for eye safety. It was definitely exhausting.”
All the work was to see how far they could push the technique. Previously, Milchberg’s lab showed that a similar method worked for distances of less than a metre. But the researchers ran into an obstacle when extending their experiments to tens of meters: their lab is too small and moving the laser is impractical. For example, a hole in the wall and a corridor become a lab space.
“There were major challenges: the massive scaling up to 50 meters forced us to rethink the fundamental physics of air waveguide generation, and we wanted a powerful laser going through a 50-foot public walkway naturally leads to major safety concerns,” says Milchberg.
Without fiber optic cables or waveguides, a light beam-whether from a laser or a flashlight – will constantly expand during travel. If the beam is allowed to spread uncontrollably, the intensity of a beam can drop to unusable levels. Whether you’re trying to recreate a sci-fi laser blaster or detect levels of pollutants in the atmosphere by energizing them with a laser and capturing the light emitted, it pays to ensure an efficient, concentrated supply of light.
Milchberg’s possible solution to this challenge of limiting light is additional light – in the form of ultra-short laser pulses. This project built on previous work from 2014 in which his lab showed they could use such laser pulses to form waveguides in the air.
The short-pulse technique uses a 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 have been ripped from their atoms. This energetic path heats the air so that it expands, leaving a path of low-density air in the wake of the laser. This process resembles a minor version of lightning and thunder where the lightning bolt’s energy turns the sky into a plasma that explosively expands the air, creating the clap of thunder; the popping sounds the researchers heard along the beam path were thunder’s little cousins.
But these low-density filament paths alone weren’t what the team needed to guide a laser. The researchers wanted a high-density core (the same as fiber optic cables for the Internet). So they created an arrangement of multiple low-density tunnels that naturally diffuse and coalesce into a moat that surrounds a denser core of undisturbed air.
The 2014 experiments used a fixed setup of only four laser filaments, but the new experiment took advantage of a new laser setup that automatically scales up the number of filaments depending on the laser energy; the filaments distribute naturally around a ring.
The researchers showed that the technique could extend the length of the air waveguide, increasing the force they could deliver to a target at the end of the corridor. At the end of the laser’s travel, the waveguide had kept about 20% of the light that would otherwise be lost from their target area. The distance was about 60 times farther than their record from previous experiments. The team’s calculations suggest they’re not yet close to the theoretical limit of the technique, and they say much higher conduction efficiencies should be easily achievable with the method in the future.
“Had we had a longer corridor, our results show that we could have modified the laser for a longer waveguide,” said Andrew Tartaro, a UMD physics student who worked on the project and is an author on the paper. “But we have our guide just for the corridor we have.”
The researchers also did shorter eight-meter tests in the lab, where they examined in more detail the physics that go on in the process. For the shorter test, they managed to deliver about 60% of the potentially lost light to their target.
The popping sound of the plasma formation was put to practical use in their tests. Besides being an indication of where the jet was, 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 that goes into making the waveguide translates into a louder bang).
The team found that the waveguide lasted only hundredths of a second before disappearing into thin air again. But that’s eons before the laser bursts the researchers sent through it: light can travel more than 3,000 km in that time.
Based on what the researchers have learned from their experiments and simulations, the team is planning experiments to further improve the length and efficiency of their air waveguides. They also plan to conduct different colors of light and to investigate whether a faster repetition rate of the filament pulse could produce a waveguide to channel a continuous high-power beam.
“Reaching the 50-meter scale for air waveguides literally paves the way for even longer waveguides and many applications,” says Milchberg. “Based on new lasers we’ll be getting soon, we have the recipe to expand our guides to a mile and beyond.”
A. Goffin et al, Optical Guidance in Air Waveguides at 50 Meter Scale, arXiv (2022). DOI: 10.48550/arxiv.2208.04240. (paper accepted for publication in the journal Physical assessment X)
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