A 50-meter laser experiment sets a record in the university’s corridor
Not every university has laser pulses powerful enough to burn paper and leather, the glowing pulses being sent down the hallway. But that’s what happened at UMD’s Energy Research Facility, an unremarkable-looking building on the northeast corner of campus. If you visit the now gray and white utilitarian hall, it will look like any other university hall – as long as you don’t peak behind the cork board and spot the metal plate covering a hole in the wall.
But for a few nights in 2021, UMD physics professor Howard Melchberg and his colleagues transformed the hallway into a laboratory: The shiny surfaces of the doors and the water fountain were covered to avoid potentially blinding reflections; Lanes connected to signage, caution tape, and private are closed laser– Absorbing black curtains. Scientific equipment and cables usually inhabit open walking spaces.
While the team members were doing their work, a crackling sound warned of the dangerously powerful path the laser had shot down the hall. Sometimes the beam’s flight would end at a white ceramic block, filling the air with louder beats and a metallic tone. Each night, the researcher would sit alone at a computer in the adjacent lab with a walkie-talkie on and make the required adjustments to the laser.
Their efforts consisted in temporarily turning thin air into fibres optical cable—or, more specifically, air waveguide—This would direct the light for tens of metres. Like one of the fiber-optic Internet cables providing efficient highways for optical data streams, the airwaveguide describes a path for light.
These airwaveguides have many potential applications related to the collection or transmission of light, such as detecting light emitted by atmospheric pollution, long-range laser communication, or even laser weapons. Using air waveguides, there is no need to loosen the rigid cables and care about the restrictions of gravity; Instead, the cable quickly forms unsupported in the air.
In a paper accepted for publication in the journal X physical review The team describes how they set a record by steering light in airwaves 45 meters long and explain the physics behind their method.
Researchers conducted record-setting atmospheric alchemy at night to avoid disturbing (or tripping up) unsuspecting colleagues or students during the workday. They had to get their safety procedures approved before they could repurpose the entrance.
“It was a really unique experience,” said Andrew Goffin, an electrical and computer engineering graduate student from UMD who worked on the project and is lead author on the resulting article in the journal. “There’s a lot of work that goes into imaging lasers outside of the lab and you don’t have to deal with it when you’re in the lab — like putting up blinds for eye safety. It was definitely stressful.”
All the work was figuring out the lengths you could push this technique. Milchberg’s lab has previously shown that a similar method works at distances of less than a meter. But the researchers encountered an obstacle in extending their experiments to tens of metres: their lab is too small and moving the laser is impractical. Thus, a hole in the wall and a hallway become a laboratory space.
“There were significant challenges: the huge scale up to 50 meters forced us to reconsider the basic physics of atmospheric waveguide generation, as well as the desire to send High power laser Down a 50-meter public gallery naturally leads to major safety issues. “Fortunately, we got excellent cooperation from both the physics and the Maryland Office of Environmental Safety.”
Without fiber optic cables or waveguides, a light beam– whether from a laser or a flashlight – it will continually expand as it travels. If the beam is allowed to propagate unchecked, the intensity of the beam can drop to unhelpful levels. Whether you’re trying to recreate a sci-fi laser blaster or detect levels of pollutants in the atmosphere by pumping them full of energy with lasers and capturing the light being released, it pays to ensure that light is delivered efficiently and focused.
Milchberg’s potential solution to this challenge of keeping light confined is extra light — in the form of ultrashort laser pulses. This project builds on earlier work from 2014 where his lab showed they could use these laser pulses to sculpt waveguides in 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 where electrons are torn from their atoms. This energetic pathway heats the air, so it expands and leaves a trail of low-density air in the laser’s wake. The process is like a mini version of lighting and thunder in which the bolt’s energy turns the air into a plasma that explosively expands the air, creating a thunderclap; The popping sounds the researchers heard along the beam’s path were from thunderclouds’ smaller cousins.
But the low-density filament paths alone weren’t what the team needed to guide the laser. The researchers wanted a high-density core (such as fiber-optic cables for the Internet). Therefore, they created an arrangement of multiple low-density tunnels that spread naturally and merge into a trench surrounding a denser core of undisturbed air.
The 2014 experiments used a specific arrangement of only four laser threads, but the new experiment made use of a new laser setup that automatically scales the number of threads depending on the laser power; The threads are distributed naturally around the ring.
The researchers showed that this technique can extend the length of an atmospheric waveguide, increasing the power it can deliver to a target at the end of a hallway. At the conclusion of the laser flight, the waveguide retained about 20% of the light that would otherwise have been lost from the target area. The distance was about 60 times greater than its record from previous experiments. The team’s calculations indicate that they are not yet close to the theoretical limit of the technology, and say that much higher steering efficiencies should be achieved using the method in the future.
“If we had had a longer entrance, our results show that we could have modified the laser to have a longer waveguide,” says Andrew Tartaro, a UMD physics graduate student who worked on the project and is an author on the paper. “But we got our clue right in our lobby.”
The researchers also ran shorter eight-meter tests in the lab where they examined the physics at play in the process in more detail. For the shorter test, they were able to deliver about 60% of the potentially lost light to their target.
The Plasma Formation’s popping sound was practically used in their tests. Besides being an indication of where the beam was, it also provided the researchers with data. They used a line of 64 microphones to measure the waveguide’s length and how strong the waveguide was along its length (more energy that goes into making the waveguide translates into a louder crackle).
The team found that the waveguide only lasted for milliseconds before dissipating back into thin air. But that’s eons for the laser bursts the researchers were sending them through: the light can travel more than 3,000 kilometers in that time.
Based on what the researchers have learned from their experiments and simulations, the team plans to conduct experiments to improve the length and efficiency of their airwaveguides. They also plan to direct different colors of light and to investigate whether a faster filament pulse repetition rate can produce a waveguide for directing a continuous high-energy beam.
“Having a 50-meter scale for waveguides literally spur the path for longer waveguides and many more applications,” says Melchberg. “Based on the new lasers we will have soon, we have the recipe for extending our guides to a kilometer and beyond.”
A. Goffin et al, Optical Guidance in Airwave Guides at 50-m Scale, arXiv (2022). DOI: 10.48550/arxiv.2208.04240. (The paper has been accepted for publication in the journal X physical review)
University of Maryland
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