150-foot laser experiment sets record in university hallway – ScienceDaily

150-foot laser experiment sets record in university hallway – ScienceDaily

150-foot laser experiment sets record in university hallway – ScienceDaily

It’s not every university where laser pulses powerful enough to burn paper and skin are sent ablaze down a hallway. But that’s what happened at UMD’s Energy Research Center, an unremarkable-looking building in the northeast corner of campus. If you visit the utilitarian white and gray hallway now, it’ll look like any other college hall—as long as you don’t peek behind a cork board and spot the metal plate covering a hole in the wall.

But for a few nights in 2021, UMD physics professor Howard Milchberg and his colleagues turned the hallway into a lab: The shiny surfaces of doors and a water fountain were covered to prevent potentially blinding reflections; connecting corridors were blocked with boards, tape and special black laser-absorbing curtains; and scientific equipment and cables normally inhabited an open space for walking.

As the team members worked, a popping sound alerted them to the dangerously powerful path the laser had cut down the hallway. Sometimes the beam’s journey ended in a white ceramic tile, filling the air with louder pops and a metallic smell. Each night, a researcher sat alone at a computer in the adjacent lab with a walkie-talkie and performed the requested laser adjustments.

His efforts were to temporarily transfigure thin air into a fiber optic cable – or, more specifically, an air waveguide – that would guide light for tens of meters. As one of the fiber optic internet cables that provide efficient roads for optical data streams, 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 by atmospheric pollution, long-range laser communication, or even laser weaponry. With an air waveguide, there’s no need to unravel solid cable and worry about gravity’s constraints; instead, the cable quickly forms without support in the air. In an article accepted for publication in the journal Physical Review X the team described how they set a record by guiding light in 45-metre-long aerial waveguides and explained the physics behind their method.

The researchers conducted their record-breaking atmospheric alchemy at night to avoid disturbing (or zapping) unsuspecting colleagues or students during the workday. They had to get their safety procedures approved before they could repurpose the hallway.

“It was a really unique experience,” said Andrew Goffin, a graduate student in electrical and computer engineering at UMD who worked on the project and is lead author of the resulting journal article. “There’s a lot of work to shoot lasers outside 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 tiring.”

All the work was to see how far they could take the technique. Previously, Milchberg’s lab demonstrated that a similar method worked for distances of less than one meter. But the researchers hit a snag when extending their experiments to tens of meters: their lab is too small and moving the laser is impractical. Thus, a hole in the wall and a corridor becoming laboratory space.

“There were big challenges: the huge scale of up to 50 meters forced us to reconsider the fundamental physics of generating air waveguides, plus wanting to send a high-power laser down a public corridor 50 meters long naturally triggers big problems security,” says Milchberg. “Fortunately, we got excellent cooperation from both physics and the Maryland office of environmental safety!”

Without fiber optic cables or waveguides, a beam of light – whether from a laser or a flashlight – will continually expand as it travels. If allowed to spread unchecked, a beam’s intensity can drop to useless levels. Whether you’re trying to recreate a sci-fi laser blaster or detecting levels of pollutants in the atmosphere by pumping them with energy with a laser and capturing the released light, it pays to ensure efficient and focused delivery of the light.

Milchberg’s potential solution to this challenge of keeping light confined is additional light – in the form of ultra-short laser pulses. This project built on previous work from 2014, in which their lab demonstrated that they could use these laser pulses to sculpt waveguides in air.

The short pulse technique uses the ability of a laser to deliver such high intensity along a path, called a filament, that it creates a plasma – a phase of matter where electrons have been ripped from their atoms. This energetic path heats the air, so it expands and leaves a path of low-density air in the laser’s wake. This process resembles a minuscule version of lighting and thunder, where the lightning energy turns the air into a plasma that explosively expands the air, creating thunder; the cracks the researchers heard along the beam’s path were tiny cousins ​​of thunder.

But these low-density filament paths on their own 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 array of multiple, low-density tunnels that naturally spread out and merge into a gap around a denser core of undisturbed air.

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

The researchers showed that the technique could extend the length of air waveguides, increasing the power they could deliver to a target down the hall. At the conclusion of the laser’s journey, the waveguide retained about 20% of the light that would otherwise have been lost from its target area. The distance was about 60 times greater than the record for previous experiments. The team’s calculations suggest that they are not yet close to the technique’s theoretical limit, and say much greater guidance efficiencies should easily be achieved with the method in the future.

“Had we had a longer corridor, our results show that we could have tuned the laser to a longer waveguide,” says Andrew Tartaro, a physics graduate student at UMD who worked on the project and is an author of the paper. “But we have our guide right for the hallway we have.”

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

The popping sound of the plasma formation was put to practical use in their tests. As well as being an indication of where the beam was, it also provided researchers with data. They used a line of 64 microphones to measure the length of the waveguide and the strength of the waveguide along its length (more energy spent making the waveguide translate to a louder pop).

The team found that the waveguide lasted just a few hundredths of a second before dissipating into thin air. But that’s an eternity for the laser bursts the researchers were sending through it: light can travel more than 3,000 km in that time.

Based on what the researchers learned from their experiments and simulations, the team is planning experiments to further improve the length and efficiency of their airwaveguides. They also plan to guide different colors of light and investigate whether a faster filament pulse repetition rate can produce a waveguide for channeling a high-power continuous beam.

“Achieving the 50-meter scale for aerial waveguides literally opens the way for even longer waveguides and many applications,” says Milchberg. “Based on the new lasers that we will soon have, we have the recipe to extend our guides up to a kilometer and beyond.”

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