“Dangerously Powerful” Laser Experiment Sets Record in University Hallway

It’s not at each college that laser pulses highly effective sufficient to burn paper and pores and skin are despatched blazing down a hallway. But that’s what occurred in UMD’s Energy Research Facility, an unremarkable wanting constructing on the northeast nook of campus. If you go to the utilitarian white and grey corridor now, it looks as if another college corridor—so long as you don’t peak behind a cork board and spot the metallic plate overlaying a gap within the wall.

But for a handful of nights in 2021, UMD Physics Professor Howard Milchberg and his colleagues reworked the hallway right into a laboratory: The shiny surfaces of the doorways and a water fountain had been lined to keep away from probably blinding reflections; connecting hallways had been blocked off with indicators, warning tape, and particular laser-absorbing black curtains; and scientific gear and cables inhabited usually open strolling area.

As members of the staff went about their work, a snapping sound warned of the dangerously highly effective path the laser blazed down the corridor. Sometimes the beam’s journey ended at a white ceramic block, filling the air with louder pops and a metallic tang. Each evening, a researcher sat alone at a pc within the adjoining lab with a walkie-talkie and carried out requested changes to the laser.

Left to proper: Eric Rosenthal, a physicist on the U.S. Naval Research Laboratory; Anthony Valenzuela, a physicist on the U.S. Army Research Lab; and Andrew Goffin, a UMD electrical and pc engineering graduate pupil, align optics at a porthole within the wall with a view to ship the laser beam from the lab down the hallway. Credit: Intense Laser-Matter Interactions Lab, UMD

Their efforts had been to quickly transfigure skinny air right into a fiber optic cable—or, extra particularly, an air waveguide—that may information gentle for tens of meters. Like one of many fiber optic web cables that present environment friendly highways for streams of optical knowledge, an air waveguide prescribes a path for gentle. These air waveguides have many potential purposes associated to accumulating or transmitting gentle, akin to detecting gentle emitted by atmospheric air pollution, long-range laser communication and even laser weaponry. With an air waveguide, there is no such thing as a must unspool stable cable and be involved with the constraints of gravity; as an alternative, the cable quickly types unsupported within the air. In a paper accepted for publication within the journal Physical Review X the staff described how they set a file by guiding gentle in 45-meter-long air waveguides and defined the physics behind their methodology.

The researchers performed their record-setting atmospheric alchemy at evening to keep away from inconveniencing (or zapping) colleagues or unsuspecting college students in the course of the workday. They needed to get their security procedures accepted earlier than they may repurpose the hallway.

“It was a really unique experience,” says Andrew Goffin, a UMD electrical and pc engineering graduate pupil who labored on the mission and is a lead creator on the ensuing 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 putting up curtains for eye safety. It was definitely tiring.”

Distributions of the laser gentle collected after the hallway journey and not using a waveguide (left) and with a waveguide (proper). Credit: Intense Laser-Matter Interactions Lab, UMD

All the work was to see to what lengths they may push the method. Previously Milchberg’s lab demonstrated {that a} comparable methodology labored for distances of lower than a meter. But the researchers hit a roadblock in extending their experiments to tens of meters: Their lab is simply too small and shifting the laser is impractical. Thus, a gap within the wall and a hallway turning into lab area.

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

Without fiber optic cables or waveguides, a lightweight beam—whether or not from a laser or a flashlight—will constantly increase because it travels. If allowed to unfold unchecked, a beam’s depth can drop to un-useful ranges. Whether you are attempting to recreate a science fiction laser blaster or to detect pollutant ranges within the ambiance by pumping them stuffed with vitality with a laser and capturing the launched gentle, it pays to make sure environment friendly, concentrated supply of the sunshine.

Milchberg’s potential resolution to this problem of maintaining gentle confined is further gentle—within the type of ultra-short laser pulses. This mission constructed on earlier work from 2014 wherein his lab demonstrated that they may use such laser pulses to sculpt waveguides within the air.

The brief pulse method makes use of the flexibility of a laser to offer such a excessive depth alongside a path, referred to as a filament, that it creates a plasma—a phase of matter where electrons have been torn free 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 tiny version of lighting and thunder where the lightning bolt’s energy turns the air into a plasma that explosively expands the air, creating the thunderclap; the popping sounds the researchers heard along the beam path were the 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 internet fiber optic cables). So, they created an arrangement of multiple low-density tunnels that naturally diffuse and merge into a moat surrounding a denser core of unperturbed air.

The 2014 experiments used a set arrangement of just four laser filaments, but the new experiment took advantage of a novel laser setup that automatically scales up the number of filaments depending on the laser energy; the filaments naturally distribute themselves around a ring.

The researchers showed that the technique could extend the length of the air waveguide, increasing the power they could deliver to a target at the end of the hallway. At the conclusion of the laser’s journey, the waveguide had kept about 20% of the light that otherwise would have been lost from their target area. The distance was about 60 times farther than their record from previous experiments. The team’s calculations suggest that they are not yet near the theoretical limit of the technique, and they say that much higher guiding efficiencies should be easily achievable with the method in the future.

“If we had a longer hallway, our results show that we could have adjusted the laser for 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 guide right for the hallway we have.”

The researchers also did shorter eight-meter tests in the lab where they investigated the physics playing out in the process in more detail. 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 beam 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 going into making the waveguide translates to a louder pop).

The team found that the waveguide lasted for just hundredths of a second before dissipating back into thin air. But that’s eons for the laser bursts the researchers were sending through it: Light can traverse 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 air waveguides. They also plan to guide different colors of light and to investigate if a faster filament pulse repetition rate can produce a waveguide to channel a continuous high-power beam.

“Reaching the 50-meter scale for air waveguides literally blazes the path for even longer waveguides and many applications”, Milchberg says. “Based on new lasers we are soon to get, we have the recipe to extend our guides to one kilometer and beyond.”

Reference: “Optical guiding in 50-meter-scale air waveguides” by A. Goffin, I. Larkin, A. Tartaro, A. Schweinsberg, A. Valenzuela, E. W. Rosenthal and H. M. Milchberg, 23 January 2023, Physical Review X.
DOI: 10.1103/PhysRevX.13.011006