Physicists have just achieved a remarkable feat, confining a self-focused laser pulse to a ‘cage’ of air for a record 45 meters (148 feet) along a university corridor. This is a major breakthrough in waveguide technology, with previous results only reaching a meter. The paper has been accepted into Physical Review X and can be found on preprint server arXiv. The results could open up possibilities for laser-based communications and weapons.
“If we had a longer hallway, our results show that we could have adjusted the laser for a longer waveguide,” says UMD physicist Andrew Tartaro. “But we got our guide right for the hallway we have.” Lasers are useful but need to be focused to prevent scattering, and waveguides are one way to do this. Optical fiber is an example, with a glass tube containing a cladding with a lower refractive index than the center, which bends the light back into the tube.
In 2014, Milchberg and his colleagues demonstrated an air waveguide, using laser pulses to create a plasma that heats the air in its wake, leaving behind a path of lower-density air, creating a filament. This lower-density air has a lower refractive index, creating a waveguide out of the air. The initial experiment was just 70 centimeters (2.3 feet) long, but the team needed a much longer corridor, so they set up their equipment at UMD’s Energy Research Facility.
After blocking the entry points, covering shiny surfaces, and deploying laser-absorbing curtains, the team created a waveguide capable of traversing the 45-meter corridor, with a laser pulse in the center retaining 20 percent of the light that would have been lost without a waveguide.
Back in the lab, the team studied a shorter, 8-meter air waveguide, finding that it retained 60 percent of the light. The experiments revealed that the waveguide is extremely fleeting, lasting just hundredths of a second, but that’s ample for something travelling the speed of light.
The research suggests where improvements can be made, such as higher guiding efficiency and length, and the team wants to try different colors of laser light and a faster filament pulse rate. “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.”