Making Optical Waveguides Out of Thin Air

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Illustration of an air waveguide. Filaments are indicated by red rods and the light, indicated by arrows, remains focused within the filaments.

Researchers from the University of Maryland, U.S.A., have found a way to make air act as an optical fiber to guide light over long distances, without the physical support needed for solid optical fibers and without loss of power (Optica, DOI: 10.1364/OPTICA.1.000005). In addition to long-range laser communications, the air waveguides could be used for pollutant detection, high-res topographic mapping and laser weapons.

The team, led by Howard Milchburg, generated a proof-of-principle long-lived air waveguide from an array of femtosecond filaments—a wall of low-density air surrounded by a core of high-density air—similar to an optical fiber’s glass core surrounded by refractive cladding. The filaments are created when pulses from a 10 Hz Ti:sapphire laser collapse the air into a narrow filament, increasing the refractive index of air in the center of the beam. The filaments heat the air as they travel, causing it to expand, and leave behind an air density depression (or hole) with a lower refractive index than the surrounding air.

To test the light-carrying ability of the air waveguide, the researchers used an air plasma spark generated from an optical source containing both continuum and spectral line emission. Light from the spark was carried along a one-meter-long air waveguide, which showed a 50 percent increase in the collected signal. The signal was strong enough for the researchers to determine the chemical composition of the air that produced the spark—crucial information for remote sensing schemes.
“It’s like you could just take a physical optical fiber and unreel it at the speed of light,” says Michberg, “put it next to this thing that you want to measure remotely, and then have the signal come all the way back to where you are."

Extrapolation of the results to a 100-meter-long waveguide suggested that the signal-to-noise ratio for optical standoff detection techniques such as lidar can potentially be increased by a factor of 104. Milchberg and his team at the University of Maryland stress that these air waveguides have two uses: to collect and transport remote optical signals; and to guide high peak and average power laser drivers to excite those sources. Their next steps will include demonstrating the air waveguides over much longer distances.

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