'Air waveguide' creates 'optical cables' out of thin air

July 23, 2014 // By Jean-Pierre Joosting
The 'air waveguide' makes air behave like an optical cable and it could provide the ability to run an optical cable or fiber to any point on earth, or even into space.

In a paper published today in the July 2014 issue of the journal Optica, Howard Milchberg, professor of physics and electrical and computer engineering at the University of Maryland, and his lab report using an "air waveguide" to enhance light signals collected from distant sources.

Because light loses intensity with distance, the range over which such tasks can be done is limited. Even lasers, which produce highly directed beams, lose focus due to their natural spreading, or worse, due to interactions with gases in the air. Fiber-optic cables can trap light beams and guide them like a pipe, preventing loss of intensity or focus.

Typical fibers consist of a transparent glass core surrounded by a cladding material with a lower index of refraction. When light tries to leave the core, it gets reflected back inward. But solid optical fibers can only handle so much power, and they need physical support that may not be available where the cables need to go, such as the upper atmosphere. Now, Milchberg's team has found a way to make air behave like an optical fiber, guiding light beams over long distances without loss of power.

Milchberg's air waveguides consist of a "wall" of low-density air surrounding a core of higher density air. The wall has a lower refractive index than the core—just like an optical fiber. In the Optica paper, Milchberg, physics graduate students Eric Rosenthal and Nihal Jhajj, and associate research scientist Jared Wahlstrand, broke down the air with a laser to create a spark. An air waveguide conducted light from the spark to a detector about a meter away. The researchers collected a strong enough signal to analyze the chemical composition of the air that produced the spark.

The signal was 1.5 times stronger than a signal obtained without the waveguide. That may not seem like much, but over distances that are 100 times longer, where an unguided signal would be severely weakened, the signal enhancement could be much greater.