Researchers Create Optical Fibers Using Air and Lasers

Researchers have found a way to enhance light signals collected from distant sources by using an "air waveguide."

The new technique, developed by Howard Milchberg - professor of physics and electrical and computer engineering at the University of Maryland - and colleagues, could help detect pollution, create high-resolution topological maps and even make laser weapons.

Light loses its intensity with distance. Even lasers tend to naturally spread and lose focus. Fiber optical cables are designed to guide light beams from sender to receiver.

Usually, fiber optic cable has transparent glass core enclosed in cladding material with a lower refractive index. The cladding material is used to confine the light beam in the glass core. When the light tries to escape, it bounces off the cladding material and stays in the glass core. Optical fibers are used in communication as well as power lines.

A major problem with using optical fibers is that they need physical support and are useless in certain environments, such as upper atmosphere.

Researchers have now found a way to make air behave like an optical fiber. The air waveguide, researchers say, can carry light beam over a greater distance without loss of power.

The air waveguides have a wall of air of higher density surrounded by a wall of air of lower density. These waveguides are created using short, powerful laser pulses. According to researchers, a powerful pulse of laser in air collapses into a narrow beam, called a filament. Previous research by the team has shown that these filaments heat air as they pass through, creating holes of low-density air. These holes even have low refractive index.

The team has earlier demonstrated that in a square arrangement, these holes form a cage of low density air. A fifth laser, when fired between the four columns of low-density air, will be trapped in the cage. The low density air reflects the light beam and keeps it focussed. Note that the "pipe" created by the beams lasted for few milliseconds. Milchberg said that for many laser applications "milliseconds is infinity."

"Any structure [even air] which has a higher density will have a higher index of refraction and thereby act like an optical fibre," said Milchberg, according to abc.net.

In the present study, researchers used a laser to break down air and created a spark. The spark was then helped by the air waveguide to reach a detector that was placed a meter away. The signal was about 1.5 times stronger than a signal obtained without a waveguide.

For the new technology to be used for practical purposes, researchers have to demonstrate that the signals can be sent over longer distances, at least 50 meters. Nonetheless, the research marks an important step in using air as an optical fiber.

"It's sort of like you have an optical fibre that you can shine into the sky, connecting your laser to the top of the atmosphere," said Ben Eggleton of the University of Sydney, according to abc news. "You don't need big lenses and optics, it's already guided along this channel in the atmosphere."

The U.S military and National Science Foundation funded the research and it is published in the journal Optica.