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Coaxing Light through the Ultimate Coaxial Cable

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J. W. Stewart
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Image: SCIENCE

ELECTROMAGNETIC FIELDS of a coaxial omniguide show that the light dispersion and polarization shifts of traditional optical fibers can be overcome.

Finding the fairest mirror of them all--one that reflects all wavelengths of light from any angle and absorbs very little of it--was no easy task. Traditional metallic mirrors met the first criterion but always sucked up a few percent of incoming rays. And newer dielectric mirrors were highly reflective, but only when the light striking them fell within a narrow range of wavelenths and hit straight on. Only a year and half ago did the fairy tale of a "perfect mirror" come true, when scientists at the Massachusetts Institute of Technology layered dielectric films in a particular way so that they behaved more like a metal.

Now those same researchers have found a way to roll their perfect mirror up into the ultimate waveguide--a tube that offers the best properties of coaxial cables and fiber optics in one. Their paper appears in the July 21 issue of the journal Science, and their new start-up, OmniGuide Communications, is already at work developing commercial applications.

In fact, the new waveguide could revolutionize the Internet and optical communications as a whole. It extends all the advantages of existing metallic coaxial cables, which are good only for such long wavelengths as radio and microwaves, down to optical wavelengths, including infrared and visible light: it retains the polarity of the incoming electromagnetic radiation and ensures that a pulse of different frequecies will keep its shape. Moreover, because of its geometry, it can bend light in a smaller area than can fiber optic cables--which could lead to significant miniaturization of integrated optical devices and greater bandwidth capabilities.

"This coaxial omniguide may be able to replace what metal does, and also do the job at wavelengths where metal doesn't work," says John D. Joannopoulos, Francis Wright Davis Professor of Physics at MIT and the leader of the team. "And the nice thing about it is that whatever you put into it, you get out. This could make a big difference where polarization is an issue."

To design the new cable, christened the coaxial omniguide, the group built on the basic layout of a coaxial cable (top left). In these waveguides, the light is confined radially between two metal cylinders (gray) and travels axially, or perpendicular to the page. As the light reflects off the metal walls, it bounces down through the cable. It is because this reflection is not 100 percent that the signals through these cables need amplification.

Initially, the researchers simply replaced the metal with dielectric layers (blue and green) rolled to have the same thickness. But the result, dubbed omniguide-A (middle left), only resolved the problems a fiber optic cable poses with polarity, not frequency. To ensure that the frequency of the incoming light would also remain unchanged (single-mode), they went back to the drawing board and created omniguide-B (bottom left). As it turns out, they needed to decrease the inner radius of the waveguiding region while keeping the thickness of the dielectric layers--meaning they had to abandon the inner layers for a single dielectric rod (blue).

The design is generic and should hold for a variety of materials and structural parameters. Indeed, it may be possible to fine-tune the coaxial omniguide for specific applications. "What's important about this is that it has opened a new direction for experimental research that was not possible before," Joannopoulos adds. "It's important to push along in this direction. This may be a breakthrough in bridging the very different requirements for transmitting infrared and radio frequencies."

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