Optical coax takes tight turns
By Kimberly Patch, Technology Research NewsAlthough they may seem a bit dull at first glance, cables are an important element of computing. They carry information among networks, devices and computer components. Their attributes affect how accurately and quickly information can be transferred. In essence, faster cables mean faster computers.
In this light, a cable designed by researchers at MIT could boost computation speeds by making it possible to build all-optical circuits.
Today, there are two major types of cables -- coaxial metal cable, and fiberglass optical cable. Coaxial cable carries information using microwaves and is literally flexible -- it can be bent at an angle without affecting the signal. Fiber-optic cable, on the other hand, uses shorter, more efficient lightwaves to carry information but if bent at too great an angle light leaks out, weakening the signal.
Small-scale use of fiber optics has been limited because fiber-optic cable cannot be bent into the angles needed for computer circuits. Traditional coaxial cable has not been used in computer circuits either, but that is because the microwaves it carries are millimeters long, which are too large to scale down. Lightwaves range 3 to 4 orders of magnitude smaller.
Coaxial cable has a second advantage over fiber optics: it's shape keeps wave signal polarization constant. Devices on the receiving end of fiber-optic cables have a more complicated task because they have to handle signals at whichever polarization state they happen to arrive in.
The best of both worlds would be transmitting light signals via a coaxial cable. The trouble is, traditional coaxial cables are metal, which absorbs light.
Researchers at MIT have gotten around the problem by designing a dialectic, or nonmetal coaxial cable that can carry lightwaves. The key to the researchers' All-Dialectic Coaxial Waveguide is a reflective coating that surrounds the wavelength-carrying, doughnut-shaped hollow of the coaxial tube. The coating boasts omnidirectional reflectivity, meaning light cannot escape even when it bounces toward the wall at a large angle. This allows the cable to be bent sharply with no signal loss.
While the minimum radius of curvature, or bend, in today's fiber-optic cables is measured in millimeters, the dialectic coaxial cable will support curvature three orders of magnitude smaller -- "down to hundreds or even tenths of micrometers," said researcher Mihai Ibanescu, a graduate student in the MIT physics department.
This ability to bend light signals makes optical circuits possible, said Ibanescu. "This miniaturization would make it possible for all-optical circuits to attain a high degree of integration.... There is still a long way to go, though. Having thinner waveguides and sharper corners is not enough; there are many other optical components that would have to be miniaturized," he added.
In addition, the dialectic coaxial cable allows for zero dispersion, said Shanhui Fan, a research scientist at MIT. The dispersion of white light into colors happens because different wavelengths of light travel at slightly different speeds. "If different wavelengths propagate differently, the pulse may spread," said Fan. Sending information long distances is more efficient when the light pulses that represent bits don't disperse, Fan said.
Optic coaxial cables could be produced within one or two years said Ibanescu.
The research was funded by the National Science Foundation, the U.S. Department of Energy, and the U.S. Army Research Office.
Ibanescu, Fan, and Yoel Fink, Edwin Thomas and John Joannopoulos, all of MIT, published a paper on the Dialectic Coaxial Waveguide in the July 21st, 2000 issue of Science. Fink, Fan and Joannopoulos published an article on the Omnidirectional Reflector in the November 27, 1998 issue of Science.
Timeline: >1 year
Funding: Government
TRN Categories: Optical Computing, Optoelectronics and Photonics; Networking
Story Type: News
Related Elements: Diagram; Technical paper, "An All-Dialectic Coaxial Waveguide" in Science, July 21, 2000; Technical paper, "A Dialectic Omnidirectional Reflector"
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July 26, 2000
Page One
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Optical coax takes take turns
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