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Optical computer components unveiled

Posted: 19 Feb 2004 ?? ?Print Version ?Bookmark and Share

Keywords:photonic microchip? cornell university? american association for the advancement of science? aaas? for silicon optical computer?

Photonic microchips were enabled by Cornell University's announcement at the American Association for the Advancement of Science (AAAS), of having fabricated all the components for silicon optical computers that use light instead of electrons.

Wires were replaced by beams of light routed on-chip through air by silicon waveguides controlled by electro-optical switches, and off-chip by a "pin hole" lens connecting to normal optical fibers.

"Photonic microchips are now a reality, not just a dream anymore. We have used nanoscale fabrication techniques to make the silicon components for optical routers, repeaters and the optical computer is just around the corner," said Michal Lipson, an assistant professor at Cornell University, in Ithaca, N.Y. "Our technology will also help to make home use of fiber-optic cables practical."

At the AAAS, Lipson revealed Cornell's cookbook for building photonic chips in nanoscale silicon by routing light through "slot waveguides" filled with air, vacuum, or new organic polymers. Working at the Cornell NonescaleFacility, Lipson has verified experimentally the theoretical prediction that nearly all of a beam of coherent light can be confined to a "slot waveguide" if the center of the guide has a much lower index of refraction than its wall.

She fabricated such waveguides in silicon with parallel strips placed 50nm to 200nm apart with a slot down for the light to travel through air. (She also discovered that silicon dioxide could form walls for light beams traveling through silicon, since is has a much lower index of refraction than silicon dioxide.)

"We found that the index of refraction of the medium in the gap has to be much lower than that of the wall, up to a ratio of about four to one," said Lipson.

Such slot waveguides have been built by Lipson and demonstrated in chips such as ring resonators, which bleed off light from a straight beam into as ring, depending on frequency, thereby enable a demultiplexer to separate different colored channels in a light beam.

So far her switches have been electro-optical, but she is also promising an all optically switched photonic chip soon.

To route optical signals off-chip the tiny on-chip lasers have to drive the vastly larger fiber-optic cables connecting chips. Instead of using a long, loss prone, taper, as is conventional, Lipson reported success on a "pin hole" lens, whereby the tiny laser is forced to focus to a point at the edge of the chip, emerging outside the chip in an expanding diameter that can be perfectly mated to a conventional fiber optic cable.

"We call it our optical solder, because we can couple an on-chip 200nm waveguides to 5?m off-chip optical fibers with 95 percent efficiency," said Lipson.

Some of Lipson's work was performed in collaboration with Alexander Gaeta group, a Cornell associate professor of applied and engineering physics. The National Science Foundation provided funding for Lipson's work.

- R. Colin Johnson

EE Times





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