Laboratories Seek Solutions to Problems of Optical Computing
Optical computers, which will work through fiber optics instead of electronics, are seen as the wave of the future. Now the National Aeronautics and Space Administration is taking the research to new heights. Literally.
NASA is flying an experiment aboard the space shuttle that could move that goal a step closer.
Optical computers will use light to transmit bits of information, and they hold the promise of extraordinarily fast computing in ever smaller packages. But the technological hurdles to reaching that goal are so complex that success may be many years away.
A number of labs around the world are struggling to find solutions to the problems, and it may well be that the first major step into optical computing will be a hybrid that combines both the electron-based systems used today and the optical circuits that almost certainly will someday be the cornerstone of computing.
In today’s computers, electrons travel between transistors on tiny metal wires, or traces. Optical computers will use photons traveling in fiber optics or a thin film to carry the data. Photons inside a cable or film will travel much faster than electrons through a hard wire, and without producing damaging heat or engaging in “cross-talk” between circuits that can scramble the message.
NASA scientists, led by Donald Frazier of the Space Sciences Laboratory at Marshall Space Flight Center in Huntsville, Ala., believe one way to get there is to fabricate thin polymer films on a bed of quartz. The film can be deposited in specific patterns by shining a laser through the quartz during the fabrication process.
By focusing a laser as narrow as 1/25,000 of an inch on the quartz, the scientists say they can “paint with light,” thus creating optical circuits. But there is a problem. The laser also generates heat, which causes convection, leading to the kind of turbulence seen above thunderstorms. That makes it very difficult to create the uniform film needed for optical circuits.
So NASA is trying an experiment during space shuttle missions, where the microgravity of orbital flight virtually eliminates convection--since there is no up or down--to try to make nearly perfect polymer films on quartz. The ultimate goal of the ongoing experiment is to learn enough about the process to be able to do the same thing on Earth. How that might be achieved is not yet known.
Other labs are taking very different approaches, and the stakes are immense. NASA estimates that an optical computer would take less than an hour to solve a problem that would take an electron-based system hundreds of hours.
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The rate of progress in optics was underscored a few months ago when Bell Laboratories unveiled an optical networking system with a 400-gigabit-per-second capacity over a single strand of fiber. That’s 400 billion bits per second, equivalent to carrying the entire traffic of the Internet over one fiber.
Eight fibers, each carrying 400 gigabits, would carry 3.2 terabits (trillion bits) per second of voice, video and data traffic, making it possible to transmit 90,000 volumes of an encyclopedia in one second. Such developments are seen as the key to handling the enormous growth rate of the Internet in the coming years.
At the personal computing level, scientists at Purdue University hope to capitalize on something that was once seen as a flaw in silicon chips. Back in the 1950s, scientists found that it was not always possible to polish silicon to the smooth specifications desired for chips. Sometimes the silicon had tiny pores.
Earlier this decade researchers found that those pores had photoluminescent properties--when current was applied, the pores emitted light.
That opened the door to combining electron and optical circuits on a single chip, but there was a problem. The pores didn’t stay around very long. Within a few weeks, the silicon oxidized, producing a glass-like surface that sealed the pores and disrupted the photoluminescent properties.
Jillian Buriak, assistant professor of chemistry at Purdue, found that by coating the porous silicon with an acid that forces bonding on the atomic level, she could protect the photoluminescence.
“What we came up with is a very clean, very easy, room-temperature, one-hour reaction that allows us to stabilize the surface,” Buriak said.
She sees that as easing the transition from electronic computers, which are based on silicon, to optical computers.
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That shouldn’t be too far off, according to scientists at Columbia University, who say they are already bonding light with electricity. The Columbia researchers say they have bonded an ultra-thin sheet of magnetic garnet, a photonic material that transmits light in only one direction, to a semiconductor.
Richard M. Osgood, professor of electrical engineering and applied physics at Columbia, managed to slice off ultra-thin sheets of garnet crystal, measuring only nine-millionths of a meter thick. Light “guides” etched into the crystal allow light to travel in only one direction, making it an effective routing device in an optical-fiber network, Osgood said.
The crystals can be attached directly to a semiconductor, “providing a bridge to an already mature technology,” he said.
That, according to many experts, is the key. Combining two very different technologies is the next logical step on the road to supercomputing for the rest of us.
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Lee Dye can be reached at leedye@compuserve.com.
