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New circuit brings optical computing closer to reality: photonics.

OPTICAL COMPUTERS, long a dream of computer scientists, have moved a step closer to reality. Researchers at AT&T Bell Laboratories, Murray Hill, Nj, have developed a 2-kbit photonic integrated circuit that they say could be used to build basic optical computing systems.

"Optical computers have been mostly on paper. Now real experimental prototypes can be built," says Leo Chirovsky, the AT&T researcher who designed the new photonic IC. "This [device] provides the first viable building blocks for optical computers,' he adds.

Optical computers use photons instead of electrons to process data. Significant improvements in speed are expected to result from optical computers because the optical integrated circuits used in them allow parallel processing at the chip level.

Other advantages of optical ICs include immunity to electromagnetic interference and the prospect for smaller-sized computers.

Although the development is seen as important, AT&T is offering the prototype IC to other companies, including its Japanese competitors, for use in experimental systems. Japanese companies, such as NEC and NTT, are considered the leaders in photonics technology - "We are happy to work with developers other companies to design [the chip] into their systems," says Edward Labuda, executive director of the Electronic & Photonic Devices Div. at Bell Labs.

Chirovsky adds hat the goal "is to stimulate systems research on an instrument level, rather than on a conceptual level where it has been."

Seeds of technology

The 2 mm x 2 mm device is made up of an array of gallium-arsenide and aluminum-gallium-arsenide multiple well structures. The chip has 2,048 elements, and subnanosecond switching speeds requiring only 2.5 pj of energy have been measured.

Each element on the chip is a symmetric self electro-optic effect device (S-SEED), a technology first developed at Bell Labs in 1987. Each element can operate as a logic gate, memory cell, or a switch. The chip has all optical inputs and outputs.

The most sophisticated photonic chip prior to AT&T's was a 1-kbit chip developed by NEC. It does not have optical inputs and functions only as a memory device. The optical 1/0 capabilities of the AT&T device mean data can be moved in and out simultaneously, greatly increasing its processing speed.

Speed is optical computing's est asset. But the speed of any computer is determined by the speed of the input and output of data, Chirovsky says. Similar to a large city that has only one-lane highways to get into and out of it, a chip's processing efficiency is reduced when its 1/0 ports form data gridlocks.

"We are trying to bring 1/0 up to speed with processing capability," Chirovsky says.

The faster 1/0 speeds allow the chip to achieve massive connectivity, and thus a high degree of parallel processing. Until now parallel processing at the chip level just has not been possible in photonics or electronics," says Chirovsky.

A key to the new device is that it overcomes some of the stumbling blocks encountered with photonic ICs. Problems with bistability and cascadability of the device have held back development of photonic ICs.

But the Bell researchers say they have overcome these problems and made a more robust chip.

"The device acts just like a transistor, operating as a three-terminal device, not just as a bistable device," Chirovsky says, "It makes it much easier [to operate] because the device is tolerant of nonuniformities and the biases which are applied to it."
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Author:Derra, Skip
Publication:R & D
Date:Feb 1, 1990
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