April 28, 2000 -- Watches tick in seconds. Basketball games are timed in 10ths of a second, and drag racers in 100ths. Computers used to work in milliseconds (1,000ths), then moved up to microseconds (millionths), and now are approaching nanoseconds (billionths) for logic operations - and picoseconds (trillionths!) for the switches and gates in chips.

"That's great in theory," says Dr. Donald Frazier of NASA's Marshall Space Flight Center. "Except that electronic signals, even with Very Large Scale Integration (VLSI) and maximum miniaturization, are bogged down by many aspects of the solid materials they travel through. So we've had to find a faster medium for the signals - and the answer seems to be light itself!"

Above: Dr. Donald Frazier monitors a blue laser light used with electro-optical materials.

Light travels at 186,000 miles per second. That's 982,080,000 feet per second -- or 11,784,960,000 inches. In a billionth of a second, one nanosecond, photons of light travel just a bit less than a foot, not considering resistance in air or of an optical fiber strand or thin film. Just right for doing things very quickly in microminiaturized computer chips.

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"Entirely optical computers are still some time in the future," says Dr. Frazier, "but electro-optical hybrids have been possible since 1978, when it was learned that photons can respond to electrons through media such as lithium niobate. Newer advances have produced a variety of thin films and optical fibers that make optical interconnections and devices practical. We are focusing on thin films made of organic molecules, which are more light sensitive than inorganics. Organics can perform functions such as switching, signal processing and frequency doubling using less power than inorganics. Inorganics such as silicon used with organic materials let us use both photons and electrons in current hybrid systems, which will eventually lead to all-optical computer systems."

"What we are accomplishing in the lab today will result in development of super-fast, super-miniaturized, super-lightweight and lower cost optical computing and optical communication devices and systems," Frazier explained.

The speed of computers has now become a pressing problem as electronic circuits reach their miniaturization limit. The rapid growth of the Internet, expanding at almost 15% per month, demands faster speeds and larger bandwidths than electronic circuits can provide. Electronic switching limits network speeds to about 50 Gigabits per second (1 Gigabit (Gb) is 10⁹, or 1 billion bits).

Dr. Hossin Abdeldayem, a member of Frazier's optical technologies research group, states that Terabit speeds (1 Terabit, abbreviated "Tb", is 10¹², or 1 trillion bits) are needed to accommodate the growth rate of the Internet and the increasing demand for bandwidth-intensive data streams. Optical data processing can perform several operations simultaneously (in parallel) much faster and easier than electronics. This "parallelism" when associated with fast switching speeds would result in staggering computational power. For example, a calculation that might take a conventional electronic computer more than eleven years to complete could be performed by an optical computer in a single hour.

"All-optical switching using optical materials can relieve the escalating problem of bandwidth limitations imposed by electronics," says Dr. Abdeldayem. "In 1998, Lucent Technologies introduced a lithographic submicron technology to further miniaturize electronic circuits and enhance computer speed. Additional miniaturization of electronic components only provides a short-term solution to the problem. There are also physical problems accompanied by miniaturization that might affect the computer's reliability. "

Drs. Frazier and Abdeldayem and their group in Huntsville, AL, have designed and built all-optical logic gate circuits for data processing at Gigabit and Terabit rates, and they are also working on a system for pattern recognition.

Left: Dr. Hossin Abdeldayem of NASA/Marshall works with lasers to develop a system for pattern recognition.

"We have also developed and tested nanosecond optical switches, which can act as computer logic gates," says Dr. Abdeldayem, who recently presented the group's research paper entitled "All-Optical Logic Gates for Optical Computing" at The Pittsburgh Conference in New Orleans, LA.

"Picosecond and nanosecond all-optical switches, which act as AND and partial NAND logic gates were demonstrated in our laboratory," explains Dr. Abdeldayem. "Such logic gates are members of a large family of gates in computers that perform logic operations such as addition, subtraction and multiplication. They are vital for the development of optical computing and optical communication. Our all-optical logic gates were made using a thin film of metal-free phthalocyanine compound and a polydiacetylene polymer in a hollow fiber"

Logic gates are the building blocks of any digital system," he continues. "An optical logic gate is a switch that controls one light beam with another. It is "on" when the device transmits light, and "off" when it blocks the light."

"Our phthalocyanine switch operates in the nanosecond regime (i.e., Gigabits per second), functioning as an all-optical AND logic gate. To demonstrate it, we waveguided a continuous (cw) laser beam co-linearly with a nanosecond pump beam through a thin film of metal-free phthalocyanine. The output was sent to a fast photo-detector and to an oscilloscope. The cw beam was found to pulsate synchronously with the pump beam, showing the characteristic table of an AND logic gate."

Right: A schematic of the nanosecond all-optical AND logic gate setup. More schematics and illustrations are available in "Recent Advances in Photonic Devices for Optical Computing" by NASA/Marshall's Hossin Abdeldayem, Donald O. Frazier, Mark S. Paley, and William K. Witherow.

"Our setup for the picosecond switch was similar, except that the phthalocyanine film was replaced with a hollow fiber coated from inside with a thin polydiacetylene film. Both collinear laser beams were focused on one end of the tube, and a lens at the other end focused the output onto a monochrometer with a fast detector attached. The product of the two beams demonstrates three of the four characteristics of a NAND logic gate."

"Optical bistable devices and logic gates such as these are the equivalent of electronic transistors," concludes Dr. Abdeldayem. "They operate as very high speed on-off switches and are also useful as optical cells for information storage."

According to Dr. Frazier, these all-optical computer components and thin-films developed by NASA are essential to the current worldwide work in electro-optical hybrid computers - and will help to make possible the astounding organic optical computers that will be the standard of future terrestrial and space information, operating and communication systems.

Web Links

"Recent Advances in Photonic Devices for Optical Computing" by NASA/Marshall's Hossin Abdeldayem, Donald O. Frazier, Mark S. Paley, and William K. Witherow. 700kb in Microsoft Word format.

Pushing the Limits of Computer Technology -- Science@NASA headline story from May 1999.

Microgravity News - Winter 1995 report about the Alliance for Nonlinear Optics.