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Would Apply to Computers, Phone Lines, TV Screens : Key to Super-Fast Optical Switch Found

Times Science Writer

A major step toward a new optical switch that promises to be more than 1,000 times faster than the best electronic switches has been taken by scientists at GTE Laboratories Inc. of Waltham, Mass.

The optical device should make it possible to transfer data between computers much faster, and enable more telephone calls to be carried over fiber optic transmission lines. The new technology, GTE scientists said, should also make it possible to develop flat-screen televisions with better defined pictures, as well as many other products.

Computers and other equipment made with the optical devices would have an additional advantage in being unaffected by electromagnetic interference. Many scientists have predicted that in the event of nuclear war, computers and communications would be disrupted by electromagnetic pulses that accompany nuclear explosions.

Plastic Is Key to Switch

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The key to the optical switch is a transparent plastic called polydiacetylene. Polymer physicist Mrinal Thakur of GTE reported here Monday at a meeting of the American Chemical Society that he and his colleagues used a new technique to grow large, virtually defect-free crystals of the plastic.

Thakur said the next step is to find ways to impose useful structures, analogous to those that make up integrated circuits, on the crystals. He said it would be about two years before prototype switches are demonstrated, and about five years before they are ready for the commercial market.

Material scientists have speculated for years that polydiacetylene would make useful optical switches. P. W. Smith of the AT&T; Bell Laboratories and others had predicted that the high switching rates could be obtained with the material. Thakur said the crystals that GTE has produced have confirmed those predictions.

Unusual Crystals

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Crystals of polydiacetylene are unusual in that their refractive index--the speed light travels through a material--changes as they are exposed to increasing amounts of light.

When light passes through an interface between materials with different refractive indexes, it is bent. An example of this effect is observed when a straight object is inserted part way into an aquarium. Viewed from certain angles, the object appears to be bent.

This change in refractive index can be used to make a switch that is analogous to a transistor. When a small amount of light impinges on the crystal, it is transmitted directly to a detector. But if the optical switch is illuminated with a bright light, such as a laser, the crystal bends the light to one side and the light does not reach the detector.

Trillion Times a Second

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The optical switch thus becomes a gate that can be turned on and off, just like a transistor.

Thakur and his colleagues at GTE have shown that the refractive index of polydiacetylene can be changed as many as 1 trillion times per second. In contrast, the best electronic switches now under development can turn on and off only about 1 billion times per second.

The slow speed of existing devices limits the benefits that can be obtained from sending telephone conversations and other information in the form of light through glass fibers

In such systems, Thakur said, “the light message travels along an optical fiber until the message must be switched, say from trunk lines to individual telephones. At this point, the message must be converted into an electrical signal, switched by a microelectronic device, and, if it is to be returned to an optical fiber, reconverted into light.”

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An optical switch made from polydiacetylene, Thakur said, would sidestep all those conversions. In addition to being faster, it also would generate less heat.


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