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Tuesday, December 27, 2005

Scientists Take Step Toward Faster Communication

By using electromagnetic waves instead of electrical current for switching, researchers have operated an optical modulator at terahertz frequencies – an accomplishment that could one day facilitate data transmission rates in the trillions of bits per second.

The work represents a key step toward a new generation of optical communication systems that would be as much as 100 times faster than current technology, bringing closer such applications as real-time telemedicine and movies on demand. While operating their terahertz modulator, the research team observed an effect that is well known in atomic physics – but until now hadn’t been seen in the semiconductor materials that make up optical modulators.

“This is just one piece, but potentially a very important piece, of a very high bit-rate optical communication system for telecommunications and other applications,” said David Citrin, an associate professor in the School of Electrical and Computer Engineering at the Georgia Institute of Technology. “The point of the experiment was to show that we can operate a modulator at terahertz frequencies, though we are still a long way from a practical device.”

Supported by the National Science Foundation, the research was reported in the October 28, 2005 issue of the journal Science.

Existing telecommunication systems depend on modulators to encode data onto beams of light that then can be carried long distances by optical fibers. Modulators work by rapidly changing their reflectivity, which varies the intensity of light beams passing through them. These variations correspond to the ones and zeroes that are the language of digital communication.

Modulators are also used as switches to reroute data streams by alternately reflecting light or allowing it to pass.

But most current modulators have a drawback – they cannot operate faster than the electronic circuitry used to control them. To boost data speeds, researchers have been seeking alternative control technologies.

“Conventional optical modulators use a voltage change to alter the properties of a material which changes the reflectivity,” Citrin explained. “Electrically switched systems are just too slow to go much beyond where we are now. But by using very high frequency electromagnetic energy to modulate the signal, the hope is that we can generate signals that have much higher data rates than what we can achieve with today’s electrical circuits.”

To gain those higher rates, Citrin and colleagues at the University of California, Santa Barbara and the NASA Ames Research Center used very high-frequency waves from a free-electron laser to control the modulator. These electromagnetic waves consist of an oscillating electric field and have the advantage of being able to move through free space without the need for circuitry.

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