Polarized light speeds messages

By Chhavi Sachdev, Technology Research News

The fiber-optic cables that make up the backbone of the world's communications network use pulses of light to transmit the ones and zeros that make up digital information. A pulse represents a one and a gap represents a zero.

Fiber can transmit a lot of information at once because light can be pulsed extremely quickly and because the cables can carry many colors, or wavelengths of light at once. Each wavelength can carry a different message. Today's state-of-the-art equipment can pulse light over a trillion times a second and can carry 160 channels at the same time.

There's more to light than color and pulse rate, however. Scientists have recently begun to look at the potential of a third variable of light: polarization. An ordinary, unpolarized light wave's electric field vibrates in all directions perpendicular to the light wave. A light wave whose electric field is oriented in only one direction is polarized.

The trouble with representing ones and zeros using light that is polarized in specific different directions is that polarization is relatively delicate: any distortions in fiber optic cables caused by even minor temperature or pressure changes can alter the polarization. These changes, or noise, makes it difficult to decipher the original signal.

Researchers from the Georgia Institute of Technology have developed a method for transmitting signals with polarized light that sidesteps the problem by using changes in polarization rather than light polarized in a certain direction. A one is represented by a change in the polarization and a zero is represented by the absence of a change.

"Disturbances of the communication channel do not matter in this scheme of communication, and we can receive the bits even when the channel is perturbed by temperature changes or mechanical disturbances," said Rajarshi Roy, one of the Georgia Tech researchers who is now a professor of physics at the University of Maryland.

The researchers' system transmits light using a laser made from a glass ring that is doped, or infused, with the metal erbium. Cycling the light within the ring generates a steady stream of light that is polarized in a specific direction. A phase modulator then changes the polarization direction; this changed light is channeled down a fiber from the ring to a receiver.

The receiver uses a pair of detectors to sense whether the light has been changed from one cycle around the transmitter ring to the next, according to Roy. “The key idea here is to use two detectors for the light after it has gone through the communication channel and to have a time delay between them that is equal to the time it takes the light to go around once in the fiber ring laser that is used as the transmitter,” he said.

“If a change in polarization is detected by comparing the two detector outputs, then we have transmitted a one; if not, we have transmitted a zero,” said Roy. The messages transmitted by this method are reliable even when the communication channel is disturbed because polarization changes caused by heat or pressure on the fiber-optic line "would affect both branches of the receiver in the same way," he said.

The system is relatively quick, according to Roy. Light takes about two hundred nanoseconds to go around the ring, and generate one bit of information. A nanosecond is a billionth of a second. In one second, light can traverse 186,000 miles, which is three-fourths the distance to the moon.

The work is intriguing, said Daniel Blumenthal, a professor of electrical and computer engineering at the University of California at Santa Barbara. "They are able to decode the bits from what looks like a noisy channel. Of course, to see if this really works, one has to perform bit error rates and measure the receiver sensitivity as a function of the received optical power," he said.

The ring laser is very different from today's fiber communications equipment, according to Roy. “A lot of development engineering would still be necessary to implement this method in a practical situation,” said Roy. The system is easy to make, however. All its components are available commercially, he said.

Roy’s research colleague was Gregory D. VanWiggeren, a scientist at Agilent Labs. The work was done while they were at the Georgia Institute of Technology. It was funded by the Office of Naval Research (ONR). The researchers published their results in the February 13, 2002 issue of the journal Physical Review Letters.

Timeline:  unknown
Funding:   Government
TRN Categories:   Optical Computing; Optoelectronics and Photonics; Telecommunications
Story Type:   News
Related Elements:  Technical paper, "Communication with Dynamically Fluctuating States of Light Polarization," Physical Review Letters, February 13, 2002.


April 17/24, 2002

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