light speeds messages
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,
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
TRN Categories: Optical Computing; Optoelectronics and
Story Type: News
Related Elements: Technical paper, "Communication with Dynamically
Fluctuating States of Light Polarization," Physical Review Letters, February
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