City of Light: The Story of Fiber Optics
City of Light: The Story of Fiber Optics
City of Light: The Story of Fiber Optics
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244 CITY OF LIGHT<br />
sent signals at four separate wavelengths through a series <strong>of</strong> six erbium amplifiers<br />
and 459 kilometers <strong>of</strong> fiber. 23 Each channel carried 2.4 billion bits per<br />
second.<br />
Other groups encoded their signals in fancy ways to try to make them<br />
resistant to degradation. <strong>The</strong> KDD team didn’t bother. <strong>The</strong>y just turned the<br />
light from each transmitter <strong>of</strong>f for zero bits and on for one bit, a coding called<br />
NRZ that was used throughout the digital telephone network. It was far from<br />
clear that they were blazing a trail for a revolution.<br />
Despite all their attractions, optical amplifiers have a serious inherent limitation.<br />
<strong>The</strong>y depend on stimulated emission, which from an engineer’s viewpoint<br />
is an analog process. At its best, an analog amplifier merely multiplies<br />
the strength <strong>of</strong> the signal it receives. If the input contains noise, it multiplies<br />
the noise along with the signal. If dispersion has stretched the input pulses,<br />
it multiplies the stretched-out pulses. In short, an optical amplifier turns up<br />
the power but does nothing to clean up the signal.<br />
In contrast, sophisticated digital electro-optic repeaters do more than amplify<br />
the signal they receive. <strong>The</strong>y regenerate the original series <strong>of</strong> digital<br />
pulses. Discrimination circuits decide if the input power represents a zero or<br />
a one. Timing circuits fit the pulses into their proper time slots. After figuring<br />
out what the received signal should be, the electronics regenerate a nice clean<br />
series <strong>of</strong> pulses. TAT-8 used such regenerators to send sharp digital signals<br />
across the Atlantic.<br />
Optical amplifiers produce some noise, but in practice the main concern<br />
was the dispersion <strong>of</strong> pulses as they traveled long distances through the fiber.<br />
Dispersion wasn’t a problem as long as single-mode fibers were transmitting<br />
near their zero-dispersion wavelength <strong>of</strong> 1.31 micrometers, with repeaters<br />
about every 50 kilometers. However, erbium amplifies at 1.55 micrometers,<br />
where single-mode fibers have higher dispersion. Realizing the promise <strong>of</strong><br />
erbium amplifiers required a way to control that dispersion.<br />
One approach is to transmit signals in a form not affected by dispersion.<br />
Linn Mollenauer <strong>of</strong> Bell Labs had already shown that special pulses called<br />
solitons could overcome dispersion by delicately balancing it against another<br />
effect called self-phase modulation that can distort light pulses in an optical<br />
fiber. While dispersion tries to stretch the duration <strong>of</strong> the pulse, self-phase<br />
modulation tries to stretch out the range <strong>of</strong> wavelengths it contains. As long<br />
as the power is high enough to trigger the distortion, and the pulse has the<br />
right shape, the tug <strong>of</strong> war between the two effects lets the pulse pass unaltered.<br />
24 Solitons needed optical amplification to keep the pulses powerful<br />
enough. Mollenauer had earlier tried an exotic amplification process, but erbium<br />
amplifiers worked better. A series <strong>of</strong> experiments made the future look<br />
bright for solitons and erbium amplifiers.<br />
A different approach is to control dispersion <strong>of</strong> the fiber itself without doing<br />
special tricks with the transmitted signals. In the early 1980s, both Corning<br />
and British Telecom had designed special single-mode fibers with complex core<br />
structures which shifted their zero-dispersion wavelengths to 1.55 micrometers.<br />
25 <strong>The</strong> emergence <strong>of</strong> erbium amplifiers renewed interest in dispersion-