City of Light: The Story of Fiber Optics

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