City of Light: The Story of Fiber Optics

EPILOGUE: THE BOOM, THE BUBBLE, AND THE BUST 245
shifted fiber for long-haul systems. The Bell Labs submarine cable group settled
on dispersion-shifted fiber and erbium amplifiers for a new generation of
high-speed systems. Neal Bergano previewed the technology in a 1991 hero
experiment which transmitted five billion bits per second through 9000 kilometers
of dispersion-shifted fiber. 26 Later AT&T used that technology for the
TAT-12 transatlantic cable switched on in 1996.
Multiplying Bandwidth
Wavelength-division multiplexing was simply a new approach to sharing a
limited spectrum. Radio stations do it by each broadcasting at a separate
assigned frequency. Each television channel likewise has its own frequency.
If the frequencies are assigned carefully, the signals don’t interfere with each
other.
The principle of wavelength-division multiplexing had been demonstrated
much earlier in fibers. It is fairly simple over the short distances that fibers
can carry light signals without amplification. The biggest problem was combining
the light signals at the input and separating them at the output. Desurvire
had shown that erbium fiber amplifiers could transmit signals at two
separate wavelengths. But as experimenters started adding new channels,
troubles mounted for both solitons and dispersion control.
Early soliton experiments looked encouraging. Bell noted that solitons at
different wavelengths ‘‘collided’’ with each other in their first experiments,
but still managed to send a pair of signals at two billion bits per second
through a chain of optical amplifiers and fiber spanning 9000 kilometers. 27
Mollenauer kept stretching the distance he could send a pair of channels, first
to 11,000 kilometers, 28 then to 13,000 kilometers. 29 He also increased the
speed per channel, eventually to 10 billion bits per second. Yet soliton collisions
posed a serious problem. They were inevitable because the speed of light
in the fiber varies with wavelength, so pulses at the faster wavelength pass
those at the slower wavelength. And they were bad news because the signals
interacted enough to upset the delicate balancing act of soliton transmission.
That gave solitons ‘‘the jitters,’’ so the output pulses didn’t fall into the proper
time slots.
Adding wavelengths also produced problems in dispersion controlled systems.
Engineers used fibers with their zero dispersion point shifted to 1.55
micrometers to keep pulse dispersion from building up over long distances.
The problem is that signals transmitted near the zero dispersion wavelength
stay in phase with each other over long distances. The light signals interact
very weakly, but crosstalk builds up if the two wavelengths stay in phase for
hundreds or thousands of kilometers.
The dilemma looked bad at first. Dispersion along a long run of fiber had
to be kept low or the pulses would run together. Yet if low dispersion kept
the pulses in phase over the whole fiber, crosstalk would scramble the signals.
Engineers solved the problem when they realized that they didn’t have to use