City of Light: The Story of Fiber Optics

234 CITY OF LIGHT
miles. Racks of equipment stood around the walls of his 20 � 30 foot laboratory;
they included eight or ten soliton transmitters operating at different
wavelengths, for experiments through a single fiber. In the middle were a pair
of massive black optical tables—built like pool tables covered with flat aluminum
plates painted a dull, reflectionless black. Fiber-optic cables dangled
from overhead runways, linking precision optics on the tables and equipment
racks to the basement, where Bell stored reels and reels and reels of fiber
forming a loop 2400 kilometers (1500 miles) long. 18
A Trillion Bits a Second
At Crawford Hill, I visited a lab where Andy Chraplyvy and Bob Tkach set a
different target, packing a trillion bits per second through a single fiber. They
don’t use solitons and they don’t seek to span the globe. Several hundred
kilometers is adequate for their mission, new technology for long-distance
service on land.
A trillion bits per second is a tremendous pipeline. Tanned and enthusiastic,
Chraplyvy told me it’s more than all the long-distance traffic that usually
flows in the whole American telephone network. 19 It’s the equivalent of
over ten million voice telephone circuits, or eight percent of all Americans
talking to each other at the same instant. Depending on how efficiently digital
video signals can be compressed, a trillion-bit fiber could carry a mindnumbing
hundreds of thousands of standard television channels, or perhaps
tens of thousands in high-definition format. You might wonder why anyone
would need that gigantic an information pipeline, but that was what people
said about fibers carrying a billion bits per second a dozen years earlier.
You can’t turn a laser on and off a trillion times a second. The record
is around a hundred billion pulses a second, but that’s tough to achieve,
even at Bell Labs. What you want are many lasers at separate wavelengths,
each modulated very fast, to send signals through the same fiber. It’s the
same concept of wavelength-division multiplexing that Bell tried nearly two
decades ago in the Northeast Corridor system, but now it’s vastly more
sophisticated and easier. Erbium-doped fiber amplifiers can amplify all the
wavelengths between about 1.53 and 1.58 micrometers. The problem is how
close can you pack the wavelengths without the signals interfering with each
other.
When I visited in July 1995, Chraplyvy and Tkach had 17 laser transmitters,
each running at 20 billion bits per second. That comes to 340 billion
bits per second, 20 more than all the long-distance traffic in North America,
going through the 9-micrometer (0.009 millimeter) core of a single fiber.
It wasn’t an easy task. ‘‘The cost of these experiments is outrageous,’’ said
Chraplyvy. He showed me a metal laboratory rack about the size of a refrigerator
holding eight laser transmitters. Their experiments demand lasers with
wavelengths that can be adjusted very, very precisely, to better than one part