City of Light: The Story of Fiber Optics

THREE GENERATIONS IN FIVE YEARS 195
crometers, although it was limited to 140 million bits per second because
pulse spreading was higher than at 1.3 micrometers. 89 The tests convinced
British Telecom to shift entirely to single-mode, and it began installing singlemode
systems in 1983.
Nippon Telegraph and Telephone and its Japanese suppliers, who opened
the long-wavelength window, also moved quickly to single-mode. However,
Bell Labs still saw submarine cables as the only place for single-mode. Bell’s
position was that single-mode was at least a decade away on land. Those who
disagreed were frowned upon. After telling a group of AT&T manufacturing
engineers about the virtues of single-mode fibers, Paul Lazay was taken aside
by a manager who informed him that his ‘‘comments on the superiority of
single-mode technology were probably a career-limiting presentation.’’ 90
Midwinter briefed Crawford Hill on British single-mode programs and left
‘‘with the overwhelming impression that they were very interested but frankly
they were very convinced that this was all very blue-sky stuff, and the real
world was graded-index and that’s what they were going to engineer the hell
out of.’’ 91 AT&T’s top priority was building a massive high-capacity digital
system to run along the nation’s busiest communications route, the Northeast
Corridor from Boston to Washington. The phone company was committed to
using graded-index fibers.
The Northeast Corridor
The Boston-to-Washington route was no ordinary link in the phone network.
Its heavy traffic came from heads of government, industry, and finance. It
was big, it was visible, and the existing system was antiquated and overloaded.
Microwave relays still carried most long-distance traffic in 1980, but AT&T
wanted to replace the old analog technology. Digital microwaves could do the
job in the wide open spaces but not along the Northeast Corridor. Traffic was
heavy, cities were close together, and the microwave spectrum was crowded.
Once the route had been planned for millimeter waveguides, which could
have provided the tremendous capacity needed. Now AT&T chose it as the
launching pad for optical fibers in the long-distance telephone network.
AT&T already had a cable right of way along the route, which had been
used for an old coaxial cable system. Huts had been built for coax repeaters
every seven kilometers (four miles), closely matching the repeater spacing
needed for graded-index fiber carrying 850-nanometer signals. Each fiber
could carry only 45 million bits per second—672 phone calls—but Bell had
designed its cable to hold up to a dozen 12-fiber ribbons. By filling the cable
with fibers and using the technology already tested in Atlanta and Chicago,
AT&T thought it could achieve both high capacity and robust reliability. The
company did not want captains of industry, senators, and congressmen waiting
for phone lines.