City of Light: The Story of Fiber Optics

The Allure of Single-Mode Fiber
SUBMARINE CABLES 205
When Alec Reeves suggested submarine fiber-optic cables in 1969, he expected
repeaters to be only 2 to 3 kilometers (1.2 to 2 miles) apart, so a
couple thousand would be needed to cross the Atlantic. He thought that high
fiber capacity—allowing transmission at 10 billion bits per second, well over
a million voice channels—would offset the problems raised by so many repeaters.
12 His estimates were based on fiber loss of 20 decibels per kilometer,
and within a few years it was clear he had been far too pessimistic.
It also became clear that pulse dispersion could not be neglected in gradedindex
fibers. That was not entirely Reeves’s fault for being overoptimistic. The
effect is proportional to distance; go ten times farther and pulses spread out
ten times farther. Dispersion that would be no problem over a couple of kilometers
could limit transmission speeds in cables spanning ten times that
distance.
Submarine cable developers also were much more concerned with reducing
the number of repeaters than Reeves had been. That pushed them to seek
the utmost in transmission capacity and repeater spacing, which led them to
reconsider single-mode fiber. The usual objections to single-mode fiber centered
on the demanding precision needed to align fibers with each other and
with light sources. Attaining that precision was hardest in demountable connectors—but
they weren’t needed in submarine cables. Alignment was easier
in the factory than in the field, and submarine cables were assembled with
repeaters in place, then loaded into cable ships. Warnings that single-mode
fibers were years from practical use didn’t frighten submarine cable developers
accustomed to spending many years perfecting new systems. The next generation
of submarine cables was not scheduled until the late 1980s; without
the best possible fibers, it might never come.
Nonetheless, as long as fiber systems had to operate at 850 nanometers,
they did not offer dramatic advantages over coaxial cable. Loss of a few decibels
per kilometer meant a repeater was needed roughly every 10 kilometers
(6 miles), just a slight improvement over the coaxial cables used in TAT-6
and -7. Serious worries about laser lifetimes offset the attractions of using a
smaller cable. Material dispersion was high at 850 nanometers, so singlemode
fibers did not offer dramatically higher transmission capacities than
graded-index.
The steady advance of fiber technology, the opening of the 1.3-micrometer
window, and the spread of digital transmission tipped the scales decisively
toward fiber for the next generation of submarine cables. Both loss and pulse
dispersion were much lower at the long wavelength than at 850 nanometers.
With the low loss at 1.3 micrometers, signals could travel 50 kilometers (30
miles) or more between repeaters. Graded-index fiber had lower dispersion at
1.3 micrometers than at 850 nanometers, but single-mode fiber had near
zero dispersion, promising much higher transmission speeds. The choice was
clear to AT&T, the Post Office, and Standard Telecommunication Labs in