City of Light: The Story of Fiber Optics

A DEMONSTRATION FOR THE QUEEN 173
digital fiber links carrying 24 telephone conversations in Scientific American. 87
Developers demonstrated digital repeaters operating at 6.3, 45, and 274 million
bits per second. The two slower ones used LEDs; the fastest one a roomtemperature
diode laser. 88 The next step was to take the technology out of
the lab for a field trial like the one of the millimeter waveguide nearing its
start in New Jersey.
AT&T, being a giant corporation, had a committee set the parameters.
Some choices were easy. The main goal was to test digital transmission in an
urban environment. Fiber-optic cables packed a lot of capacity in a small
diameter, so they could alleviate crowding in underground ducts—saving the
phone company the untold millions needed to dig up the streets and lay new
ducts. Fibers also could stretch much farther between repeaters than the 6000
feet (1.8 kilometers) possible with wires.
Committees need something to debate, and this one concentrated on transmission
speed. Some people wanted fibers to replace the 1.5 million bit per
second T1 lines that AT&T had been installing since 1962; that way they
could interface directly with existing systems. Another suggestion was to operate
at the 6 million bit per second T2 speed, where electronics were readily
available. However, executive director Warren Danielson pushed for the 45
million bit per second T3 rate, which had never been used for lack of a cable
to carry it. He argued the slower speeds didn’t improve on wires and that the
higher speed would share the high cost of the laser and fiber over more
channels. The others agreed. 89
Ira Jacobs’s group at Holmdel spent 1975 assembling equipment for tests
at a suburban Atlanta AT&T plant. They packed Corning and AT&T fibers
into plastic-coated ribbons of 12 fibers, and stacked a dozen ribbons together
to make a pencil-thin 144-fiber core for the cable. 90 They threaded 650 meters
(2100 feet) of that cable through ducts buried under a parking lot, then
spliced fiber ends together to test transmission over distances up to 10.9 kilometers
(6.8 miles). 91 They put a separate fiber cable through environmental
torture tests in a duct where they could adjust the temperature and humidity.
On January 13, 1976, they turned on the Atlanta fiber system and spent the
next several months waiting for something bad to happen.
The communications world watched. The British Post Office and others
had their own tests in the works, but no one had the resources to match Bell
Labs. Bell might not lead in inspiration, but it excelled in perspiration. Its
careful and cautious engineers would poke and probe the new technology
and spot any problems.
They did find one problem. A distressingly large fraction of the lasers died
during the test. That was not unexpected, nor was it fatal. Laser lifetimes
were improving steadily, and Bell was hedging its bets by developing LEDbased
transmitters. Otherwise, ‘‘things went quite smoothly,’’ Jacobs recalls.
Fiber loss was smaller than expected, as was the variability among components.
92 Bell had hoped to get 100 good fibers, but 138 of the 144 were good.
Average loss was six decibels per kilometer, below the planned eight deci-