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