City of Light: The Story of Fiber Optics

THREE GENERATIONS IN FIVE YEARS 187
Looking farther into the future, Bell Labs set new fiber goals. One target
was submarine cables using single-mode fibers and gallium arsenide lasers;
another was long-distance systems operating in the 1.1 to 1.3 micrometer
window.
In Britain, the field trials at Martlesham Heath and Hitchin-Stevenage were
similarly encouraging. The Post Office decided to lay eight-fiber cables along
routes totaling about 500 kilometers (300 miles), including the London-
Reading route where the millimeter waveguide had almost gone. 56 Like Bell
Labs, they began looking to future single-mode and long-wavelength systems.
Some annoying glitches did nag at engineers evaluating the field trials. It
proved impossible to predict reliably how much light would be transferred
from one fiber to another at Hitchin-Stevenage, even when their specifications
seemed identical. Something was inducing noise at the junctions. Initially
baffled, Richard Epworth sat down to analyze the problem at Standard Telecommunication
Labs. Common sense said that ‘‘bad’’ lasers were the most
likely cause, and switching lasers usually cured the problem. However, the
lasers that seemed best in laboratory measurements fared the worst in the
system.
Epworth finally realized that the problem lay in the coherence of laser light.
Coherent light waves can interfere with each other if they travel slightly
different paths, producing light and dark zones. Illuminate a small area with
coherent light and you see shifting grainy patterns called ‘‘speckle.’’ It’s an
effect well known to laser specialists, which makes laser-illuminated holograms
look grainy. Epworth realized the same effect could occur when many
modes interfered with each other at the end of a multimode fiber. The more
coherent the light, the more pronounced the speckle pattern and the more
intense the ‘‘modal noise’’ caused by minute shifts in laser wavelength or
fiber position. From a theoretical standpoint, it resembled the modal problems
that plagued the millimeter waveguide. 57
Modal noise hit the STL test hard because it used more coherent lasers
and fibers with 30-micrometer cores instead of the 50 or 62.5 micrometer
cores used in American trials. The coherence increased speckle intensity, and
the small cores meant that loss of only a few speckles could cause noise.
When Epworth first described the problem in 1978, he recommended shifting
to less coherent light sources, 58 or using fibers that carried more modes. However,
it soon became clear that only single-mode fibers could eliminate modal
noise. 59 It was a message many people did not want to hear.
Back to Single-Mode
Single-mode fibers had a bad reputation because light coupling was difficult.
They also didn’t offer much advantage over graded-index fibers at 850 nanometers,
where high material dispersion largely offset any attractions of
single-mode transmission.