City of Light: The Story of Fiber Optics

SUBMARINE CABLES 209
filled a cable with water and put an uncabled fiber in the same water to
compare what happened to transmission. After eight months, loss of the cabled
fiber increased dramatically at 1.08 to 1.24 micrometers and beyond 1.4
micrometers. Attenuation of the uncabled fiber had hardly changed. 26
Those were the wavelengths where loss was mysteriously rising in other
fibers. The shorter wavelengths were where hydrogen molecules absorbed
light. The longer wavelengths matched the absorption of the tiny amounts of
residual water that remained trapped in virtually all silica fibers. That pointed
to hydrogen as the problem. Somehow the affected fibers were soaking it up.
Yet the changes were inconsistent; they affected some fibers and some cable
structures but not others.
Developers in Japan, America, and Britain set up task forces to attack the
problem. Once they focused on hydrogen, they began to see patterns. The
only fibers affected were those containing phosphorus, which softened the
glass, making it easier to draw. British work showed that phosphorus was
the bad actor; it reacted with hydrogen, forming chemical bonds that absorbed
light. The problem didn’t show up with a dash of phosphorus but
became serious if concentration reached about one percent.
Cable structure was important because some types accumulated hydrogen.
The high voltages carried by wires to power undersea repeaters caused metal
in the cable to react with the traces of water that inevitably seeped into the
cable. 27 Sometimes submarine cables collected so much hydrogen that it could
be flared off when the cable was hauled to the surface and opened for repairs.
28 Moisture also could build up in cables on land. The extent of the
problem depended on where the hydrogen collected in the cable; STL’s original
design had a problem because an internal barrier trapped hydrogen in
the same region as the fiber. 29
With the culprit identified, developers attacked the hydrogen problem on
all fronts. Glass makers purged phosphorus from their preforms and their
vocabulary. 30 Fiber drawers applied new coatings to keep hydrogen from seeping
into fibers. Cable makers added hydrogen seals, eliminated plastics that
might release hydrogen, and sealed their cables tighter to keep water out. 31
Engineers hooked measurement instruments to phosphorus-free fibers in
hydrogen-free cables and watched carefully. They saw no changes. The great
hydrogen scare was over. Hearts resumed beating naturally; sighs of relief
echoed in Japan, Britain, France and America. Submarine fiber cables pushed
onward.
A Go-ahead for Submarine Cables
Plans for the massive TAT-8 project unfolded with little heed of the hydrogen
scare. The cable builders had no other options. In December 1982, an international
consortium 32 requested bids for a fiber-optic cable running from
Tuckerton, New Jersey, to a point off the European coast, where it split into
separate cables to France and England. AT&T, Standard Telephones and Ca-