City of Light: The Story of Fiber Optics

8 CITY OF LIGHT
speech—300 to 3000 hertz (or cycles per second), if you like specifications.
That’s only part of the human ear’s nominal range of 20 to 20,000 hertz, so
sound quality isn’t good. But it does the job because you don’t need high
fidelity to understand speech.
Phone wires can carry other audio signals as well as speech. The push
buttons on a Touch-Tone phone whistle musical notes at audio frequencies.
Modems and fax machines warble their codes as tones in the audible speech
range. Our ears don’t understand their electronic speech, but the telephone
network carries their signals nonetheless, and other modems and fax machines
decode the sounds.
The phone wires in your walls are strands of copper, coated with plastic
insulator and grouped in pairs (or sometimes fours). When they have to go
a long way, they’re often wound around each other, making what the phone
industry calls twisted pairs. The problem with standard twisted-pair phone
wires is that they were designed to carry a single conversation, which doesn’t
amount to much information. Adjust and condition them properly, and if all
goes well they can carry the equivalent of a few dozen conversations a few
miles, or of many more over a thousand feet. But that’s it. So far, no more
information can fit through the pipe.
Modern telephone systems contain electronics that convert the whistles,
warbles, and words into digital signals within a few miles of your home. Each
‘‘voice channel’’ becomes a series of 64,000 bits per second. More electronics
interleave those bits with the bits that encode dozens, hundreds, or even
thousands of other telephone calls. Phone companies have a hierarchy of
levels, each one combining more digitized voice channels to generate a
higher-speed signal. Sometimes the slower signals go through special metal
cables or are relayed by microwave towers, but typically they go through
optical fibers.
Most modern fibers concentrate light signals in tiny cores about nine micrometers—0.009
millimeter—across. That might seem a tiny pipe, but in
fact it has a huge information capacity. Its small size keeps short pulses of
light from spreading out and interfering with each other as they travel down
the fiber. With the best state-of-the-art transmitters phone companies can
buy, a single fiber can carry more than one trillion bits per second, equivalent
to 13 million telephone conversations. 2 Pairs of fibers carry two-way traffic,
one carrying signals in each direction.
The clarity of glass fibers is another advantage. Depending on the design,
they can carry signals tens of miles without any internal amplification. That’s
vital if you’re building a cable crossing a continent or an ocean, or a cabletelevision
network serving a small city. Cable television network use coaxial
cable (‘‘coax’’), with a central wire surrounded by a plastic layer that is encased
in a metal sheath, and usually covered with a protective jacket. Coax
thinner than a pencil hooks home video systems together; thicker coax—
costlier but able to carry signals farther—links cable companies to homes.
Yet even those costly coaxial cables can’t carry signals very far. Cable companies
have to install signal-amplifying repeaters about every half kilometer