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OCR for page 38
~ - ~
Right: Ultrapure glass fibers
encased t ~protective ribbons
are t ~ainstay of fiber
..................
Fiber-Oplic
~ me
Munson
...............................
information revolution
. I 'of' t2he 1960s created such an
ticcom= ~.~7ton>~t - ~1 Avalanche of data that
futurists saw it soon over
whelming the telephone lines
and radio waves that carry
information around the globe.
That vision and the
invention of lasers spurred
research into the tremendous
potential of light, which can
carry thousands of times more
information than either
electric signals or radio waves
The work led in 1970 to a very
transparent glass thread
whose appearance quickly
opened the door to fiber-optic
communication, a technology
able to channel the flood of
information into the twenty
first century.
Today's optical fiber is thinner than a
human hair. Yet it carries so much informa
tion that four fibers two in each direction
in a trans-Atlantic telephone cable are
handling up to 40,000 calls at once. The cable,
38
called TAT-8 and laid in 1988, is the first
transoceanic fiber-optic cable. TAT-7, the
copper cable laid in 1983, carries less than a
fourth as many calls and is twice as thick.
The huge capacity of optical fibers translates
into lower construction cost per telephone
circuit. The first trans-Atlantic copper cable,
for example, cost more than $1 million per
circuit to install in 1956; 32 years later, TAT-8
cost less than $10,000 per circuit. l
Waves of light carry more information
because they have higher frequencies-
measured in wave cycles per second, or
hertz than either radio waves or the electric I
waves on copper telephone wires. High-
frequency waves can be switched on and off
faster than low-frequency waves, so they can
I E N G I N ~ E R I N G A N D T H E A D VA N C E M E N T O F H U M A N W E L FA R E
OCR for page 39
be divided into more pulses per second. The
pulses and gaps between them represent the
l's and O's of digital information. The
infrared laser light used in optical telephone
cables has a frequency of about 100 minion
megahertz, 100 million times higher than a
typical AM radio signal and 100 billion times
higher than an electric telephone signal.
Fiber-optic communication became
practical with the development of fibers that
carry light pulses a substantial distance
F I B E R - O P T I C C O M M U N I C A T I O N
The green light of a an argon
laser flows through several
miles of experimental optical
fiber wound around a spool.
Similar hair-thin optical fibers
are the heart of cables that
carry 40,000 or more
simultaneous telephone calls
on beams of laser light under
oceans and across continents.
39
OCR for page 40
Below: Fiber-optic cables
carry many more communica-
tion signals than do copper
cables of similar size. Fiber
cable laid in the trans-Atlantic
link is less than half the size
of copper cable used in an
earlier link but carries more
than four times as many
telephone calls.
before needing a repeater. Their cores are
made of silica glass, one of the most transpar-
ent solid substances known. Impurities in
early glass fibers weakened pulses so much
that light lost 99 percent of its energy after
just 20 meters. But a new chemical process,
pioneered in the 1970s, now produces cores
of such high transparency that fibers in TAT-
8 need only 101 repeaters to boost signals
expensive than lasers. Although their signals
deteriorate over long distances, LEDs are
efficient for short links between, for example,
a compact-disc player and stereo amplifier or
computers in a local network.
Optical fibers can handle many times
more information if they carry light pulses of
several frequencies at the same time. The
bandwidth of light (its range of frequencies)
it..-'- .;
-
The C.S. (Cable Ship) Long
Lines laid more than 3,600
miles of the TAT-8 fiber-optic
cable, from the United States
to a point off the coast of
Europe. From there, British
and French ships laid branch
cables to their respective
countries. In all, nearly 4,200
miles of cable were laid.
40
across the Atlantic. TAT-7, the copper cable,
needs 662.
The debut of a practical fiber coincided
with the development of other technologies
needed for fiber-optic communication. In
1970 a room-temperature, semiconductor
laser was built that could transmit well-
defined pulses through long-distance optical
fiber. Photodetectors were developed that
could handle the torrent of pulses pouring
out the other end and convert them into
electric signals. For short-distance systems,
tiny light-emitting diodes (LEDs) were
devised that use less power and are less
is about 10 billion megahertz from infrared to
ultraviolet and can be divided into millions
of signal channels, each at a slightly different
frequency. Radio waves cover a relatively
tiny bandwidth, leaving little room for
multiple channels. The bandwidth is even
smaller for electric channels.
Most optical fibers today carry light of
only one frequency, although they are
capable of transmitting several more. So far, I
it has proved economical to use multifre-
quency systems only in special cases of
growing demand where limited numbers of
fibers have already been installed. For I
example, fiber pairs that were carrying
24,000 voice circuits between Chicago and
Philaclelphia have been upgraded to transmit
E N G ~ N E E R ~ N G A N D T H E A D VA N C E M E N T O F H U M A N W E ~ FA R E
OCR for page 41
two frequencies, so they now carry 48,000
circuits.
Fiber-optic communication has other
advantages over radio and electric channels.
Light signals do not produce the external
halo of electromagnetic waves that surround
electric telephone lines and cause "cross talk"
between them. Optical fibers are also
relatively secure from unwanted eavesdrop-
pers, who can easily intercept radio waves or
tap electric telephone signals without being
detected.
Optical fibers have been quietly weaving
into communication networks since the first
commercial system opened in Chicago in
1 1977. Since then more than 1.5 million miles
of optical fiber have been installed across the
United States; copper cables are no longer
laid for main lines between U.S. cities. An
undersea fiber-optic cable that U.S. and
Japanese cable ships laid across the Pacific
was activated in 1989. Optical fibers have
also been adapted for sensing movement in
gyroscopes, linking industrial lasers to
cutting and drilling tools, and threading light
~ into the human body for examinations and
I laser surgery.
I A future engineering challenge lies in
! building a computer that uses light instead of
electricity. Theoretically, an optical computer
I would be 1,000 times faster than the best
modern supercomputers. Another challenge
closer to home is in stringing fibers to
individual users from the optical main lines
that now exist. At present, copper wires
connect homes to central telephone
exchanges, where electric signals are convert-
ed into light pulses that are channeled into
the main lines. Telephone authorities in the
United States, Great Britain, Japan, France,
Canada, and West Germany have been
running optical fibers to homes on an
experimental basis since 1980. Once econom-
ic and regulatory obstacles are overcome, the
pace of replacing copper wires with optical
fibers to individual homes in the United
States should pick up. Economical optical
switches and converters for such "end-to-
end" systems still need to be developed.
But early ~ the twenty-first century, many
people could be enjoying hundreds of
television shows, movies, newspapers, and
books brought into their homes through a
single, hair-thixl optical fiber.
F ~ B E R - O P T I C C O M M U N ~ C A T ~ O ~ i
A technician in Philadelphia
checks a device that uses
microprocessors to pinpoint a
malfunction among signal
regenerators along the
heavily amused fiber-optics line
between New York City and
Washington, D.C.
Left: Optical fibers can be
threaded into the ureter, the
tube leading from the bladder
to the kidney, to bring in
laser light that dislodges
kidney stones. The photo here
shows a laser chipping a
kidney stone into fragments
small enough to pass through
I the ureter naturally.
41
Representative terms from entire chapter:
radio waves
