How does a fiber optic cable work?

Fiber optic cables make long-distance telephone communication and Internet access much more affordable and accessible. Get Marshall's explanation of fiber optic cables in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:00):
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(00:27):
h I am Marshall Brain with today's question, how does
a fiber optic cable work? Over the last thirty years
or so, fiber optic cables have taken over and transformed
the long distance telephone industry. Optical fibers are also a
huge part of making the Internet available around the world.
When fiber replaces copper for long distance calls and Internet traffic,

(00:49):
it dramatically lowers costs. To understand how a fiber optic
cable works, imagine an immensely long drinking straw or flexible
plastic pipe. For example, imagine a pipe that's several miles long.
Now imagine that the inside surface of the pipe has
been coated with a perfect mirror. Now imagine that you

(01:10):
are looking into one end of the pipe several miles away.
At the other end, a friend turns on a flashlight
and shines it into the pipe. Because the interior of
the pipe is a perfect mirror, the flashlights light will
reflect off the sides of the pipe, even though the
pipe may curve and twist, and you will see the
light at the other end. If your friend were to

(01:30):
turn the flashlight on and off in a Morse code fashion,
your friend could communicate with you through this pipe. That
is the essence of a fiber optic cable. Making a
cable out of a mirror tube would work, but it
would be bulky and it would be very hard to
coat the interior of the tube with a perfect mirror.
A real fiber optic cable is therefore made out of glass.

(01:52):
The glass is incredibly pure, so that even though it's
several miles long, light can still make it through. Imagine
glass so transparent that a window several miles sticks still
looks clear. The glass is drawn into a very thin
strand with a thickness comparable to that of a human hairror.
The glass strand is then coated in several layers of plastic.

(02:16):
By coating the glass in plastic, you get the equivalent
of a mirror around the glass. Strand this mirror creates
total internal reflection, just like a perfect mirror coating on
the inside of a tube does. You can experience this
sort of reflection with a flashlight and a window in
a dark room. If you direct the flashlight through the

(02:36):
window at a ninety degree angle, it passes straight through
the glass. However, if you shine the flashlight at a
very shallow angle nearly parallel to the glass, the glass
will act as a mirror and you will see the
beam reflect off the window and hit the wall opposite
on the inside of the room. Light traveling through the
fiber bounces at shallow angles like this and stays completely

(02:59):
within the fiber. To send telephone conversations through a fiber
optic cable, analog voice signals are translated into digital signals.
A laser at one end of the pipe switches on
and off to send each bit. Modern fiber systems with
a single laser can transmit billions of bits per second.
The laser can turn on and off several billion times

(03:22):
per second. The newest systems use lasers with multiple colors
to fit multiple signals into the same fiber. Modern fiber
optic cables can carry a signal quite a distance, perhaps
sixty miles or a hundred kilometers. On a long distance line,
there is an equipment hut every forty or fifty miles.
The hut contains equipment that picks up and retransmits the

(03:45):
signal down the next segment at full strength. Do you
have any ideas or suggestions for this podcast? If so,
please send me an email at podcast at how stuff
works dot com. For more on this and thousands of
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sure to check out the brain stuff blog on the
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