A Fiber-Optic Relay SystemA Fiber-Optic Relay System

To understand how optical fibers are used in communications
systems, let's look at an example from a World War II movie
or documentary where two naval ships in a fleet need to
communicate with each other while maintaining radio silence
or on stormy seas. One ship pulls up alongside the other. The
captain of one ship sends a message to a sailor on deck. The
sailor translates the message into Morse code (dots
and dashes) and uses a signal light (floodlight with
a venetian blind type shutter on it) to send the message to
the other ship. A sailor on the deck of the other ship sees
the Morse code message, decodes it into English and sends the
message up to the captain.

Now, imagine doing this when the ships are on either side of
the ocean separated by thousands of miles and you have
a fiber-optic communication system in place between the two
ships. Fiber-optic relay systems consist of the following:

* Transmitter - Produces and encodes the light signals
* Optical fiber - Conducts the light signals over
a distance
* Optical regenerator - May be necessary to boost the
light signal (for long distances)
* Optical receiver - Receives and decodes the light
signals

Transmitter
The transmitter is like the sailor on the deck of the sending
ship. It receives and directs the optical device to turn the
light "on" and "off" in the correct sequence, thereby
generating a light signal.

The transmitter is physically close to the optical fiber and
may even have a lens to focus the light into the fiber.
Lasers have more power than LEDs, but vary more with changes
in temperature and are more expensive. The most common
wavelengths of light signals are 850 nm, 1,300 nm, and 1,550
nm (infrared, non-visible portions of the spectrum).

Optical Regenerator
As mentioned above, some signal loss occurs when the light is
transmitted through the fiber, especially over long distances
(more than a half mile, or about 1 km) such as with undersea
cables. Therefore, one or more optical regenerators is
spliced along the cable to boost the degraded light signals.

An optical regenerator consists of optical fibers with
a special coating (doping). The doped portion is "pumped"
with a laser. When the degraded signal comes into the doped
coating, the energy from the laser allows the doped molecules
to become lasers themselves. The doped molecules then emit
a new, stronger light signal with the same characteristics as
the incoming weak light signal. Basically, the regenerator is
a laser amplifier for the incoming signal.

Optical Receiver
The optical receiver is like the sailor on the deck of the
receiving ship. It takes the incoming digital light signals,
decodes them and sends the electrical signal to the other
user's computer, TV or telephone (receiving ship's captain).
The receiver uses a photocell or photodiode to detect the
light.