How Fiber Optics Work

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 maintainingradiosilence 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 intoMorse 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

Thetransmitteris 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. Themost common wavelengthsof light signals are 850 nm, 1,300 nm and 1,550 nm (infrared, non-visible portions of thespectrum).

Optical Regenerator

As mentioned above, somesignal lossoccurs when the light is transmitted through the fiber, especially over long distances such as with undersea cables. Therefore, one or moreoptical regeneratorsis spliced along the cable to boost the degraded light signals.

Anoptical regeneratorconsists of optical fibers with a special coating (doping). The doped portion is "pumped" with alaser. When the degraded signal comes into the doped coating, the energy from the laser allows the doped molecules tobecome 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

Theoptical receiveris 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'scomputer,TVortelephone(receiving ship's captain). The receiver uses aphotocellorphotodiodeto detect the light.