Most of the data sent over a LAN is in the form of electrical signals. However, optical fiber links use light to send data. Something is needed to convert the electricity to light and at the other end of the fiber convert the light back to electricity. This means that a transmitter and a receiver are required.

The transmitter receives data to be transmitted from switches and routers. This data is in the form of electrical signals. The transmitter converts the electronic signals into their equivalent light pulses. There are two types of light sources used to encode and transmit the data through the cable:

• A light emitting diode (LED) producing infrared light with wavelengths of either 850nm or 1310 nm. These are used with multimode fiber in LANs. Lenses are used to focus the infrared light on the end of the fiber
• Light amplification by stimulated emission radiation (LASER) a light source producing a thin beam of intense infrared light usually with wavelengths of 1310nm or 1550 nm. Lasers are used with single-mode fiber over the longer distances involved in WANs or campus backbones. Extra care should be exercised to prevent eye injury

Each of these light sources can be lighted and darkened very quickly to send data (1s and 0s) at a high number of bits per second.

At the other end of the optical fiber from the transmitter is the receiver. The receiver functions something like the photoelectric cell in a solar powered calculator. When light strikes the receiver, it produces electricity. The first job of the receiver is to detect a light pulse that arrives from the fiber. Then the receiver converts the light pulse back into the original electrical signal that first entered the transmitter at the far end of the fiber. Now the signal is again in the form of voltage changes. The signal is ready to be sent over copper wire into any receiving electronic device such as a computer, switch, or router. The semiconductor devices that are usually used as receivers with fiber-optic links are called p-intrinsic-n diodes (PIN photodiodes).

PIN photodiodes are manufactured to be sensitive to 850, 1310, or 1550 nm of light that are generated by the transmitter at the far end of the fiber. When struck by a pulse of light at the proper wavelength, the PIN photodiode quickly produces an electric current of the proper voltage for the network. It instantly stops producing the voltage when no light strikes the PIN photodiode. This generates the voltage changes that represent the data 1s and 0s on a copper cable.

Connectors are attached to the fiber ends so that the fibers can be connected to the ports on the transmitter and receiver. The type of connector most commonly used with multimode fiber is the Subscriber Connector (SC connector). On single-mode fiber, the Straight Tip (ST) connector is frequently used.

In addition to the transmitters, receivers, connectors, and fibers that are always required on an optical network, repeaters and fiber patch panels are often seen.

Repeaters are optical amplifiers that receive attenuating light pulses traveling long distances and restore them to their original shapes, strengths, and timings. The restored signals can then be sent on along the journey to the receiver at the far end of the fiber.

Fiber patch panels similar to the patch panels used with copper cable. These panels increase the flexibility of an optical network by allowing quick changes to the connection of devices like switches or routers with various available fiber runs, or cable links.

Lab Activity Lab Exercise: Fiber-Optic Cable Purchase This lab will introduce the variety and prices of network cabling and components in the market. The student will gather pricing information for fiber patch cables and fiber bulk cable.