Basic Fiber-Optic Communication System

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A fiber-optic communication system uses light pulses to transmit information over optical fibers. An optical fiber is a very thin and long plastic or glass medium that can carry light along its length. Thus, a basic fiber-optic communication system consists of the following: 1) an optical transmitter for transmitting the light pulses; 2) fiber-optic cables that carry the light signals; 3) optoelectronic repeaters and optical amplifiers to strengthen the signals at various points of the transmission; and 4) an optical receiver that receives the light pulses and convert them into a useable electrical signal.

A simple optical transmitter may just employ a light-emitting diode (LED) as its light source, which is just a p-n junction that emits light when it is forward-biased. LED's used for communications are usually fabricated from gallium arsenide (GaAs) or gallium arsenide phosphide (GaAsP). Being low-cost, LED's are an inexpensive solution for simple fiber-optic communication applications. Unfortunately, the emitted light of an LED is incoherent and has a relatively wide spectral width, resulting in high fiber dispersion and lower bit rate-distance product. Thus, LED light transmission over optical fibers is inefficient, limiting their application primarily to local area network communications.

A better optical transmitter for a fiber-optic communication system is one that uses semiconductor lasers as light source, which can deliver a much higher power output. The coherent and highly-directional light output of a semiconductor laser allows it to be coupled into fiber-optics at a much higher efficiency than LED's. Semiconductor lasers also exhibit less chromatic dispersion and can be modulated at higher frequencies, resulting in higher bit-rate-distance product. By the way, semiconductor lasers are also produced by diodes (called laser diodes), which produce light by stimulated emission instead of spontaneous emission as seen in LEDs.

Modulation in optical transmitters is achieved using circuits that control the current flowing through the diode, pulsing the diode 'on' and 'off' at high frequencies to digitally encode the information being transmitted.

An optical fiber basically has three layers: 1) a core; 2) a cladding around the core; and 3) a protective outermost layer known as a buffer. The core and the cladding are often fabricated using very pure and high-quality silica glass (although plastic may also be used), with the cladding designed to have a lower refractive index so that it can guide the light along the core by total internal reflection. Two optical fibers are connected together by splicing (fusion or mechanical), a process that requires microscopic precision to align the cores of the two fibers being connected. Optical fibers are assembled into fiber-optic cables that are easy to deploy and maintain.

Since fiber-optic cables are not perfect media for light transmission, attenuation and distortion of the light signals passing through them occurs. This is why opto-electronic repeaters are used to convert the weakened light pulses into electrical signals, convert them back into light pulses, and then re-transmit them into the fiber-optic lines as a stronger light signal. New systems are now using optical amplifiers to strengthen weak light pulses directly without having to convert them first into electrical signals.

The optical receiver of a fiber-optic communication system typically consists of a photodetector (its main component) and amplifiers/signal processors. The photodetector converts the light signals it receives into electrical signals. Photodetectors are semiconductor devices that generate a flow of current when photons strike their surface. Amplifiers and signal processors are used to condition the electrical signals generated by the photodetectors to make them more useable by later stages of the network system.