Basics Optical Fiber Communication Systems

Optical Fiber System Components

The simplest optical fiber communication systems have three main subsystems. These are:

• Optical Transponder
• Optical Fiber Cable
• Optical Repeater or Amplifier

These are discussed in sections below:

Optical Transponder

An optical transponder includes a single mode laser, a modulator, a photodetector and a demodulator. The laser is a small device made from semiconductor material which emits a spectrally pure optical signal in the infrared band. Lasers are typical made from Gallium Arsenide or Indium Gallium Arsenide / Indium Gallium Arsenide Phosphide semiconductor alloys. The laser wavelengths are around 850nm for short connections (typically within buildings); 1310 nm and 1550 nm waveeengths are used for longer distance communication links, and links where the data rate is above 1 Gbit/s.

Optical Fiber Cable

Optical fibers are made from very high grade silica glass, which has been altered to change its optical properties. There are two main types of optical fiber. Single mode fiber and multimode fiber. Single mode fiber is used where the data rate is greater than 1 Gbit/s, or where distances are greater than a few hundred meters. Multimode cable is used for short connections within a building, for data rates up to 1 Gbit/s. Multimode cable is slightly cheaper than single mode cable, and is a little easier to work with.

Modal dispersion: Multimode fiber has a wider core than single mode cable (50 to 120 microns versus 5 to 15 microns). The light travelling within the multimode cable core can take many paths (modes); each path has a slightly different path length. Therefore after a few tens of meters, different parts of the signal will arrive at the end of the cable at slightly different times. This effect becomes important at high data rates, as the optical pulses within a signal are very close together. Spreading of the pulses can cause 1's and O's in a signal stream to blur together, causing errors.

Single mode fiber has a very narrow core, and only supports one mode (path). Therefore multimode dispersion does not limit the distance and data rate (bandwidth) of a single mode fiber system. Single mode fiber systems are used where the bandwidth x distance > 1 Gbit/s x 500 meters. (This is a very rough rule of thumb). Allows check with your own expert, and with equipment manufacturer's specifications.

Attenuation: Attenuation is a term used for the absorption of the optical signal by the optical fiber. Attenuation is measured in dB per km (or miles). A single mode fiber will typically have an attenuation of less than 0.2 dB per km, at 1310nm or 1550nm wavelengths. However, fusion splices where fiber are carefully welded together, or connectors will also add to the attenuation. If a fiber is tightly bent, light will also may lost, and the fiber may be damaged.

Optical Repeaters and Amplifiers

Over longer distances, the optical signal will become distorted by other dispersion effects (including chromatic and polarization mode dispersion), and become weaken by attenuation. Therefore even single mode fiber systems may need to repeaters or amplifiers every 20 km or so.

Optical repeaters convert the signal back to electrical domain, and clean up the pulses, removing the effects of dispersion. They then retransmit the optical pulses. These repeaters are often 3-R repeaters: they receive, regenerate and retransmit.

Optical amplifiers are devices which allow the optical signal to be boosted without converting it back to electrical domain. While they compensate for loss, they do not correct for dispersion. Also optical amplifiers add background noise, which will degrade the signal to noise ratio by about 6 dB. Despite these issues, optical amplifiers are more convenient and less expensive than 3-R regenerators. And they allow systems to be easily upgraded to support multiple wavelengths.