When it comes to optical transmission, it is easy for discussion about data rates, bandwidth, loss, fiber core size and the distances supported to be misleading. To make the right choice, it pays to know the language. I would like to identify and explain some of the terminology one might use if he or she is a fiber optic infrastructure specialist.
Channel or link loss is the total path loss or attenuation between the transmitter and receiver. It is the sum of various loss mechanisms: scattering, microbending, macrobending and interconnection. These limit the maximum system length and the number of connections allowed.
Scattering causes the intrinsic fiber loss dominated by what is known as Rayleigh scattering, which results from variations in density and compositions of the glass. This loss varies with the wavelengths of light applied.
A microbend is a local deflection of the optical fiber axis, with amplitude much less than the optical fiber diameter. Microbends can cause a light ray to strike the fiber core-cladding interface at an extensive angle allowing it to escape and increase loss. A macrobend is a bend or loop in the fiber with a radius of curvature of several millimeters or more, causing power loss from the core and inducing additional loss.
Interconnection loss associated with splices and connectors can be divided into intrinsic and extrinsic loss. Intrinsic mechanisms are a result of manufacturing tolerances on fiber core diameter, ovality, eccentricity and numerical aperture. Extrinsic mechanisms depend on the connection hardware and its ability to control separation between the fiber ends. Transverse, axial and angular misalignment of the fiber cores result in connector insertion loss.
Within optical fiber, bits of data are represented by pulses of light. Each pulse of light will spread, or disperse, over time as it travels along the length of the fiber. When the spreading pulses overlap, Intersymbol Interference (ISI) can result. The less ISI caused by dispersion, the greater the fiber’s capacity to transmit information. There are two main types of dispersion in optical fiber in the LAN environment: chromatic dispersion and modal dispersion.
Chromatic dispersion describes the tendency for different wavelengths to travel at different speeds in a fiber. If operated at wavelengths where chromatic dispersion is high, optical pulses tend to broaden as a function of time or distance and cause intersymbol interference. Although multimode fiber exhibits relatively high chromatic dispersion at the 850 nm wavelength, the use of controlled launch lasers vertical cavity surface emitting lasers (VCSEL) in gigabit networks and the distances in the LAN minimize the effects.
In multimode fiber systems, the majority of the dispersion is caused by modal dispersion. Modal dispersion exists because the different light rays (modes) have a different optical path length along the fiber, therefore rays entering at the same time will not arrive at the far end of the fiber at the same time. Light travels faster in the low-index regions near the cladding and slower in the high-index regions near the center of the core. This dispersion effect limits the bandwidth as shown in the illustrations.
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