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Single-Mode Fibers
In the context of optical fiber technology, a mode is the term for a ray of light. Single-mode fibers are designed to carry a single ray of light. Single-mode fibers are used when information must be transmitted over large distances. Although all fibers lose some data, the data loss over distance in a single mode fiber is much lower. Single-mode fibers can also carrying a higher bandwidth than multi-mode fibers. One drawback of single-mode systems is that equipment to support them is more expensive than that required for multi-mode systems, although the fiber itself is typically cheaper. Single-mode index fibers are typically made from glass (silica).
Multi-Mode Fibers
Multi-mode optical fibers are sometimes called MM fibers. MM systems are cheaper to set up than single-mode systems. They have the disadvantage of lower bandwidth and can only transmit data reliably over a relatively short distance; the bandwidth drops significantly as the distance increases, from around 10 gigabits per second over a distance of 985 feet (approximately 300 meters). to 100 megabits for distances up to 6,562 feet (approximately 2 kilometers). Within these limitations, however, MM cables are reliable enough for backbone systems within buildings. MM cables are typically seen in local area networks (LANs).
The Index of Refraction
Another difference between kinds of optical fiber is the index of refraction. This is the speed at which light travels through a medium. The index of refraction is typically different at the center of the fiber compared to the the outer part of it. Single-mode glass fibers are called step-index, because the change in the refractory index between the center and the cladding is very sharp. Multi-mode fibers can have other indexes.
Mutimode Graded Index Glass and Step Index Glass
In multi-mode graded index glass, the diameter of the fiber is large and the change in refractory index between the center of the fiber and the cladding is gradual. The beam of light travels most easily through the center of the fiber. If any light strays towards the cladding, is is bent back towards the center. This means that less data is lost and the fiber has a lower bandwidth. Multi-mode step-index glass has a very narrow diameter and the change in the refractory index is sharp. It is cheaper to make but has a lower bandwidth.
Multi-Mode Plastic-Clad Silica
Plastic-clad silica fibers consist of silica with a plastic coating. They typically have a larger diameter than other fibers. Cables made from these fibers are cheap and perform acceptably over short distances, although they should not be used where data needs to be transmitted over a longer distance.