Multi-mode optical fiber
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Multi-mode optical fiber (multimode fiber or MM fiber) is a type of optical fiber mostly used for communication over shorter distances, such as within a building. It can carry 1 Gbit/s for typical building distances; the actual maximum data rate (given the right electronics) depends upon the distance. Multi-mode fiber has a higher light-gathering capacity than single-mode optical fiber, making splicing less difficult, but its limit on speed × distance is lower. Because multi-mode fiber has a larger numerical aperture than single-mode fiber, it supports more than one propagation mode, resulting in larger modal dispersion and consequently higher pulse spreading rates, limiting information transmission capacity.
Multimode fibers are more useful for carrying larger amounts of power very short distances than single mode fibers. In such fibers, mode-filling becomes important, and mode scrambling attempts to fill the fiber to capacity, achieving an equilibrium mode distribution that utilizes all available fiber modes and has a more uniform energy density. These fibers are used when an intense beam is needed, as in optical pumping, laser welding, cutting, and marking. For data applications, however, multi-mode fiber's higher attenuation is another factor limiting the length of multimode fiber links (in addition to dispersion).
The earliest fiber optic cables used a technique termed multi-mode transmission. This is where the light signals from the laser are broken up into a number of paths along the length of the fibre and are reflected off the fiber wall. The amount of reflection that occurs dictates the quality of the signal.
The equipment used for communications over multi-mode optical fiber is less expensive than that for single-mode optical fiber. Typical transmission speeds/distances limits are 100 Mbit/s up to 2 km (100BASE-FX), 1 Gbit/s for distances up to 500-600 meters (1000BASE-LX - single-mode and less often multi-mode, 1000BASE-SX - multi-mode), and 10 Gbit/s for distances up to 300 meters (10GBASE-SR).
There are two main types of multimode optical fiber, step index and graded index. Step index fiber has a sharp change in index of refraction between the core and the cladding. Graded index fiber has a more gradual transition between the two. Unfortunately both of these categories suffer from Rayleigh scattering, which means that only wavelengths between 650 and 750 nm can be carried over significant distances.
