Multi-mode optical fiber (multimode fiber or MM fiber) is a type of optical fiber mostly used for communication over shorter distances, e.g. 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. It is easier to connect than single-mode optical fiber, but its limit on speed x distance is lower. Multi-mode fiber has a larger core than single-mode fiber, which allows it to support more than one propagation mode, or path within the fiber.

Multimode fibers are useful for carrying larger amounts of power than single mode fibers. In such fibers, mode-filling becomes important, and mode scrambling attempts to fill the fiber to capacity, utilizing all available fiber modes and more uniform energy density. These fibers are used when an intense beam is needed, as in optical pumping, laser welding, cutting, and marking.

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 is 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.

Optical fiber