Multi-mode optical fiber is a type of optical fiber mostly used for communication over short distances, such as within a building or on a campus. Multi-mode links can be used for data rates up to 800 Gbit/s. Multi-mode fiber has a fairly large core diameter that enables multiple light modes to be propagated and limits the maximum length of a transmission link because of modal dispersion. The standard G.651.1 defines the most widely used forms of multi-mode optical fiber.

The equipment used for communications over multi-mode optical fiber is less expensive than that for single-mode optical fiber. Typical transmission speed and distance limits are 100 Mbit/s for distances up to 2 km (100BASE-FX), 1 Gbit/s up to 1000 m, and 10 Gbit/s up to 550 m.

Because of its high capacity and reliability, multi-mode optical fiber generally is used for backbone applications in buildings. An increasing number of users are taking the benefits of fiber closer to the user by running fiber to the desktop or to the zone. Standards-compliant architectures such as Centralized Cabling and fiber to the telecom enclosure offer users the ability to leverage the distance capabilities of fiber by centralizing electronics in telecommunications rooms, rather than having active electronics on each floor.

Multi-mode fiber is used for transporting light signals to and from miniature fiber optic spectroscopy equipment (spectrometers, sources, and sampling accessories) and was instrumental in the development of the first portable spectrometer.

Multi-mode fiber is also used when high optical powers are to be carried through an optical fiber, such as in laser welding.

Multi-mode optical fiber features a larger core diameter (typically 50–100 μm), allowing multiple light modes to propagate simultaneously. This design simplifies alignment and installation, making MMF cost-effective and ideal for short- to medium-distance data transmission in enterprise networks, data centers, and campus environments. MMF supports high data rates—up to 100 Gbps—over distances typically ranging from 300 to 550 meters, depending on fiber type (OM3, OM4, OM5). Additionally, MMF can utilize lower-cost light sources such as light-emitting diodes (LEDs) and vertical-cavity surface-emitting lasers (VCSELs), reducing overall system cost while maintaining reliable performance.

LEDs and VCSELs operate at the 850 nm and 1300 nm wavelength, whereas single-mode fibers used in telecommunications typically operate at 1310 or 1550 nm. However, compared to single-mode fibers, the multi-mode fiber bandwidth–distance product limit is lower. Because multi-mode fiber has a larger core size than single-mode fiber, it supports more than one propagation mode; hence it is limited by modal dispersion, while single mode is not.

The LED light sources sometimes used with multi-mode fiber produce a range of wavelengths and these each propagate at different speeds. This chromatic dispersion is another limit to the useful length for multi-mode fiber optic cable. In contrast, the lasers used to drive single-mode fibers produce coherent light of a single wavelength. Because of the modal dispersion, multi-mode fiber has higher pulse spreading rates than single mode fiber, limiting multi-mode fiber's information transmission capacity.