MMF

Multimode fiber (MMF) is an optical fiber with a core diameter large enough to carry multiple distinct light paths, or modes, simultaneously. The standard core size is 50 micrometers in modern installations, though 62.5 micrometer variants exist in older networks. This contrasts with single-mode fiber (SMF), which has a core of roughly 8 to 10 micrometers and carries only one mode. The wider core of MMF makes it easier to couple light into the fiber, reducing alignment precision requirements and cost at the light source.

MMF comes in two main types defined by refractive index profile: step-index and graded-index. Step-index MMF has an abrupt change at the core boundary and is rarely installed new; it exhibits severe modal dispersion that limits bandwidth over distance. Graded-index MMF, where the refractive index decreases gradually from core center to edge, compresses the arrival times of different modes and is the standard in modern data center and campus networks. OM1 (62.5 micrometer, 220 MHz km) and OM2 (50 micrometer, 500 MHz km) are older standards; OM3 (50 micrometer, laser-optimized, 2000 MHz km) and OM4 (4700 MHz km) are current.

Where MMF fits in real installations

MMF is practical for runs up to 300 meters at 10 Gbps and shorter distances at higher speeds. The typical application is intra-building backbone cabling, server-to-switch connections in data centers, and shorter campus links. A 10 Gigabit Ethernet link over OM3 MMF reaches about 300 meters; the same link over OM4 extends to 400 meters. Single-mode fiber is required for longer distances, metropolitan area networks, and carrier services because its minimal dispersion permits signals to travel many kilometers without regeneration.

Modal dispersion is the primary performance limiter in MMF. Because different modes travel at different velocities through the fiber, they arrive at the receiver at slightly different times, broadening the pulse. This sets a hard limit on how far the signal can travel before the broadened pulses overlap and become unrecoverable. Chromatic dispersion, caused by material and waveguide effects, also degrades MMF performance but is less severe than modal dispersion. Temperature swings and macrobending (bending radius under 10 cm in some cases) degrade signal quality in installed MMF.

Connectors for MMF are typically LC or SC type, with the same physical size as SMF connectors but lower precision requirements due to the larger core. Transceivers for MMF are substantially cheaper than SMF equivalents because the light sources (usually vertical-cavity surface-emitting lasers, or VCSELs, for OM3/OM4) and receivers need less precision. This cost advantage, combined with ease of termination, explains MMF's continued dominance in data center environments despite the growth of SMF in campus and long-haul applications.