Fiber whose large core supports hundreds of modes, so cheap LED and VCSEL sources at 850 nm and 1300 nm can couple light into it easily. The modes travel different path lengths and arrive at slightly different times, and this modal dispersion caps bandwidth and reach: a 10G link runs about 300 m on OM3 and 400 m on OM4. Graded-index profiles curve the rays back toward the center and let the longer outer paths run faster through lower-index glass, which shrinks that spread.
Those runs between the core switches are only 80 meters, so multimode with SR optics is fine and the transceivers are half the price.
Never patch multimode into singlemode or the reverse: the loss on the large-to-small direction is typically well over 10 dB and the link simply will not come up.
Also heard as
- MM
- MMF
- multi-mode
Do not mix it up with
- Singlemode fiber: Singlemode carries one mode through a 9 µm core and reaches tens of kilometers, while multimode trades reach for cheaper optics.
Related terms
OM grades
The OM1 to OM5 performance classes for multimode fiber, each rated for bandwidth and reach at a given data rate.
Singlemode fiber
Optical fiber with a core so small that light travels in a single path, used for long-haul, metro, FTTH and campus backbones.
Numerical aperture
A measure of how wide a cone of light a fiber can accept and guide, written as a number such as 0.20.
Mandrel wrap
Multimode test jumper wound around a smooth rod to strip out high-order modes and stabilize loss readings.
Core
The central glass region of an optical fiber where the light actually travels.