Multimode fiber
termStands for: Multimode optical fiber (OM1-OM5)
The short-reach fiber of buildings and data centers - a wide core, cheaper optics, distances of hundreds of meters.
Multimode's 50-micron core (62.5 in legacy OM1) carries many light paths at once; their spread (modal dispersion) is what caps distance as speeds rise. Grades OM1 through OM5 mark the progression - OM3 and OM4 (aqua) are the laser-optimized workhorses for 10-100 Gb/s inside facilities, OM5 (lime) adds wideband support for multi-wavelength short reach. The trade against single-mode is classic: cheaper transceivers, shorter reach.
The Fiber Optic Association gives the core sizes, 'almost always 50 or 62.5 microns', inside a 125-micron cladding. A core that wide admits many modes, paths of slightly different lengths, so a pulse spreads as it travels, and the faster the signal the shorter the distance before pulses blur. The grades track the fight against that spreading: one Cisco 10GBASE-SR module reaches 33 m on OM1, 82 m on OM2, 300 m on OM3 and 400 m on OM4, all at 850 nm.
Above 10 Gb/s, speed came from parallel fibres as much as from faster light: 100GBASE-SR4 uses four lanes each way on an MPO connector, and 400G SR8 uses eight pairs, both to about 100 m on OM4. OM5 is, in Fluke Networks' words, 'a version of the OM4 fiber with additional bandwidth characterization at 953 nm', for short-reach wavelength multiplexing.
The jacket says the grade: orange for OM1 and OM2, aqua for OM3 and OM4 (violet for OM4 in much of Europe), lime green for OM5.
Also known as: multimode, mmf, om3, om4, om5