Multimode fibre optic cable is optical cable with a 50 µm or 62.5 µm core and 125 µm cladding that carries many modes of light, mainly at 850 nm. It is graded OM1 to OM5, works with low-cost VCSEL transceivers and suits links inside buildings, data-centre rows and short campus routes, typically up to a few hundred metres depending on speed and grade.
What multimode fibre is
A multimode fiber optic cable guides light through a glass core of 50 µm or 62.5 µm, inside the same 125 µm cladding and 250 µm coating used for single-mode fibre. The notation 50/125 or 62.5/125 gives the core and cladding diameters. The core is wide enough for light to travel along many paths, called modes, at the same time.
Those paths have slightly different lengths, so the parts of one light pulse arrive at the far end at slightly different times and the pulse spreads. This is modal dispersion. At high data rates, neighbouring pulses start to overlap and the receiver can no longer tell them apart, which is what limits multimode reach.
Modern multimode fibre reduces the effect with a graded-index core: the refractive index is highest at the centre and falls towards the edge, so light on the longer outer paths travels faster and catches up with light near the centre. How well a fibre does this is expressed as modal bandwidth in MHz·km, and that figure separates the OM grades. For how multimode compares with single-mode on reach and cost, see single-mode vs multimode fibre.
The OM1 to OM5 family
ISO/IEC 11801 names the cabled multimode grades OM1 to OM5. All share the 125 µm cladding, so they use the same connector ferrules, but they differ in core size, modal bandwidth and the optics they were designed for.
| Grade | Core/cladding | Usual jacket colour (TIA-598) | Typical use |
|---|---|---|---|
| OM1 | 62.5/125 µm | Orange | Legacy LED-based links at 100M and 1G, older buildings |
| OM2 | 50/125 µm | Orange | Legacy 1G links, extensions of existing OM2 plant |
| OM3 | 50/125 µm | Aqua | Laser-optimised: 10G to 100G, existing data-centre plant |
| OM4 | 50/125 µm | Aqua, or heather violet | Laser-optimised: the usual default for new multimode links at 10G to 400G |
| OM5 | 50/125 µm | Lime green | Wideband: multi-wavelength shortwave optics between 850 and 953 nm |
An OM3 cable or OM4 cable looks the same as any other 50/125 cable apart from the jacket colour and print, so read the legend rather than relying on colour. The supported distance for each Ethernet speed on each grade is set by IEEE 802.3, and our OM3 vs OM4 vs OM5 guide lists those reaches.
The 850 nm and 1300 nm windows
Multimode systems use two wavelength windows where glass attenuation is low and practical light sources exist.
- 850 nm: the main window today. Almost all current multimode Ethernet and Fibre Channel optics transmit here. TIA-568.3-D allows a maximum cabled attenuation of 3.0 dB/km at 850 nm for OM3, OM4 and OM5, and 3.5 dB/km for OM1 and OM2.
- 1300 nm: attenuation is lower, with a TIA-568 maximum of 1.5 dB/km. It was used by LED-based optics such as 100BASE-FX and is still met in older plant and some industrial equipment. Multimode links are often tested at both wavelengths.
On the short links multimode serves, attenuation is rarely the limit. Bandwidth and the channel's connector loss usually decide whether a link works.
Laser-optimised fibre and VCSEL optics
Early multimode links used LEDs, which fill the whole core with light. OM1 and OM2 were specified for that overfilled launch. Faster optics use vertical-cavity surface-emitting lasers (VCSELs), which launch light into a smaller part of the core. If the fibre's index profile has small defects near the centre, the modes excited by a laser arrive at different times, an effect called differential mode delay.
OM3, OM4 and OM5 are laser-optimised: the index profile is controlled and measured for differential mode delay so the fibre delivers a specified effective modal bandwidth with a VCSEL source. That is what lets an 850 nm VCSEL run 10G and above over tens to hundreds of metres. VCSELs are cheap to make and easy to couple into a 50 µm core, so multimode optics have generally cost less per port than single-mode optics.
Where multimode is used
- Inside buildings: riser and horizontal links between floors and telecom rooms, and links within equipment rooms.
- Data-centre rows: server, top-of-rack, leaf and spine links, often as MPO trunks feeding parallel optics such as 40GBASE-SR4, 100GBASE-SR4 and 400GBASE-SR8. Our guide to 400G and 800G parallel optics cabling covers the fibre counts involved.
- Campus links: short building-to-building runs at 1G and 10G, within the grade's reach. Longer routes are normally single-mode.
- Equipment and control links: industrial controllers, broadcast racks and machines with installed multimode ports.
Multimode makes sense where many short links share a lower optic price. Where links run beyond the multimode limits, or the speed plan moves to single-mode optics, single-mode is the better long-term choice.
Cable constructions and connectors
Multimode fibre is built into the same cable types as single-mode:
- Patch cords: simplex or duplex, commonly 2.0 mm or 3.0 mm, in LSZH or PVC. See multimode patch cords.
- Distribution and breakout cable: tight-buffered fibres for risers and equipment rooms, with a fire rating such as LSZH or OFNR or OFNP for US buildings.
- MPO trunks: 8 to 144 or more fibres terminated in MPO connectors, for data-centre backbones.
- Armoured and indoor-outdoor cable: for routes exposed to crush loads, rodents or moisture.
LC duplex is the standard connector for duplex transceivers and dense panels. SC remains common in older enterprise plant, and ST appears in legacy and industrial installations. MPO-12, MPO-8, MPO-16 and MPO-24 connectors carry parallel optics, where each lane uses its own fibre. Multimode connectors are normally PC or UPC polish, including multimode MPO; angled APC polish is a single-mode practice. Connector housings commonly follow the fibre: beige for OM1 and OM2, aqua for OM3 and OM4, with violet or lime green used by many makers for OM4 and OM5.
Bend-insensitive multimode fibre is available in OM3, OM4 and OM5 grades for crowded trays and cassettes. It mates with standard fibre of the same grade.
Mixing 50 and 62.5 µm fibre
Do not join 62.5 µm and 50 µm fibre in one channel. Light passing from the larger 62.5 µm core into a 50 µm core cannot all be captured, so the joint adds a large loss in that direction, and the two fibres have different bandwidth behaviour. Mating patch cords of the wrong core size is a common cause of failed links in older buildings that carry both.
Mixing laser-optimised 50 µm grades is different. OM3, OM4 and OM5 share the same core and can be connected, but the channel must then be designed to the reach of the lowest grade in it. Single-mode and multimode must never be mated in a working link. The one recognised bridge is the mode-conditioning patch cord used with 1000BASE-LX optics on multimode plant, and it must match the core size of the installed fibre.
What to specify when ordering multimode assemblies
- Fibre grade (OM3, OM4 or OM5, or OM1 and OM2 for legacy extensions), core size, and whether bend-insensitive fibre is required.
- Connector at each end, for example LC duplex to SC, or MPO with fibre count and pin gender.
- Polarity for duplex and MPO assemblies.
- Length, length tolerance and breakout leg lengths.
- Cable construction, outer diameter, jacket material, colour and fire rating for the country of installation.
- Maximum insertion loss per connector, the test wavelengths (850 nm, and 1300 nm if needed) and the test report you require.
- Labelling and jacket marking.
Radix Fibrotech builds multimode patch cords, MPO trunks and breakouts to drawing in Pune. Send the details above with a request for a quote and ask for the loss values that apply to your build.
Questions buyers ask
What is multimode fibre optic cable used for?
Short links inside buildings, data-centre rows and short campus routes. It connects servers, switches and storage, carries MPO trunks for parallel optics such as 100GBASE-SR4, and serves equipment that has multimode ports. Longer routes normally use single-mode fibre.
What is the difference between OM3 and OM4 cable?
Both are laser-optimised 50/125 µm multimode fibres with the same connectors. OM4 has higher effective modal bandwidth at 850 nm, so the same optic runs further on it. OM3 is aqua, and OM4 is aqua or heather violet depending on the market.
Can I connect 62.5 µm and 50 µm multimode patch cords?
It should be avoided. Light passing from a 62.5 µm core into a 50 µm core suffers a large loss at the joint, and the fibres differ in bandwidth. Match the patch cords to the core size of the installed cable, which is printed on the jacket.
What colour is multimode fibre cable?
Under TIA-598, OM1 and OM2 are orange, OM3 and OM4 are aqua, and OM5 is lime green. Heather violet is widely used for OM4, especially in Europe. Outdoor cable is often black whatever the fibre, so read the printed legend.
Which wavelengths does multimode fibre use?
Mainly 850 nm, where current VCSEL-based optics transmit. The 1300 nm window was used by older LED-based optics and is still used for testing. OM5 adds a specification up to 953 nm for optics that send several wavelengths over one fibre.