Multimode fiber (MMF) is optical fiber with a relatively large core, 50 µm or 62.5 µm across, that carries light along many paths at once. Multimode links are typically used for short distances inside buildings, server rooms and data centers, where they pair with short-reach optics. It is graded OM1 through OM5, and how far a given speed will run depends on the grade, the speed and the optics.
At a glance
- A larger core (50 or 62.5 µm) carries many modes of light, which limits reach as speeds rise.
- Grades run from OM1 and OM2 (legacy) through OM3, OM4 and OM5 (laser-optimized 50 µm fiber).
- Typical uses are links within a building, a data center row or a short campus run, not carrier handoffs.
- Distance limits depend on speed and optics: higher speeds over older grades may reach only tens of meters.
- Multimode needs multimode optics and patch cords; mixing it with single-mode is a common turn-up failure.
What problem it solves
Inside a building or data center, most fiber links are short: switch to switch in the same room, or between floors. Multimode fiber was designed for that job. Its larger core is easier to align and works with lower-cost light sources, which historically made multimode optics cheaper than single-mode optics for short runs. Many buildings and data centers have years of multimode cabling in place, so knowing what grade you have decides which optics and speeds you can use without recabling.
How it works
Light enters the core and travels along many paths (modes), bouncing at different angles. Those paths arrive at slightly different times, which spreads each pulse. The faster the signal, the less spreading it can tolerate, so reach shrinks as speed rises. Later grades (OM3 onward) are optimized for laser light sources to reduce that effect.
| Grade | Core size | Typical use | Common jacket color |
|---|---|---|---|
| OM1 | 62.5 µm | Legacy building cabling, 1G and below | Orange |
| OM2 | 50 µm | Legacy building cabling, 1G | Orange |
| OM3 | 50 µm (laser-optimized) | 10G and some higher-speed short-reach links | Aqua |
| OM4 | 50 µm (laser-optimized) | Data center links at 10G to 100G, longer than OM3 | Aqua, or violet in some markets |
| OM5 | 50 µm (wideband) | Short-reach links using several wavelengths per fiber | Lime green |
Colors are a common convention, not a rule; the printed legend on the jacket is the reliable source.
Distance. Reach depends on speed and optics. A grade that comfortably carries 1G a few hundred meters may carry 10G a much shorter distance, and 40G or 100G multimode optics may need OM3 or better and parallel fibers through MPO connectors. Read the optic’s datasheet against your grade and the path’s measured loss.
Optics. Multimode links use short-reach optics, such as 1000BASE-SX or 10GBASE-SR, in an optical transceiver. Higher-speed short-reach optics may use multiple fibers per direction or several wavelengths per fiber.
Connectors. Multimode cables typically end in LC, SC or ST connectors with a UPC polish, or MPO for parallel optics. See fiber optic connector for the types.
When it matters for buyers
- Upgrading switches. Moving from 1G to 10G or faster on existing multimode cabling may work, or may not reach, depending on the grade. Find out what is installed before buying optics.
- Building fit-out. If you are cabling a new office as part of your LAN infrastructure, decide deliberately between multimode and single-mode for the backbone rather than inheriting the contractor’s default.
- Data center and colocation space. Short in-row links often run on multimode. Cross-connects to carriers are usually single-mode, so confirm before ordering patch cords.
- Inherited sites. Older orange-jacketed cabling is often OM1 or OM2, which can limit speeds even if the switches support more.
If you are planning cabling for a new space, our wired and wireless LAN infrastructure solution page covers how we help scope it.
Questions to ask vendors
- Which OM grade is installed (or proposed), and can you show the test results?
- What speeds and optics will this cabling support at the distances we actually have?
- Should any of these runs be single-mode instead, given our plans for the next several years?
- What connector type and polish are used at each patch panel?
- Are there 62.5 µm and 50 µm runs mixed anywhere on the path?
- If we need 40G or 100G later, will the cabling support parallel optics (MPO) or wavelength-multiplexed optics?
How it differs from single-mode fiber
Single-mode fiber has a much smaller core (around 9 µm) that carries one path of light, so it reaches much farther at high speeds and is what carriers typically deliver fiber circuits on. Multimode is typically chosen for short runs where its optics have historically cost less. The two are not interchangeable: fiber, patch cords and optics generally need to match.