Multimode fiber is a type of fiber optic cable with a relatively large core, typically 50 or 62.5 micrometers in diameter, that allows light to travel along multiple paths simultaneously. It’s the dominant cabling choice inside buildings, data centers, and campus networks where distances stay under about 550 meters. Compared to single-mode fiber, which uses a tiny core and a single light path for long-haul connections, multimode fiber pairs with less expensive light sources and transceivers, making it the cost-effective option for shorter runs.
How Light Travels Through Multimode Fiber
Inside a multimode fiber, light doesn’t follow a single, straight path. The core is wide enough that light enters at many different angles, creating dozens or even hundreds of distinct paths called “modes.” Each mode bounces through the core at a slightly different angle and travels a slightly different total distance to reach the other end. A mode that zigzags sharply across the core takes a longer path than one that travels nearly straight down the center.
This difference in path length creates a problem called modal dispersion. Because each mode arrives at the far end at a slightly different time, a sharp light pulse sent into one end of the cable spreads out into a broader, softer pulse by the time it reaches the other end. The longer the cable, the worse this spreading gets. Modal dispersion is the fundamental reason multimode fiber has shorter distance limits than single-mode fiber: at some point the pulses blur together and the receiver can no longer distinguish one bit of data from the next.
To reduce modal dispersion, modern multimode fibers use a graded-index core. Instead of a uniform glass core with a sharp boundary, the refractive index gradually decreases from the center outward. This causes light traveling at steeper angles to speed up near the edges, partially compensating for the longer path. The result is tighter pulse arrival times and significantly higher bandwidth than older step-index designs.
The Five OM Grades
Multimode fiber is classified into five standard grades, labeled OM1 through OM5. The grades reflect increasing bandwidth capacity, which directly determines how fast and how far data can travel.
- OM1 uses a 62.5-micrometer core and offers bandwidth above 200 MHz·km at 850 nm. It was designed for LED light sources and supports 10 Gigabit Ethernet only to about 33 meters. It cannot support 40G or 100G Ethernet at all.
- OM2 also uses a 62.5-micrometer core but with improved bandwidth above 500 MHz·km. It extends 10G Ethernet to around 150 meters, though it’s still locked out of 40G and 100G standards.
- OM3 was the first grade designed specifically for laser sources rather than LEDs, using a 50-micrometer core with bandwidth above 1,500 MHz·km. It supports 10G Ethernet to 300 meters, and 40G or 100G Ethernet to 100 meters.
- OM4 pushes the 50-micrometer core further, with bandwidth above 3,500 MHz·km. It handles 10G Ethernet out to 550 meters and extends 40G and 100G Ethernet to 150 meters.
- OM5 matches OM4’s bandwidth at 850 nm but adds optimized performance across a wider wavelength range, from 850 nm up to 940 nm. This makes it suitable for a newer transmission technique that sends data on multiple wavelengths simultaneously through a single fiber pair.
OM1 and OM2 are considered legacy grades. New installations almost always use OM3, OM4, or OM5.
Light Sources and Why They Matter
Early multimode systems relied on LEDs as their light source. LEDs are inexpensive and simple, but they inject light across the entire core at many angles, exciting a large number of modes and producing significant dispersion. This limited both speed and distance.
In the late 1990s, a type of semiconductor laser called a VCSEL (vertical-cavity surface-emitting laser) changed the picture. VCSELs can be modulated at much higher rates than LEDs while remaining relatively cheap to manufacture. They also launch light into a more controlled set of modes, reducing dispersion. The introduction of VCSELs drove the development of OM3 and later grades, which were engineered to take full advantage of this tighter, more focused light source. Today, every standards-compliant multimode transceiver from 1G up through 400G Ethernet uses VCSELs operating at 850 nm.
Because VCSELs launch light differently than LEDs, the way fiber bandwidth is measured changed too. Older fibers were rated using a measurement called overfilled launch bandwidth, which mimics the broad light spread of an LED. Modern OM3, OM4, and OM5 fibers are rated using effective modal bandwidth, a metric that better reflects how a VCSEL’s focused beam actually interacts with the fiber.
Distance Limits by Speed
The practical reach of multimode fiber depends on both the fiber grade and the data rate. Higher speeds require cleaner signals, which means shorter maximum distances. Here’s how the most common Ethernet speeds break down:
At 10 Gigabit Ethernet, OM3 reaches 300 meters and OM4 reaches 550 meters. Those distances cover the vast majority of runs inside a building or across a campus. At 40 Gigabit and 100 Gigabit Ethernet, the limits tighten considerably: OM3 supports 100 meters and OM4 supports 150 meters. Those shorter distances still work well within a single data center floor, which is where most 40G and 100G connections live.
If your cable runs exceed these limits, the options are to use single-mode fiber (which can reach tens of kilometers) or to place networking equipment at intermediate points to regenerate the signal.
Common Connectors
The LC connector is the standard choice for most modern multimode installations. It’s a small-form-factor design that fits into high-density patch panels, making it well suited for data centers and telecom rooms where port count per rack unit matters.
For 40G and 100G links that use parallel optics (sending data across multiple fiber strands at once), MPO/MTP connectors are the norm. A single MPO connector can terminate 12, 24, or even 36 fibers in one plug, replacing a bundle of individual LC connections with a single click-in connector. This dramatically simplifies cabling in environments running thousands of high-speed links.
Older connector types like SC and ST still show up in legacy installations, but they’re rarely specified for new builds. SC connectors are bulkier than LC, and ST connectors use a twist-lock mechanism that’s fallen out of favor as port density has increased.
OM5 and Wideband Transmission
OM5, standardized around 2016, was designed to support a technique called shortwave wavelength division multiplexing, or SWDM. Traditional multimode systems send all data on a single 850 nm wavelength. SWDM splits traffic across four wavelengths ranging from 850 to 940 nm, all traveling through the same fiber pair. This effectively multiplies the fiber’s capacity without adding more physical strands.
With SWDM, a 40G or 100G connection can run over a simple two-fiber LC duplex link instead of requiring an 8- or 20-fiber MPO connection. That’s a significant reduction in cabling complexity. OM5 fiber is essentially OM4 that has been additionally optimized to maintain high bandwidth across that wider wavelength window. It still meets all OM4 specifications at 850 nm, so it’s backward compatible with existing OM4 transceivers.
Cost Advantage Over Single-Mode
The main reason multimode fiber dominates short-distance networking is cost, and most of that savings comes from the electronics rather than the cable itself. Single-mode fiber uses precision laser sources that must emit light at a very specific, narrow wavelength and align with a core just 8 to 9 micrometers wide. Multimode transceivers use VCSELs that are cheaper to produce and easier to couple with the larger 50-micrometer core.
The fiber cable itself is only slightly less expensive for multimode than single-mode. But when you multiply the transceiver savings across hundreds or thousands of switch ports in a data center, the total cost difference is substantial. That economic reality keeps multimode firmly in place for in-building and short-reach data center connections, while single-mode handles the longer links between buildings, campuses, and cities.