Multimode fiber: the Fact File

Optical fibers are based on the principle of total internal reflection: The light that travels inside the fiber core is reflected back into a core when it reaches the boundary between a cladding and a core.

The light signals propagate through the core of the optical fiber, but the way they propagate differs depending on the fiber type.

The fiber type name is self-explanatory; the propagation path is single (singlemode fiber) or multiple (multimode fiber).

Note: The reality is that singlemode fiber actually transports eight to 10 modes, which, in order of magnitude, is considered “single” compared to the 1,300 modes a multimode fiber can transport. The mode in which light travels depends on:

• Geometry
• The index profile of the fiber
• The wavelength of the light

The geometry factor is heavily dependent on the size of the core:

This graphic should help you visualize the physical differences between the fiber types. All have 125-micron outer diameters (the cladding dimension). They differ in the size and construction of their cores. The larger diameter core of 62.5 and its larger numerical aperture (NA) make it the best choice for LED-based systems, because it gathers more light from the larger emission pattern of typical LEDs. A 50-micron size is less efficient for LEDs, but the narrower emission pattern of VCSELs (laser diode) couples equally well into both 50 and 62.5 micron. The lower light gathering ability of 50 micron means it carries fewer modes and, as a result, has higher bandwidth overall than 62.5. This is taken to the extreme with singlemode fiber.

When should we choose one type over the other? This depends, as always, on your requirements.

Over the course of natural technological evolution, five different fiber classes have been developed. As we’ve seen in the history section, the first multimode fiber had a 62.5-micron core (OM1). It was used for many years, along with the more capable 50-micron version (OM2). Those two types are now rarely used.

The OMx nomenclature comes from the ISO 11801 standard and ranges from OM1 to OM5. We will now describe each and compare their construction and supported distances and applications.

OM1

Designed originally for all multimode applications using LED transmitters, OM1 is based on a 62.5-micron core. Indoor cables are identifiable by the traditional orange color jacket. OM1 just supports 10G up to 33 m (10GBASE-SR). In the context of enterprises today, many consider OM1 usable just for expansions or repairs of old installations that require low-bandwidth applications.

OM2

Quite similar to OM1, but with a 50-micron core. Supports 10G up to 82 m given its slightly higher bandwidth.

OM4

As an evolution of the OM3 specifications, the 50-micron OM4 fiber type has gained substantial popularity. OM4—with more than double the effective bandwidth at 850 nm—introduced extended reach for existing applications (gigabit and multigigabit applications) and enabled future applications. It’s worth noting that CommScope’s LazrSPEED 550 was the precursor to this standard.

Backwards compatible with OM3, it supports 10G up to 550 m. The jacket color is also aqua. See the supported distances chart in the next section to get specs for other applications.

OM5

To learn more about the history of MMF and better understand the implications of the OM5 innovation, check out this white paper.

Market evolution
How has the market reacted to the tech evolution? See the chart below. Admittedly, the more capable the fiber type, the higher the price—so that plays a role in the market.

For tips you may find useful when designing your network, you can consult this design guide.

DMD (differential mode delay) describes the difference in delay time between the latest and earliest arriving pulses in multimode fiber. This “pulse spreading” limits bandwidth and is the main reason conventional OM1/OM2 multimode fiber cannot properly support 10 Gbps.

In multimode fiber the dispersion is caused by modal and chromatic dispersion. Modal dispersion exists because the different light rays (modes) have a different path length; therefore, rays entering at the same time will not leave the other end of the fiber at the same time. The modal bandwidth is controlled by the pulse broadening due to the differences in many propagating modes (300 to 900).

The modern multimode fibers have a structure that reduces the effect of the modal dispersion. These fibers, called “graded index fibers,” are used in all SYSTIMAX® SCS multimode fiber products.

With multimode fiber transmission, distances of up to 2 km are possible; however, as speeds increase, the distance support is reduced. Adding connectors (which reduce the signal power) also limits the distance that applications can support. Higher bandwidth and better connectivity provide optimum performance from MMF systems.

For singlemode systems, the glass core diameter has been reduced to a size where only one path (mode) can propagate through the fiber. The modal dispersion is no longer present. However, dispersion still exists in a singlemode fiber. Different wavelengths travel at different speeds in glass. These wavelength differences give rise to “chromatic dispersion.” Chromatic dispersion is related to the light source, which normally has a range of wavelengths.

SC and FC connectors were once the kings of the fiber connectivity realm, but have been unable to keep up with increasing requirements for density, performance, scalability, and ease of deployment.

The industry instead adopted LC and MPO connectors and has used them for some time—and for very good reasons. Get a clearer perspective of the advantages of LC and MPO connectors in the video below. Connectors will be covered in more detail as a separate main topic on this page.

On top of what you have read in the previous sections, we need to have a look into what the future may bring. Some data centers are looking to make the jump to 400G. A key consideration is which optical technology is best. The optical market for 400G is being driven by cost and performance as OEMs try to dial in to the data centers’ sweet spot. You can find more information here.

One other aspect to consider is the number of fiber cores that will be needed in the future to support high-demand applications. Even using the multiplexing capability of OM5, some applications like 400G or 800G will demand multiple fiber cores per link—and a DC has a huge number of links. So higher fiber counts may become a must in DCs. Learn more about this in our expert’s article.

The need for data and power across fiber

Network devices are popping up everywhere in buildings and across campuses—devices like small cell sites, Wi-Fi access points, IP cameras, building access and inventory controls, and countless others.

While these new applications improve user connectivity, operational technology efficiency and property-wide safety and security, they also present you with a new and growing challenge: getting high-bandwidth data and power connectivity to every device in every location, indoors and out, with the low-latency performance needed to leverage edge network architecture.

A powered fiber solution makes it relatively easy to meet those challenges in a single, easy-to-handle cable run, with options suited to weatherized outdoor deployments or plenum-rated indoor installations.

A powered fiber solution combines high-performance, low-latency fiber-optic data connectivity with a copper low-voltage dc power connection. This helps enable the connection of many powered remote devices without the need for new conduit, bulky extra cable runs or expensive electricians. With the powered fiber cable solution, your network can gain access to a vast and growing ecosystem of network-connected applications, including:

As a medium for transmitting data, fiber is more powerful than copper. However, installation is more complex as a result, and skilled expertise is required. CommScope’s PartnerPRO® Network may be a useful tool on your path toward certification and expertise.

If you or your team are looking to learn more formally about multimode fiber, we have two courses that enable students to learn at their own pace. Both are available online in multiple languages.