Choosing the Right Multimode Fiber for Data Communications

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The Evolution of Multimode Fiber Designs
While 50 µm multimode fiber might seem new, in the 1970s, when optical fiber
was introduced, standard 50 µm fiber (OM2) was the most popular of the early fiber
types available, and was used for both long haul and short reach applications.
After the introduction of single-mode fiber in the 1980s for long haul telephony
applications, multimode fibers were applied to short-reach interconnects, such as building
and campus backbones needing support over distances from 300 meters to 2000 meters.
As with many technology choices, there were trade offs between 50 µm multimode and
the singlemode fiber systems. The light-emitting diode (LED) sources used for
multimode applications had a very large “spot” and the 50 µm fiber did not fully couple
the available power into the 50 µm core. Consequently, 50 µm fiber used with 850 nm
wavelength LEDs was limited in distance. Receivers were not always able to detect an
adequate light power at the distant end of the backbone. Still, network designers were
reluctant to install single mode fibers as the power sources remained more expensive and
there was no need in most premises applications for the long link lengths the more
expensive singlemode facilities would provide.
62.5 µm multimode fiber (OM1) was introduced in 1985 to solve these two
problems. Because more light from LEDs could be coupled into its larger core, OM1
fiber could support 2 km campuses at 10 Mb/s. At the same time, its higher numerical
aperture, which can be thought of as the fiber’s “light gathering” ability, made it easier to
cable. Through much of the 1980’s and 90’s one multimode fiber, 62.5 µm core FDDI
fiber became a defacto standard among the vast majority of LAN installations. Despite
continual upgrades in LAN bandwidth requirements, FDDI grade fiber remained a