An old fiber type – 50 µm multimode – is making a comeback. Why? To meet increasing network speeds! This high bandwidth fiber makes it ideal for Gb/s and 10 Gb/s networks. 50 µm multimode fiber offers three times the bandwidth of standard 62.5 µm fiber at 850nm; 500 MHz-km vs. 160 MHz-km. The bandwidth advantage of 50 µm multimode fiber comes from its smaller core size. Modal dispersion is the principal bandwidth-limiting factor in multimode fiber. The smaller 50 µm core allows fewer modes of light to travel down the fiber, resulting in less modal dispersion. The inherently lower modal dispersion in 50 µm fiber results in higher bandwidth.
The high bandwidth of 50 µm fiber enables higher speeds and longer link lengths. Using standard 62.5 µm multimode fiber in a Gigabit Ethernet system operating at 850nm limits link lengths to 275 meters, while for 50 µm fiber the distance is 550 meters.

The bandwidth advantage of 50 µm fiber is the key reason why the Optical Internetworking Forum (OIF) is including only 50 mm multimode fiber in their standard. The distance recommendation for new high-performance laser-optimized 50 µm multimode fiber is 300 meters at 10 Gbps.

Didn’t We Use 50 µm 20 Years Ago?
Many of us have the feeling of déjà vu. Although 62.5 µm multimode fiber is the North American standard, the first optical fiber commonly installed was the 50 µm variety, developed by Corning in 1976. For several reasons, however, larger core multimode fibers were preferred, which led to the development and introduction of 62.5 µm multimode fibers (and several others - 100/140 µm and 85/125 µm - that never achieved prominence in North America).
Until recently, multimode transceiver technology used the light-emitting diodes (LED´s). While inexpensive, LED´s require a relatively large numerical aperture (NA). 50 µm fiber had a lower NA than 62.5 µm fiber, 0.20 and 0.27, respectively. Corning and others developed 62.5 µm multimode fiber, which offered ample bandwidth and the larger NA. A large numerical aperture was also preferable for other reasons at that time: the transceiver output power was lower, and connector polishing and alignment were not as advanced as they are today. As a result, 62.5 µm fiber and LED´s were adopted in high data rate multimode fiber systems. Even though 62.5 µm fibers were the predominant fiber type used, 50 µm fibers have always been recognized as an alternative.
Interestingly, several other companies, notably Japan and Germany, stayed with 50 µm multimode fibers and never adopted 62.5 µm as a standard.

Why Now Change to 50 µm Multimode Fiber?
Advances in the components used in multimode fiber systems have removed the obstacles blocking the adoption of 50 µm fiber. Numerical aperture is no longer a critical concern when using LED´s, because of improvements in output power and fiber alignment. Connectors have also been enhanced. Today´s LED´s and connectors perform equally well with both 50 µm and 62.5 µm fibers.

The most important factor, however, has been the development of low-cost 850 nm lasers, such as vertical cavity surface emitting lasers (VCSEL´s). VCSEL´s provide multi-gigabit speeds at lower cost than Fabry-Perot lasers, which are commonly used with single-mode fiber and operate at 1300 nm. Comparatively inexpensive light sources, VCSEL´s are ideal for use with multimode fiber when optimized for the shorter wavelength, and 50 µm fiber has much better performance with these sources than 62.5 µm fiber.
Ultimately, high performance 50/125 µm fibers will be less expensive than 62.5/125 µm multimode fibers, since the core size is much smaller. Today, standard performance 50/125 µm fibers are less expensive than 62.5/125 µm designs. For 10 Gb/s systems, only the higher performance 50/125 fiber can be used and they are more expensive than 62.5/125 fibers.

Conclusion

With its inherently higher bandwidth and longer reach compared to that of 62.5 µm fiber, 50 µm fiber is the appropriate choice for new builds and existing plant upgrades. As greater information-carrying capacity becomes ever more critical, 50 µm multimode fiber will continue to gain wide acceptance as the cost-effective solution for premises networks.