The Lightwave Guest Blog

What’s happening with bend-insensitive multimode fiber?

By David Mazzarese

Multimode fiber is replacing copper as the connectivity option of choice in high-end data centers. With 10 Gbps as the preferred method of transmission, OM3 and OM4 multimode fibers have proven to be the most cost-effective way to transmit high data rates over the 10 to 550 meters typically seen in data centers. Meanwhile, data center links have begun to migrate from 10 Gbps to 100 Gbps as 10-Gbps server ports have become available.

It has become apparent that meeting these challenges will require parallel data transmission using multiple fibers for the 100-Gbps data stream. The recent IEEE 802.3ba Ethernet standard calls for 10 multimode fibers transmitting 10-Gbps each to reach 100 Gbps. The standard interface is a 24-fiber or two 12-fiber MPO connectors, so a 100-Gbps link typically requires 24-fiber cable.

TIA 42.12, IEC SC86A and ISO/IEC SC25 have recently developed OM4 multimode fiber standards to support these new applications. Market acceptance of OM4 fiber has been explosive since the adoption of IEEE 802.3, which included OM4 fiber as one of the approved media types.

What will follow OM4? One candidate is bend-insensitive multimode fiber (BI-MMF). These products were first introduced as a way to improve cable management in large data centers. Discussions about BI-MMF in standards groups began in April 2010. Since then, it has become apparent that the specifications for standard OM3 and OM4 fiber are not sufficient for these new bend-insensitive multimode fiber designs. Studies show that several “leaky” mode groups propagate up to several hundred meters in BI-MMF and need to be accounted for in fiber standards.

A TIA TR 42.12 task force led by OFS has been assigned with understanding these new fiber types. Its work includes clarifying the numerical aperture, core diameter, and bandwidth of BI-MMFs. Work to date has shown that fiber profiles vary significantly from manufacturer to manufacturer, unlike standard multimode fiber. These profiles are all very different from the embedded base of standard multimode fiber. The interaction of all these different designs is not well understood at this time and is being evaluated by the task force.

All this uncertainty makes it likely that many bend-insensitive multimode fibers being installed today may have bandwidth and connection losses that vary significantly from the standard multimode fibers in the embedded base. Until the fundamental transmission properties can be agreed upon and the interaction of these fibers with the embedded base is well understood, we at OFS believe it would be wise to hold off on deploying this new class of optical fibers.

David Mazzarese is fiber systems engineering manager at OFS.

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