OM4 multimode fiber nears widespread standardization
By David Mazzarese
Completion of the OM4 specification for multimode optical fiber cables will have a ripple effect on other standards.
The standardization of next-generation multimode fiber has seen a great deal of activity in 2009. A key milestone was achieved on May 20, when TIA TR42.12 finished comment resolution on the multimode fiber specification for OM4 optical cables; TIA 492 AAAD, “Detail specification for 850-nm laser-optimized, 50-µm core diameter/125-µm cladding diameter class Ia graded-index multimode optical fibers of OM4 performance,” was published in October 2009 to define those specifications. This document helps clarify some of the common terms used to describe laser-optimized multimode fiber as well as the relationship of the differential mode delay (DMD) specification to the bandwidth observed in vertical cavity surface-emitting laser (VCSEL) systems.
The new fiber supports an effective modal bandwidth (EMB) of 4,700 MHz*km at 850 nm when used with 10-Gbps Ethernet-compliant VCSELs. It also supports a 3,500 MHz*km OFL bandwidth at 850 nm, and a 500 MHz*km bandwidth at 1,300 nm, to support legacy applications. This specification is critical, as it supports IEC/ISO SC 25, which has already described a cabled optical fiber with 4,700 MHz*km EMB. The IEC/ISO fiber is intended for use in 850-nm short-reach systems and is backward compatible with OM1, OM2, and OM3 fibers. The standardization of this product has led application groups, including IEEE and Fibre Channel, to incorporate this product into their current standards development efforts, enabling longer transmission distances than OM3 fiber.
Great care was taken to ensure that the same level of scrutiny used to develop the widely adopted OM3 fiber was used in the development of OM4 standards. Each technical attribute was carefully considered to ensure that this product would provide reliable, high-speed 10-to-100-Gbps optical links in installed systems. A goal of both IEC and TIA has been to develop a harmonized specification that will be adopted worldwide. With the published TIA TR42.12 recommendation TIA 492 AAAD, standardization should follow rapidly in IEC 86A WG1.
IEEE 802.3ab is including OM4 fiber in its upcoming 40/100-Gigabit Ethernet standard. Multimode fiber plays a critical role in providing lower-cost interconnects in high-speed data center applications, an important application space for this new standard. OM4 cable will support 125-m links at 40 and 100 Gbps. The 125-m link distance is significant, as more than 90% of data center links are shorter than that. Fibre Channel is also updating its application tables to include extended link distance support using OM4 fiber.
David Mazzarese is the global manager for optical fiber technical marketing for OFS (www.ofsoptics.com), where he coordinates standards efforts for the company. He actively participates in many standards organizations, including TIA, IEC, and ITU-T. He holds a PhD in chemical engineering and an MS in electrical engineering from the University of Massachusetts at Amherst.