MMF standards satisfy high-speed networks

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Beginning in the late 1990s, both the Telecommunications Industry Association’s FO-4 Engineering Committee and International Electrotechnical Commission’s Technical Committee 86 on Fiber Optics started the process of updating their multimode-fiber (MMF) standards. That was motivated by high-speed system developments in IEEE 802.3, the International Telecommunication Union (ITU-T), and InterNational Committee for Information Technology Standards (INCITS) T11.2 requiring advanced laser-based transceivers. Today, a comprehensive and harmonized set of MMF test methods and specifications exists to support current application-network standards as well as anticipated future ­upgrades.

A new 50-µm laser-optimized fiber capable of supporting multigigabit applications is described in TIA 492AAAC and IEC 60793-2-10 and specifically designed to support operations using cost-effective 850-nm VCSELs.

MMF had always been characterized for performance using an overfilled launch (OFL) bandwidth metric that was appropriate for systems using LEDs. However, the OFL bandwidth technique could not predict performance with lasers. That led to the development of test methods to more accurately predict MMF performance with lasers.

Restricted mode launch (RML) was added to the bandwidth test method in TIA FOTP-204 and IEC 60793-1-41, and new procedures were developed to measure differential mode delay (DMD) in TIA FOTP-220 and IEC 60793-1-49. The DMD measurement data can be used in two ways:

• Provide a simple pass/fail metric in the form of a DMD mask targeted to performance at 10 Gbits/sec over 300 m.

• Provide a calculated effective modal bandwidth value (EMBc) that accounts for light propagation in the fiber when combined with a defined laser source. EMBc represents a scalable bandwidth metric that can be applied to any bit rate or link length of interest.

MMF that might eventually support gigabit speeds should be measured for laser-based performance using the new industry standards developed to ensure compatibility with these high-speed networks. Th 192112

Steve Swanson is responsible for standardization strategy at Corning and is chair of TIA FO-4. He can be reached at swansonse@corning.com.

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