Testing Wide Band Multimode Fiber - WBMMF
November 5, 2015 / Seymour Goldstein / General
Multimode fiber (MMF) dominates in local area networks and data centers. Electro-optic converters (e.g. transceivers) used for transmitting and receiving data are less expensive than those used for transmitting over single-mode fiber. Network capacity has increased over the years through the use of parallel transmission – using multiple fibers and multiple transceivers. As the need for capacity continues to increase, the use of multiple fibers and multiple transceivers needed for parallel transmission may increase network cost.
A new MMF, known as wideband multimode fiber (WBMMF), extends capacity of 50 µm fiber such as OM4 by using wavelength division multiplexing (WDM). OM4 is fiber type (e.g. A1a.3) that provides optimized operation at 850 nm using VCSELs. By multiplexing four wavelengths spaced closely together, a single strand of WBMMF can be used to increase data capacity by a factor of four. The transceivers used for WBMMF applications are expected to be VCSEL based, in the 850 nm to 950 nm ranges.
Over the last two years, members of the Telecommunications Industry Association (TIA), a standards organization representing manufacturers and suppliers of communication networks, have developed a draft standard for WBMMF, TIA-492-AAAE. During the recent October balloting phase of the WBMMF standard, a Fluke Networks comment was introduced suggesting that testing installed cabling using WBMMF is undefined and may cause confusion. While the comment was out of scope for the ballot, it did raise the testing issue.
Today, testing installed MMF is normally done using LEDs operating at 850 nm and 1300 nm employing an encircled flux launch. Using the same rationale for testing WBMMF as that used on MMF, testing at the operating wavelength extremes of 850 nm and 950 nm, poses a potential problem - LEDs are not currently manufactured at 950 nm. In this case, testing at only 850 nm seems sensible.
The current opinion of TIA experts is that if a proper launch is used, such as encircled flux, the difference in connection attenuation between different wavelengths will be minimized. It has been demonstrated in other standards bodies that this assertion is true. If there was difference, 850 nm would typically show slightly more attenuation, demonstrating a worst case measurement. Regarding fiber attenuation, again 850 nm suffers the highest attenuation but longer wavelengths such as 1300 nm are more prone to bending losses.
In summary, testing WBMMF at the two wavelength extremes of 850 nm and 1300 nm, provided by existing test equipment, should provide adequate and prudent testing. Testing at additional wavelengths in the WBMMF band is not advised using VCSELs nor is it justified. VCSELs are not permitted for optical attenuation testing according all the testing and structured cabling standards. The spectral widths of VCSELs do not meet the requirements of ANSI/TIA-526-14-C for example.