Oh, flux. There's more to the story.

Recently in some online postings as well as an article that appeared in the April issue of Cabling Installation & Maintenance magazine, we have brought you information about the recently adopted TIA-526-14-B standard that specifies encircled flux as the launch condition for installed multimode fiber cable. The TIA TR-42 Committee approved TIA-526-14-B in October 2010.

After seeing the article in our April issue, Jim Hayes, founder of The Fiber Optic Association and VDV Works, wrote to me. Within his letter, Hayes noted that the International Organization for Standardization (ISO), which first standardized the encircled-flux launch condition, did so based on research conducted by a single individual. Additionally, the research "only looked at multimode loss up to 2 dB." Hayes added that based on tests he (Hayes) conducted with another individual, at loss values higher than 2 dB, the technology used to achieve the EF launch condition "failed to even make two OTDRs agree, and the results were vastly different depending on the configurations of the cable plant."

Hayes also said that much of the EF analysis conducted when the TIA was considering adopting the EF specifications, was done using simulations of cable plants. "Real world data was scarce," he said. In the meantime, there was poor correlation among labs when international round-robin testing was initially conducted. Another round-robin test is now underway.

In his letter, Hayes noted CI&M's recent coverage of bend-insensitive fiber (pro and con), commending the magazine for "noting that there was still controversy in the marketplace." He added, "I think you should have treated encircled flux with the same skepticism."

The Fiber Optic Association has published and maintains a Reference Guide to Fiber Optics. The guide's section on Modal Effects on Multimode Fiber Loss Measurements includes information on encircled flux and its adoption by the TIA. That section describes EF as "a more precise method of defining mode fill," and "a more sensitive way of defining power." That section of the guide later explains that EF has been incorporated into several multimode testing standards, adding, "It is intended to create a more reproducible modal condition for testing that is similar to the CPR/mandrel wrap method ... Since EF is new (as of 2/2011), testing sources for EF [have] not been demonstrated to be well correlated between labs or manufacturers, so using it is not yet widely accepted. It will probably not be a widely used method before 2012."

You can read that section of the Reference Guide to Fiber Optics here.

This take on encircled flux differs from information we have published in the past, including this article from a year ago. Despite the questions that Hayes raises about the validity of EF across a range of loss values, it remains the specified launch condition in TIA-526-14-B as well as IEC 61280-4-1 edition 2, from which the TIA adopted it. We intend to follow the practical application of encircled flux, and describe its real-world use for testing installed multimode fiber plants.