The Telecommunications Industry Association (TIA) recently published a standard for measuring installed multimode fiber-optic cabling; the standard uses the encircled flux metric for the launch condition on such tests.
TIA-526-14-B Optical Power Loss Measurements of Installed Multimode Fiber Cable Plant was "back-adopted" from part of an International Electrotechnical Commission (IEC) standard. That specification is titled IEC 61280-4-1 edition 2, Fibre-Optic Communications Subsystems Test Procedure Part 4-1: Installed Cable Plant - Multimode Attenuation Measurement. The IEC spec is where encircled flux was initially defined and referenced for multimode fiber testing.
"This is a very important document that has come out of TIA," said Bob Jensen, chair of TIA's Engineering Committee TR-42 Telecommunications Cabling Systems. "A lot of work has been done on encircled flux."
The TIA's "back-adoption," as it is called, of the IEC spec means there is complete harmonization between the two. The TIA document has a foreword, which includes additional information to that in the IEC standard.
The technical detail behind the need for the encircled-flux launch condition is complex to put it mildly. But it has implications that have directly affected many fiber-optic system installers on the jobsite. Seymour Goldstein, principal engineer with Fluke Networks' media test business unit, explained it this way in an article published in 2010. "Imagine this scenario. An installation team installs a fiber plant, tests each fiber, and delivers the results to the building owner showing that all fibers have passed a certification test for fiber link loss. For a typical fiber loss, the installation technician measures 2.0 dB using a light source and power meter. The building owner then decides to spot check a few fibers using his own equipment, which includes a different light source. He gets a different result. He may measure 3.0 dB for the same link that measured 2.0 dB for the installation crew. Which light source is right? It depends on several factors."
One such factor is the light source's launch conditions. The specification of encircled flux into IEC 61280-4, and its adoption into TIA-526-14-B, are meant to eliminate the variability of light-source launch conditions.
The following three paragraphs are taken directly from the standard's scope.
This part of IEC 61280-4 is applicable to the measurement of attenuation of installed fiberoptic cabling using multimode fiber, typically in lengths of up to 2,000 m. This cabling can include multimode fibres, connectors, adapters and splices.
Cabling design standards such as ISO/IEC 11801, ISO/IEC 24702 and ISO/IEC 24764 contain specifications for this type of cabling. ISO/IEC 14763-3, which supports these design standards, makes reference to the test methods of this standard.
In this standard, the fibre types that are addressed include category A1a (50/125 m) and A1b (62,5/125 m) multimode fibres, as specified in IEC 60793-2-10. The attenuation measurements of the other multimode categories can be made, using the approaches of this standard, but the source conditions for the other categories have not been defined.