Encircled Flux (EF) controls how the light is launched into the fiber under test from a multimode source. Failing to specify the launch conditions correctly can result in a measurement uncertainty as high as 60%. Even if you use reference grade test cords and strict referencing procedures, your reported optical loss of a fiber link will depend on how that light was launched into the fiber. With an
overfilled launch, you could end up with a pessimistic result, as the modes are being lost in the cladding at launch. Using a mandrel is supposed to prevent these modes from entering the cladding at the point of test – but is not always effective. Likewise, an underfilled launch results in an optimistic result since many of the modes are not excited. Prior to EF, ANSI/TIA defined the launch condition using a technique called Coupled Power Ratio or CPR. This method turned out to be inadequate and was replaced with EF in 2010. ISO/IEC replaced CPR with Modal Power Distribution or MPD in 2006. Whilst better than CPR, it too was deemed not adequate for measuring low loss multimode fiber optic systems.