Encircled Flux (EF) controls how the light is launched into the fiber under test from a multimode source.
Why is it important to define this?
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. Take a look at three different launch conditions below:
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.
What standards specify this?
The two most common industry standards ANSI/TIA-568.3-D and ISO/IEC 11801:201` defer launch conditions to ANSI/TIA-526-14-C and IEC 61280-4-1 respectively. ANSI/TIA-526-14-C is actually an adaptation of IEC 61280-4-1. The big difference being that EF is only required for 850 nm on 50/125 µm fiber in the C revision with 1300 nm and 62,5/125 µm recommended. IEC 61280-4-1 will continue to require an EF launch for 850 nm and 1300 nm, for all multimode fiber types.
Options for being EF compliant?
TIA-TSB-4979 describes two options. The first is to put a launch conditioner on your existing LED multimode source, often referred to as Method 1. Whilst the launch conditioner is said to work on VCSEL sources, such sources are not permitted in either ANSI/TIA or ISO/IEC standards.
Modal Launch Conditioner (Method 1)
Method 1 has the advantage of not needing to replace your existing test equipment. However, the launch conditioners are considered bulky and expensive. Depending on where you are located, they can be difficult to get hold of too.
The second option is to use a matched source and test reference cord, often referred to as Method 2. This method requires investment in new equipment, but the matching test reference cords are cheaper, less bulky and easier to get a hold of. Fluke Networks CertiFiber Pro falls into Method 2 of TIA-TSB-4979 and is fully compliant to ANSI/TIA-526-14-C and IEC 61280-4-1 for both multimode wavelengths and all fiber types. Cords are available for certifying SC, ST, LC and FC using the preferred 1 Jumper Reference method on 50/125 µm and 62,5/125 µm fiber.
Matching test reference cord (Method 2)
1. April 2014: Fluke Networks now only ships EF compliant solutions for 850 nm and 1300 nm for all multimode fiber types.