Ever tried measuring the insertion loss of a Multi-Mode connector using a single-mode launch fiber? You tend to get really good results! It’s like squirting a 1/2 inch garden hose into a 3 inch fire hose from a foot away. It’s not difficult to catch nearly all of the water, getting all the water into another 1/2 inch hose however, requires much more precise alignment.
I think we would all agree that testing MM with SM launch fiber was cheating, but there is a sort of intermediate situation, that of an under-filled MM launch.
If all the light passes along the center of the MM core, with not much around the edge, slight offsets won’t lose all that much light. On the other hand, an overfilled launch fiber, with lots of light around the edge will send light into the cladding will likely lead to a poor DUT measurement as the smallest of offset will lose significant light. A standard is required so measurements made on different test sets at least have a chance of agreeing.
The IEC 61280-4-1 Standard sets out to define the distribution of light (flux) at the launch into the DUT. Encircled flux is just a rather grand way of saying the amount light inside each circle of a given size within the core. “The IEC 61280-4-1 standard recommendations are based on the defined lower and upper boundaries of EF values at four predefined radii of the fiber core (10, 15, 20, and 22 mm), and for each wavelength (850 and 1300 nm)” I wish I could show you a picture of this, but I either infringe on someone’s copyright or spend too much time creating my own - Google will find you lots.
This is all very well, creating a standard for how it should be, but how is it to be measured, implemented, and maintained? Mode conditioners are available (though you might need lots) offering to change any distribution into compliance with IEC 61280-4-1, verifying this may not be easy, especially when there are many combinations of source, cable and switch to contend with. Measuring Modal distribution is not simple and accurate measurements require complex equipment, usually beyond the reach of a jumper production house.
At Fiber Optic Center, we take the approach of starting out with an IEC 61280-4-1, EE compliant instrument in the first place and taking care to use only modally transparent components (switches and fibers etc.) between there and the device under test. The VIAVI MAP, Passive Component Tester, is compliant “off the shelf” and will give you consistent EF compliant measurements without resorting to the complication of extra inline conditioners. A range of modally compliant switches are available for testing multi-fiber cables.
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