In fibre optic loss testing, the most important condition is properly setting the 0-decibel (dB) reference with the light source, power meter and reference cables. Conditioning the light from the light source to and through the launch cable has never been an exact science.
When you couple a fibre launch cable to a source, there is a lot of variability in what the light output of the launch cable looks like; that variability can make a big difference in a loss measurement. When using different light sources 50% differences in loss measurement were common, so something had to be done. It is however complex because of the core structure in the MM fibre and the variability of the light-emitting diodes (LED).
This complexity has led to a bit of controversy over how to condition sources properly so international standards groups came up with a new way of testing “encircled flux” (EF), which was adopted into U.S. and international standards.
Different types of light sources produce different types of launch conditions. For example, a light emitting diode (LED) overfills a multimode fibre with too many mode groups while a laser under fills a multimode fibre with not enough mode groups. Over filling a fibre tends to produce link-loss measurements that are too high while under filling a fibre tends to produce link-loss measurements that are too low.
In other words, certification tests involving under filled launches that can obscure actual high-loss events such as misaligned connectors, which can lead to false “pass” results, that may ultimately hamper cabling infrastructure performance.
So Encircled Flux (EF) is a multimode launch-condition metric that serves to alleviate these problems. Its first intention is to reduce link-loss variation when using different light sources so test results are similar, independent of the supplier. This variation has been limited to ± 10% for link loss higher than 1 dB. EF was developed to keep up with components used in high-speed networks, e.g. 10/40/100-Gbit Ethernet. It was not until high-speed transmission over multimode fibre became a reality that EF measurements became important.
In conclusion, when testing MM fibre networks use an EF compliant source with a tuned reference cord that strips out the unwanted modes.