Improve multimode fibre loss measurements with Encircled Flux

Previous methods and assumptions about fibre testing no longer hold true.

That's the word from network test solutions specialists Fluke Networks, who says the introduction of new technologies requiring low-loss fibre optic components means loss budgets and test limits are becoming increasingly smaller.

Fluke Networks provides innovative solutions for the testing, monitoring and analysis of enterprise and telecommunications networks and the installation and certification of those networks.

As networking technology evolved and loss budgets decreased, link-loss measurements have become more demanding, Fluke Networks explains. "New networking applications require more accurate and reproducible multimode attenuation measurements from different field test instruments."

As a result, industry experts have concluded that narrowing launch condition variability was required, especially when testing for 1 Gigabit or higher supportability over multimode fibre.

Installers are now required to use Encircled Flux compliant light launch conditions, a relatively new addition to standards that experts say is a major improvement over previous methods, in order to reduce uncertainty in link loss measurements shown by different test equipment when testing multimode optical fibre.

Encircled Flux is a complex method of measuring the source output.

"Encircled flux is a way to control the amount of light that exits a connector on a Test Reference Cord used to perform insertion loss measurements on a multimode fibre optic permanent link or channel," explains Wayne Allen, APAC Product Marketing Engineer, Fluke Networks.

Allen says Encircled Flux can greatly improve link-loss measurement consistency in comparison to previous launch conditions and standards.

Fluke Networks explains how different testers yield different results, for example when testing a multimode fibre optic link with light sources from two different equipment vendors. "By simply using a different light source you may notice that the loss measurement is different by as much as 50%," Allen says.

Without proper controls, Fluke Networks says multimode light sources will inject incorrect amounts of light into multimode fibre under test.

"Modal power distribution is a way of explaining how many modes are supported by a multimode fibre," Allen explains. "Typically, multimode fibre supports hundreds of modes while a single mode fibre supports one mode.

This modal power distribution is commonly referred to as a 'launch condition'. When light sources operate under different launch conditions, link-loss measurements will vary proportionately, resulting in different and confusing test results.

Different types of light sources produce different types of launch conditions, Fluke Networks says. For example, LED overfills a multimode fibre with too many mode groups while a laser under fills a multimode fibre with not enough mode groups. Overfilling 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 can obscure actual high-loss events such as misaligned connectors, which can lead to false 'pass' results, that may ultimately hamper cabling infrastructure performance," Allen explains.

Fluke Networks says tighter launch conditions leads to more accurate and repeatable measurements.

Encircled Flux compliance reduces loss measurement variation to a goal of +/- 10% . Reducing variability by up to 75% compared to the preceding standard, it is the most recent standard that increases multimode testing accuracy and repeatability, Fluke Networks says.

There are now external solutions available for field-testing purposes. Launch controllers are specially constructed test-grade reference cords fitted with modal conditioners. These launch controllers work by restricting the number of mode groups launched from the test cord to within Encircled Flux specifications, ensuring that the resulting measurements are precise and repeatable according to the standards.

Encircled Flux is a major improvement in tightening multimode link-loss measurements to within a 10% variability target. As a result, it improves upon older methods such as modal power distribution (MPD) and coupled power ratio (CPR).

"Because test instruments meeting the Encircled Flux standard will provide the most consistent and reliable results when performing certification testing, it is recommended that network engineers and designers update their test specifications to reference the utilisation of Encircled Flux-compliant sources," adds Allen.

For Certification of Multimode Links, the Fluke Networks CertiFiber PRO (MM and Quad modules) will meet the Encircled Flux launch requirements provided that the recommended Test Reference Cords are used.

It is a combination of the source Fluke Networks has chosen and the Encircled Flux Test Reference Cords that make the measurement system Encircled Flux compliant.

To request a demo of Fluke Network's Versiv unit, which is is Encircled Flux compliant, please click here. For more information on Encircled Flux or to learn more about Fluke Networks, please visit the Fluke Networks website.