Encircled Flux

In August, 2013 the Telecommunications Industry Association (TIA) has released TSB-4979, “Practical Considerations for Implementation of Encircled Flux Launch Conditions in the Field.” The document provides specifications that define the launch condition of test sources into multimode fiber that have been standardized and are mandatory for testing optical attenuation of installed multimode fiber cabling per TIA-568.

Multimode fiber presents a challenge for certification testing, in that different types of light sources can produce different modal conditions. For example, LEDs can overfill a fiber with too many mode groups, while a laser may underfill the fiber. The result is that the results of certification tests on the same fiber can vary widely, depending upon the light source used. These variations become more significant as data rates increase.

Encircled flux is a way of characterizing launch conditions so that certification testing can be performed with more accurate and repeatable results. TSB-4979 describes implementation methods for field test equipment to meet standard encircled flux launch conditions. In fact, it describes two methods.

The first method can be used with any light source from any test supplier; TIA considers the method creates what it calls a “universal controller.” The second method is a light source and launch cord combination (the launch cord could include a controlling device) that together meet encircled flux requirements. TSB-4979 calls this combination a “matched controller.” In addition to defining the two methods, TSB-4929 reviews the pros and cons of each implementation method. The document also provides uncertainty of measurements and best practices advice.