The attenuation of a passive multimode component such as a connector is important for assessing its performance as a standalone product and as part of an installed fiber-optic link. The value measured depends on the modal condition of the light launched into the component. Unfortunately, significant disparities can be found in the definitions of modal condition in documents written by various standards committees around the world (IEC 86B; JTC1 SC25 WG3, which publishes ISO/IEC 11801 for Generic Cabling of Customer Premises; TIA FO-4; and CENELEC). The result is attenuation values that depend on the particular test method being used.
IEC SC86B WG4 is attempting to improve and harmonize the definition of modal condition found in the various standards. Currently, all published standards define the modal condition of the source by coupled power ratio, or CPR. That is followed by a description of a mode filter consisting of several fiber wraps around a specified mandrel size. The mode filter removes some of the higher-order modes excited by the source, but is ineffective if the source does not initially overfill the fiber. Furthermore, the number of fiber wraps and the mandrel size vary among the different standards.
The first improvement made by WG4 was to define the modal condition at the location of the component under test, thereby combining the effect of both source and mode filter. Further refinements include actual measurement of the relative filling of the various mode groups in the fiber by measuring the near-field of the light that would be launched into the component under test. Hopefully, a template definition of mode filling can be applied as a “go-no-go” test. Standardization in this area is long overdue and should be of great benefit to the industry.
Ton Bolhaar is responsible for technology and standards at Tyco Electronics and convenor of IEC SC86B WG4, Fiber-optic Test and Measurement Methods. He can be reached at T.Bolhaar@tycoelectronics.com.