Why do the Encircled Flux testers use an FC connector?
Knowledge Base - FAQ
Why do the Viavi Encircled Flux modules for the Viavi Certifier 40G use an FC connector?
We achieve EF (Encircled Flux) by having EF at the connector then using a "modally transparent" launch cord. This requires a very tight core alignment at the output port of the tester. The FC connector type provides the best core alignment.