EXFO iOLM - Manual (Page 15)

Introducing the intelligent Optical Link Mapper
intelligent Optical Link Mapper 9
Launch, Receive, and Loop Fiber
The loopback measurement is designed to test duplex cables.
In a loopback measurement, one end of the first fiber is connected to the
iOLM through a launch fiber, while the other end is connected to the
second fiber using a fiber called a loop. Usually, a receive fiber is used on
the proximal end of the second fiber. The loop fiber can be viewed as a
receive fiber for the first fiber (the one the iOLM is connected to) and a
launch fiber for the second fiber.
The loopback process performs one single acquisition for both fibers, then
automatically splits the result, providing independent measurements for
both fibers in the duplex cable.
The automatic split of the initial measurement can only be performed if the
length of the launch, loop and receive fibers are properly specified in the
application.
See Configuring your iOLM on page 42 for more details.