Fluke CableAnalyzer DSX-5000, Technical Reference Handbook, Page: 419 / 700

Chapter 11: The OptiFiber Pro OTDR Module
Bi-directional SmartLoop Tests
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To switch between results for fibers A and B, tap on the
EventMap screen.
Note
When you save SmartLoop results, the tester saves the
results in two records, one for each fiber in the link.
Bi-directional SmartLoop Tests
The bi-directional SmartLoop test gives you bi-directional OTDR
results for both fibers in a link. The tester also automatically
calculates averages of the two results and includes the averaged
values in the test record.
Why Do Bi-directional Tests?
OTDR tests from both ends of a fiber can give different results
because some properties of fiber and fiber components can cause
differences between the loss measurements for each direction.
For example, the loss of an event is the difference between the
backscatter levels before and after the event. If a segment after a
connector or splice has a higher backscatter value than the
segment before, the tester shows different values of loss for the
connector or splice from the two directions.
For splices, which have very little loss, the difference in the
backscatter of the two fibers can cause an apparent gain in power
across the splice in one direction. On multimode fibers, the loss of
a splice can be different for the two directions, even if the
backscatter of the fibers before and after the splice are the same.
This difference is caused by differences in the modal distribution
of light in the launch cords and differences in how multimode
light propagates down the fiber in each direction.
For these and other reasons, bi-directional averages of connector
and splice loss in both directions are more accurate than
measurements from one direction.