EXFO FTB-5600 Distributed PMD Analyzer - Manual (Page 134)

Optimizing Measurements
128 FTB-5600
Use an Appropriate OTDR Pulse Length
Use an Appropriate OTDR Pulse Length
A recommended pulse length of 100 ns should be used for most
acquisitions. However, when a depolarization effect for this pulse may be
too big because of short beating lengths from FUTs (that are often observed
in the field fibers), a short pulse length, for example, 50 ns, may be used to
reduce the depolarization effect and improve SNR so as to ensure an
acquired signal level that is larger than a coherent noise level, but it will
result in a reduced measurable range.
On the other hand, if the FUT has a small depolarization, use a long pulse
length, for example, 275 ns, for the acquisition may be helpful to increase
the measurable range or reduce the acquisition time.
Below you can see an example of a depolarization on a fiber: