EXFO FastReporter Data Post-Processing Software - Manual (Page 75)

Working with OTDR Files
Data Post-Processing Software 69
Creating Bidirectional OTDR Files
Creating Bidirectional OTDR Files
Analyzing Bidirectional Measurements
If two OTDR measurements were acquired in opposite directions on the
same fiber span, the Create Bidirectional Files tool allows you to match
the corresponding events. FastReporter performs a bidirectional analysis
and generates an event table with the averaged loss for each event; that is,
the average of the losses obtained from both directions.
Bidirectional analysis is the recommended method for splice loss
measurements on singlemode fibers by the Telecommunications Industry
Association (test procedure EIA/TIA FOTP-61 Measurement of Fiber or
Cable Attenuation Using an OTDR).
This method removes the so-called “gainers” (increase in the optical
power) and exaggerated losses and provides accurate measurements. This
analysis is particularly useful to test the quality of a link, especially if it
comprises several sections with different types of fibers or fiber from
different manufacturers.
Gainers and exaggerated losses result from the joining of two fibers of
different mode-field diameters (MFD) The mode-field diameter of a fiber
corresponds to the size of the area where light is dispersed across its core
and cladding.
Mismatch of MFDs will contribute to differences in backreflected signals
that are not related to the loss at the splice point, that is to the true loss
seen in transmission. In this case, a unidirectional OTDR measurement will
show an apparent increase (gainer) or decrease (exaggerated loss) in
signal, depending on the direction of measurement.
Bidirectional averaging of OTDR splice loss measurements provides the
most accurate splice loss results.