EXFO FTB-7000 OTDR Series for FTB-500 - Manual (Page 241)

OTDR 227
14 Analyzing Bidirectional Traces
Note: The OTDR Bidirectional Analysis utility is available only from the
Applications tab of ToolBox
If two OTDR traces are acquired in opposite directions on the same fiber
span, the OTDR Bidirectional Analysis utility allows you to match the
corresponding events.
The application performs a bidirectional analysis and generates an events
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 fibers 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 signal
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 trace 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.
You can also analyze OTDR traces that use a multiwavelength feature.
To work with the OTDR Bidirectional Analysis utility, you must acquire and
save the traces before the analysis.