EXFO FTB-7000 OTDR Series for FTB-500 - Manual (Page 83)

Testing Fibers in Advanced Mode
OTDR 69
Setting the IOR, RBS Coefficient, and Helix Factor
Setting the IOR, RBS Coefficient, and Helix
Factor
You should set the IOR (group index), RBS coefficient and helix factor
before performing tests in order to apply them to all newly acquired traces.
However, you can also set them at a later time in the Trace Info pane to
reanalyze a specific trace (see Viewing and Modifying Current Trace
Settings on page 153).
Note: In Auto mode, you can change the IOR (group index), RBS coefficient and
helix factor parameters after an acquisition only if you have activated the
Enable Current Trace Settings Editing function (see Testing Fibers in Auto
Mode on page 57). You can always view these parameters for a specific
trace by selecting the Trace Info pane.
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The index of refraction (IOR) value (also known as group index) is
used to convert time-of-flight to distance. Having the proper IOR is
crucial for all OTDR measurements associated with distance (event
position, attenuation, section length, total length, etc.). IOR is provided
by the cable or fiber manufacturer.
The test application determines a default value for each wavelength.
You can set the IOR value for each available wavelength. You should
verify this information before each test.
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The Rayleigh backscatter (RBS) coefficient represents the amount of
backscatter in a particular fiber. The RBS coefficient is used in the
calculation of event loss and reflectance, and it can usually be
obtained from the cable manufacturer.
The test application determines a default value for each wavelength.
You can set the RBS coefficient for each available wavelength.
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The helix factor takes into consideration the difference between the
length of the cable and the length of the fiber inside the cable. Fibers
within a cable are spiraling around the cable core. The helix factor
describes the pitch of that spiral.