EXFO OTDR 2 - Manual (Page 32)

Preparing Your OTDR for a Test
24 OTDR
Setting the IOR, RBS Coefficient, and Helix Factor
Setting the IOR, RBS Coefficient, and Helix
Factor
You should set the IOR (group index), backscatter 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 but in that case, you
have to reanalyze the trace if you change the backscatter coefficient (see
Viewing and Modifying Current Measurement Settings on page 119).

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.

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.

The helix factor represents the ratio between the length of the cable
and the length of the fiber inside the cable. Since fibers within a cable
are spiraling around the cable core, the fiber length is different from
the cable length.
By setting the helix factor, the length of the OTDR distance axis is
always equivalent to the physical length of the cable.
The helix factor is expressed as a percentage. For example, a helix
factor of 1 % implies that the fiber is 1 % longer than the cable. If you
specify a helix factor of 1 %, the displayed length would be reduced by
1 % according to the cable length.