EXFO FG-750 - Manual (Page 144)

Setting Up Your RTU
130 FG-750
Managing System Setting Values
Defining Default Helix Factor, IOR, and RBS Values
You can define the IOR (group index), RBS coefficient and helix factor that
the RTU will use when it creates the optical routes.

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.

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 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.
By setting the helix factor, the length of the OTDR distance axis is
always equivalent to the physical length of the cable (not the fiber).