Exfo OTDR 2 User Manual download pdf (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.

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Exfo OTDR 2 User Manual Page 32