Fiber
Optic MiniCourse: Reading An OTDR Trace
Distance Scale
The
distance scale is calculated by measuring the time the
OTDR test pulse takes to make the round trip to a point
in the cable, dividing it in two to get the one-way
time, and then converting it to distance using the index
of refraction of the fiber stored in the OTDR settings.
The index of refraction is a measure of the speed of
light in the fiber and must be correct for the distance
calculated by the OTDR to be correct.
Distance = speed X time
Speed of light in a fiber = speed of light in a vacuum /
index of refraction
The typical index of refraction of optical fiber is ~
1.46 so the speed of light in the fiber is about 2/3 the
speed of light in a vacuum.
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