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 oneway
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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