Chapter 1
Principle of Measurement
Principle of reflectometry measurements
2
User Manual 770000102/08
Principle of reflectometry measurements
Optical time domain reflectometry consists in injecting a light pulse into
one end of the optical fiber to be analyzed and observing, at the same
end, the optical intensity passing through the fiber in the opposite direc-
tion to the propagation of the pulse.
The signal detected is exponentially diminishing in form, typical of the
phenomenon of backscattering, with superimposed peaks due to reflec-
tions from the ends of the fiber or other variations in the refractive index.
Information
yielded by the
measurement
From a backscatter trace it is possible, in particular, to determine the
position of a section of fiber within a link. The measurement result must
reveal:
–
the attenuation
–
the location of faults, by their distance from a point of origin,
–
attenuation with respect to distance (dB/km)
–
the reflectance of a reflective event or a link.
Fig. 1
Trace showing typical backscattering
To locate faults, a reflectometer measures only time. Consequently,
group velocity must be introduced in order to determine the distance
of the location. This is done by introducing the refractive index of the
fiber into the instrument.
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