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To locate faults, a reflectometer measures only time. Consequently, group
velocity must be introduced in order to determine the distance of the loca-
tion. This is done by introducing the refractive index of the fiber into the
instrument.
Validity of Measurement
UTI-T, in recommendations G.650, G.651 and G.652, give backscatter measurement as
an alternative method for measuring attenuation, the method of reference being the cut
fiber.
The field of application of backscatter is not limited, but the conditions for application of
this method are nevertheless stipulated:
•
injection conditions: Fresnel reflections must be limited at fiber input.
•
a high-power source (laser) should be used.
•
receiver bandwidth should be chosen to achieve a compromise between pulse
rise time and noise level.
•
backscatter power should be represented on a logarithmic scale.
Reflectance
Reflectance is a value with which the coefficient of reflection of a reflecting optical
element can be quantified. It is defined as the ratio of the power reflected by the element
over the incident power.
These reflections are due to variations in refractive index all along the optical link in
certain telecommunications applications. If they are not controlled, they may degrade
the performance of the system by perturbing the operation of the emitting laser (espe-
cially DFB lasers) or may generate interference noise in the receiver by multiple reflec-
tions.
The reflectometer is particularly well suited to the measurement of discrete reflectances
on an optical fiber link. To calculate the coefficient of reflection, it is necessary to
measure the total amplitude of the Fresnel reflection generated and then to apply a
conversion formula to obtain the reflectance value.
This formula takes into account:
•
the total amplitude of the reflection measured by the reflectometer.
•
the pulse width used to measure the amplitude of the reflection (in nanoseconds)
•
the backscatter coefficient of the fiber used:
User Manual
Chapter 1 Principles of measurement
Principle of reflectometry measurements
78000010220 Rev. 024
3
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