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Validity of
Measurement
Reflectance
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 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 (especially DFB lasers) or
may generate interference noise in the receiver by multiple reflections.
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:
–
typical values of the backscatter coefficient for a pulse of 1 ns
and
–
for a single-mode fiber:-79 dB at 1310 nm
–
for a multi-mode fiber:-70 dB at 850 nm
User Manual
770000102/08
Chapter 1 Principle of Measurement
Principle of reflectometry measurements
-81 dB at 1550 nm and 1625 nm
-75 dB at 1300 nm
3
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