Viavi 8100 V2 Series User Manual page 42

Chapter 1 Principles of measurement
Principle of the optical power and attenuation measurements
Laser light
source of the
Platform 8000
Attenuation A
To perform accurate measurements, the following conditions are vital
• Use one of the light sources of the LTS or a light source which is stable both in
time and as a function of temperature.
• Make sure that all connections and fibers and the receiving cell are perfectly
clean.
• Use a reference link between the laser source and the test subject. If several
measurements are to be made under identical light injection conditions, this refer-
ence fiber must not be disconnected during the period while measurements are
taking place.
Insertion loss method
1 The power meter is first connected to the laser source via the reference fiber: P1
is measured.
2 Then the fiber to be tested is inserted between the reference fiber and the power
meter: P2 is measured.
The difference between P2 and P1 gives the attenuation of the fiber under test.
It is preferable to use the same type of connector at both ends of the fiber being
tested, to ensure the same connection conditions for measuring P1 and P2.
Accuracy of measurements
• A high degree of accuracy is often required. It is then necessary to perform a
preliminary calibration without the fiber under test to eliminate the losses due to
connections as far as this is possible. To do this, use the «Reference Value» func-
tion.
For measurements in the laboratory, where both ends of the fiber are on the same site,
the repeatability of attenuation measurements is better than 0.1 dB. For measurements
in the field, where the two ends are on different sites, variations from one measurement
to another are of the order of ± 0.2 dB (using a relative measurement).
6
1
Reference
fiber
= P1 - P2
(dB) (dBm)
User Manual
Link under test
(dBm)
78000010220 Rev. 024
2
Power meter