Viavi T-BERD/MTS 2000 User Manual page 25

Laser light
source
Attenuation A = P1
(dB)
To perform accurate measurements, the following conditions are necessary
• Use 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.
Principle of optical power and attenuation measurements
1
Reference
Link under test
fiber
- P2
(dBm) (dBm)
User Manual
Chapter 1 Principle of Measurement
77000010208 Rev. 012
2
Power meter of the
MTS/T-BERD
2000/4000 V2
5