Principle Of Bi-Directional Measurement - Viavi 8100 V2 Series User Manual

Chapter 1 Principles of measurement

Principle of bi-directional measurement

• typical values of the backscatter coefficient for a pulse of 1 ns and
– for a single-mode fiber: -79 dB to 1310 nm
– for a multi-mode fiber: -70 dB to 850 nm
NOTE
To measure the widest range of reflection coefficient, it is necessary to insert
a variable optical attenuator between the reflectometer and the link to be
tested. This attenuator enables the level of the trace to be adjusted so as to
avoid saturation of the reflectometer by the reflection to be evaluated.
Principle of bi-directional measurement
If fibers with different mode-field diameters (core size etc.) are joined, the resulting
OTDR trace waveform can show a higher backscattering level. This is due to the
increased level of backscattered signal reflected back to the OTDR in the downstream
fiber.
Figure 2 Normal splice
This phenomenon can occur when jointing different types of fiber in multi-mode or 2
fibers with different backscattering coefficients.
4
-81 dB to 1550 nm and 1625 nm
-75 dB to 1300 nm
User Manual 78000010220 Rev. 024