Viavi 8100 V2 Series User Manual page 44

Chapter 1 Principles of measurement
PMD principle
Figure 5 Fixed Analyzer Method used to measure the PMD
Broadband
light source
(SLED)
The method used to measure PMD is the Fast Fourier Transform Method (FFT).
From the spectrum, the mean period of the amplitude modulation is measured.
The Fast Fourier Transform Method into a time distribution will give a Gaussian curve
and the mean DGD value is determined from this curve (for fiber links with strong mode
coupling).
It is not necessary to modify the polarization angle of the analyzer when strong mode
coupling is used. For weak mode coupling, an angle could be selected to get the
maximum amplitude of the modulation.
The instrument should have a higher dynamic range than the link itself. A 35 dB dynamic
range is usually enough for most of the applications, and 45 dB should be used for very
long distance networks.
The measurement range of the PMD should be linked with the transmission rate. For
WDM applications, it should be between 0.1 ps to 60 ps so that measurement can be
carried out for bit rates between 2.5 and 40 Gbit/s. The table below indicates the
maximum permitted PMD values for various bit rates.
Bit rate (Gbit/s)
2.5
10
40
10Gbps Ethernet
• Tables at the end of chapter gives information about the appropriate standards
and limits.
1.This is standardized by the ANSI/TIA/EIA FOTP-113 Polarization Mode Disper-
sion Measurement for Single-Mode Optical Fibers by the Fixed Analyzer Method.
8
Fixed
Polarizer
Link under test
Maximum PMD
(ps)
40
10
2.5
5
User Manual
Rotatable
Polarizer
PMD coefficient (ps/√ km)
400 km cable length
< 2
< 0.5
< 0.125
-
78000010220 Rev. 024
Optical Spectrum
Analyzer (OSA)