Pmd Principle; Method Used To Measure The Pmd - Viavi 8100 V2 Series User Manual

Chapter 1 Principles of measurement

PMD principle

PMD principle
The transmission rate and range are two of the most important parameters of fiber optics
paths and must therefore be optimized. And, since more and more paths (including
those already installed) are being used for transmitting Wavelength Division Multiplex
(WDM) signals or for bit rates of 10 Gbit/s, it is becoming all the more important to deter-
mine the Polarization Mode Dispersion (PMD).
PMD, which is the basic property of single-mode fibers, in particular affects the magni-
tude of the transmission rate. It results from the difference in propagation times of the
energy of a given wavelength, which is split into two polarization layers that are at right
angles to each other (as shown in the below diagram). The main causes of this birefrin-
gence are non-circularities of the fiber itself and external stress on the fiber (macro-
bending, micro-bending, twist and temperature variations).
Figure 4 Example of a time delay between two polarization layers
DGD
V
2
V
1
The PMD is also referred to as mean value of all Differential Group Delays (DGD) in
picoseconds (ps) or as the DGD coefficient in ps/√km.
The mean DGD causes the transmission pulse to broaden when transmitted along the
fiber, generating distortion, which in turns increases the bit-error-rate (BER) of the optical
system. The consequence is that the PMD limits the transmission bit rate on a link. It is
then important to know the PMD values to calculate what are the bit rate limits of the
links.

Method used to measure the PMD

1
The method used to measure the PMD is based on the Fixed Analyzer Method which
requires a broadband polarized source at one extremity, and a polarized (variable)
Optical Spectrum Analyzer (OSA) at the other extremity.
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