EXFO FastReporter 2 - Manual (Page 373)

Measuring Polarization Mode Dispersion: Theory
Post Processing Software 363
It is essential to keep in mind that DGD fluctuates in time and can be either
smaller or larger than its rms value or PMD. This results in a statistical
probability that a pulse (information bit) is broadened, and leads to the
eventual impaired ability of the receiver to efficiently decode the
information. This adverse PMD effect makes it a critical phenomenon in
limiting transmission of high-bit-rate information.
In the case of PMD in a long fiber, there is a specific state called input PSP.
In this state, when the input SOP of the signal is aligned with one of its
axes, it will propagate through the fiber without any spreading or distortion
of the signal. This phenomenon is defined as this specific input SOP such
that the output SOP is independent of optical frequency. Again, the worst
case occurs when the signal is equally split between both input PSPs.
For long telecom fibers with random coupling of energy between modes
(that is. L >> h, where h is the coupling length), PMD grows as the square
root of the distance, whereas PMD of strong HiBi fiber (negligible mode
coupling) is directly proportional to the distance. Therefore, the PMD
coefficient for negligible mode coupling is expressed as ps/km, while
the PMD coefficient for random mode coupling is defined as ps/km
1/2
.