Document #: GP-UM-PMD-1000-21
Page 106 of 122
Figure 29 Illustrations of the PMD effect on an optical signal.
A) The SOP of the input signal is aligned 45 degrees from the PSP of the fiber link, causing the worst-case
signal distortion. In this case, if the DGD is larger than the width of one bit, then the DOP=0 because the
two orthogonal polarization components have the same power, with no phase relationship.
B) and C) The SOP of the input signal is aligned with the slow or fast PSP axis, respectively. In these
cases, no signal distortion occurs except the slight late or early arrival, respectively, of the pulse. The DOP
of the signal remains at 1. In all 3 cases, the PMD of the link remains the same, but the effect on the signal
is different due to the different input polarization states.
Several methods with varying success rates exist to mitigate PMD effects for 40G
deployment. The first is to select fiber routes with low PMD coefficients. Such a method
requires extensive survey of all available fiber routes to identify and cherry-pick those
with sufficiently low PMD. However, such an approach has three potential drawbacks:
First, the PMD of a fiber route changes with time as the environmental conditions around
the fiber change. Such changes can include land movements caused by earthquake, flood,
or mud-slides. A route that is good today may become bad in the future. Second,
interconnected fiber routes may always have bad fiber sections with high PMD, rendering
the selection of low PMD fiber routes throughout the system impractical. Finally, the
supply of fiber routes with low PMD values will eventually be exhausted as 40G
deployments increase.
The second method is to use bandwidth-efficient modulation formats, such as DPSK or
DQPSK. Such modulation formats reduce the effective bandwidth of a 40Gb/s channel to
that of a 20Gb/s or even a 10Gb/s channel. Consequently, the impact of PMD on the
signal is greatly reduced, though never eliminated.
The third approach is electronic PMD compensation, in which forward error correction
(FEC) or other algorithms are used to reduce the PMD effect on the signal after the
PMD
RX
t
Power
PMD
RX
PMD
RX
t
Power
t
Power
DGD
A)
B)
C)
DOP=0
DOP=1
DOP=1
