Document #: GP-UM-PMD-1000-21
Page 109 of 122
Importance of polarization optimization:
The PMD effect on a system is highly
polarization dependent. When the input state of polarization (SOP) is aligned or counter-
aligned with the principal state of polarization (PSP) of the fiber link or the PMD source,
the PMD has no effect on the signal, and therefore no effect on system performance, as
shown in Figure 29. By contrast, when the input SOP is aligned 45 degrees from the PSP
of the fiber link or PMD source, the PMD has its maximum effect on the transmitted
optical signal, and hence its worst-case effect on system performance. The effect of PMD
on data can be characterized by the degree of polarization (DOP) of the optical signal,
because PMD causes depolarization of the optical signal, as shown in Figure 29: the
smaller the DOP, the larger the PMD effect. In general, the DOP of the signal passing
through a medium with nonzero 1
st
order PMD can be expressed as:
1
)]
0
(
)
(
1
)[
(
4
)
(
2
2
2
+
−
−
=
R
R
DOP
τ
γ
γ
τ
(1)
where
τ
is the DGD value of the medium,
γ
is the power distribution ratio of the two
polarization components with respect to the principal state of polarization (PSP) of the
DGD element (
γ
= ½ when the powers of the two polarization components are equal or
when the input SOP to the medium is oriented 45 degrees from its PSP) and
)
(
τ
R
is the
self-correlation function of the optical signal, which decreases as
τ
increases. Clearly, the
DOP reaches a minimum value of
)
0
(
)
(
R
R
τ
when
γ
= ½ or when the input SOP is 45
degrees from the PSP of the DGD medium. At this SOP, the PMD has its worst effect on
the quality of the signal. Conversely, when
γ
= 1 or when the input SOP is aligned with
the PSP, the DOP remains at its maximum value of 1, regardless of the DGD value.
When higher order PMD is present, the PSP is different for different wavelength
components of the signal and Eq. (1) may no longer hold. Nevertheless, there always
exists an input SOP at which the PMD has its most severe impact on the quality of the
signal, and at which the DOP of the signal at the output end is therefore at a minimum.
There also exists an input SOP at which the PMD has the least effect on the quality of the
signal, and at which the output DOP may therefore be much higher.
