From Optical Bit Error Rate
PMD has been phenomenologically explained by extensive vectorial analysis that
considers the propagation of principal states of polarization. In a graphical representation
of polarization states, known as the Poincaré sphere, PSPs are located at
diametrically opposite points on the sphere’s surface; points on the equator of the
sphere represent linear polarization, polar points represent circular polarization, and
points on the hemispheres represent elliptical polarization states (the northern hemisphere
represents right-handed states and the southern left-handed states). As the
noncircularity of a fiber core causes PSPs, each polarization state at different speed
and phase, an issue emerges at the connecting point of two fibers where in most
practical cases, core noncircularity is not matched. Hence, slow and fast states from
one segment are coupled into another core with different orientations of polarization.
Thus, at the interface, as the polarization states from one fiber are coupled into
the next, input PSPs are transformed into different output PSPs. This is known as
polarization mode coupling (PMC). As a consequence, over a fiber span with several
connections, PMC becomes a random event that affects DGD and PSPs randomly
to further complicate PMD compensation.
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