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If linearly polarized light is launched into a PMF fiber such that light is coupled into both the fast
and slow axes, propagation through the PMF will delay one polarization component with re-
spect to the other. The delay creates a phase difference, d, between them. The magnitude of
the phase difference, and the proportion of light carried by each fiber polarization component,
determines whether the output SOP is linear, elliptical, or circular.
Due to the cyclical relationship between the phase difference and the polarization state, the two
polarization states will evolve to be periodically in and out of phase as the light propagates
down the fiber. The polarization components are in phase with the same direction of linear po-
larization when d=0° and 360°. The shortest length of fiber that results in a change in phase of
360° is called the polarization beat length. When the wavelength of the light is l, and the differ-
ence in refractive indices is dn, the polarization beat length is:
L = l / dn
If the light has propagated one beat length through the fiber, the light polarized along the slow
axis lags the light in the fast axis by a distance equal to the wavelength of the light in the fiber.
While polarization beating is more easily identified in PMF, it also occurs in SMF. However, as
SMF has low and variable levels of birefringence, the beat length also varies along the length of
the fiber.
To ensure a linear polarization state coupled into the PMF is preserved at the output, the
launch conditions must be carefully controlled. It is essential that either the fast or slow axis of
the PMF is aligned with the polarization direction of the input light. Stated another way, the azi-
muthal orientation of the light signal must be aligned to either the slow or fast axis of the PMF.
If this is not done, light will be coupled into both the slow and fast axes of the fiber and the large
difference in refractive index between them will induce a short beat length of a few millimeters.
The result will be frequent changes in the state of polarization along the length of the fiber, in-
stead of the desired maintenance of the polarization state.
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