From Optical Bit Error Rate
2.13.6 Polarization-Mode Dispersion
All fibers have some residual birefringence and a core that is not perfectly circular
over their entire length. Fiber birefringence and core noncircularity cause an optical
(monochromatic) signal to be separated into two orthogonally polarized signals, or
principal states of polarization (PSP), each traveling at a different speed and phase.
The same thing happens to each pulse of a modulated optical signal; the pulse is separated
into two pulses, each traveling at a different speed. Thus, when the two signals
recombine, because of the variation in time of arrival, a pulse spreading occurs. This
phenomenon is particularly noticeable in single-mode fiber transmission at ultrahigh
bit rates (above 2.5 Gbps) and is known as polarization-mode dispersion (PMD).
By definition (see ITU-T G.650), PMD is measured by the average differential
group delay time (DGD) over wavelength between two orthogonally polarized
modes (measured in ps). PMD is maximized if both PSPs are equally and maximally
excited, resulting in maximum DGD and pulse spreading. Conversely, PMD vanishes
or is greatly minimized if only one of the two states is excited. Similarly, the
polarization mode dispersion coefficient is defined as the PMD divided by the
square root of fiber length (measured in ). Optical fibers have a polarization-
mode dispersion coefficient of less than (see ITU-T G.650, G.652,
G.653, and G.655).
As a consequence of the DGD definition, if DGD is measured by the average
difference of time arrival, Δτ, between the two orthogonally polarized modes, and
the two polarized modes are related to the birefringence of the fiber, Δng, then DGD is
|Δτ = (ΔngL)/c|
where c is the speed of light, L is the length of the fiber, and ng is the refractive index
variation corresponding to the group velocity of the orthogonal polarization states.
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Topics of Interest
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