|Polarization Mode Dispersion (PMD)|
Source: David R. Goff. Fiber Optic Video Transmission, 1st ed. Focal Press: Woburn, Massachusetts, 2003
and other private writings.
Polarization is a property of light that
causes it to be broken down into two waves that are 90° out of phase
with each other. The most common place that polarization is
encountered in everyday life is in polarized sunglasses. Polarized
sunglasses are designed such that they only pass one polarization of
light. When sunlight reflects off of a surface, it becomes
polarized. Polarized sunglasses are designed such that they only
pass the polarization not associated with reflections, thus most
reflected light almost disappears.
When light is injected into an optical fiber it usually splits into the two different polarization planes and each polarization travels down the fiber. PMD occurs in optical fiber because light in each of the polarization planes within an optical fiber can travel at slightly different speeds, leading to a distortion of the pulse shape. Figure 1 shows how differences in velocity between two polarizations of light traveling down a fiber can lead to pulse distortion/broadening as the light on the fast axis races ahead of light on the slow axis. PMD is an important factor in high bit rate transmission systems - typically at transmission rates greater than 10 Gbit/s.
Figure 1 - Polarization Mode Dispersion (PMD)
|PMD was one of the biggest risk elements associated with the introduction of 10 Gb/s transport speeds and stands as one of the key barriers to the leap to 40 Gb/s. It is one of the newest and least understood aspects of optical fiber. It only became a serious issue when data rates reached OC-192 (10 Gb/s). One of the biggest challenges with PMD is the fact that it varies with time. Most other fiber distortions and nonlinearities are relatively stable over time. Various techniques exist to correct for PMD. Optical techniques (See Figure 2) usually involve applying some sort of stress to a fiber at one or more points to change the polarization. Electrical techniques (See Figure 3) also exist to correct PMD distortion after the photodetector and amplifier.||
Figure 2 - Optical PMD Compensator
Figure 3 - Electrical PMD Compensator
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