PDF of this tutorialPMD–induced problems can be reduced simply by shortening the optical transmission distance of a system. For example, in the San Francisco to Los Angeles route, an optical-electronic receiver function could be placed about halfway between the two cities (see Figure 8).
Figure 8. Transmission System with Mid-Span Regeneration Block Diagram
The sole purpose of the receiver at this mid-span site is to decode the optical signals before they are corrupted by dispersion. After the bits are decoded, the electronic signals are then merely retransmitted by another optical transmitter, continuing their journey to their final destination where they will be decoded again and routed or switched electronically.
This method of a mid-span receiver/transmitter function, typically known as regeneration, has been widely used in fiber-optic transmission since its inception. However, regeneration has always been considered a poor alternative because the only reason the transmitted bits are converted to electronic signals at the regeneration site is because of the dispersion of the optical bits; the electronic bits are not routed, switched, multiplexed, or, in most cases, even monitored: they are merely regenerated. Therefore, from a network point of view, a regeneration site is an inefficient and costly optical-electronic conversion site.
Adding to the expense and inefficiency of a regeneration site is the fact that most long-haul transmission systems are now multiwavelength, dense wavelength division multiplexing (DWDM) systems. In this application, the transmission link must first be demultiplexed, then regenerated, then multiplexed again (see Figure 9). This is a very costly operation compared to the preferred alternative of a multiwavelength amplifier.
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Figure 9. Detail of the Regeneration Site
From a network and cost perspective, a more efficient method of addressing the PMD problem is to fix the effects of PMD while the transmission is in an optical state, before a receiver tries to decode the bits. This method is known as PMD compensation, and it is shown in Figure 10.
Figure 10. Transmission System with a PMD Compensator Block Diagram
A PMD compensator (PMDC), deployed at the destination of the transmission system, can reduce the effects of the dispersion in the fiber and ensure that the optical bits are correctly decoded by the receiver before they are to be routed and switched.
