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Unformatted text preview: 328 Transmission System Engineering To understand how PMD can be compensated optically, recall that PMD arises due to the fiber birefringence and is illustrated in Figure 2.7. The transmitted pulse consists of a fast and a slow polarization component. The principle of PMD compensation is to split the received signal into its fast and slow polarization compo- nents and to delay the fast component so that the DGD between the two components is compensated. Since the DGD varies in time, the delay that must be introduced in the fast component to compensate for PMD must be estimated in real time from the properties of the link. The PMD effect we have discussed so far must strictly be called first-order polarization-mode dispersion. First-order PMD is a consequence of the fact that the two orthogonal polarization modes in optical fiber travel at slightly different speeds, which leads to a differential time delay between these two modes. However, this differential time delay itself is frequency dependent and varies over the band- width of the transmitted pulse. This effect is called second-order PMD. Second-order PMD is an effect that is similar to chromatic dispersion and thus can lead to pulse spreading. PMD also depends on whether RZ or NRZ modulation is used; the discussion so far pertains to NRZ modulation. For RZ modulation, the use of short pulses enables more PMD to be tolerated since the output pulse has more room to spread similar to the case of chromatic dispersion. However, second-order PMD depends on the spectral width of the pulse; narrower pulses have larger spectral widths. This is similar to the case of chromatic dispersion (Section 5.7.2). Again, as in the case of chromatic dispersion, there is an optimum input pulse width for RZ modulation that minimizes the output pulse width [SKA00, SKA01]. In addition to PMD, some other polarization-dependent effects inuence system performance. One of these effects arises from the fact that many components have a polarization-dependent loss (PDL); that is, the loss through the component depends on the state of polarization. These losses accumulate in a system with many com- ponents in the transmission path. Again, since the state of polarization uctuates with time, the signal-to-noise ratio at the end of the path will also uctuate with time, and careful attention needs to be paid to maintain the total PDL on the path to within acceptable limits. An example is a simple angled-facet connector used in some systems to reduce reections. This connector can have a PDL of about 0.1 dB, but hundreds of such connectors can be present in the transmission path. 5.8 Fiber Nonlinearities As long as the optical power within an optical fiber is small, the fiber can be treated as a linear medium; that is, the loss and refractive index of the fiber are independent 5.8 Fiber Nonlinearities 329 of the signal power. However, when power levels get fairly high in the system, we have to worry about the impact of nonlinear effects, which arise because, in reality,...
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This note was uploaded on 01/15/2011 for the course ECE 6543 taught by Professor Boussert during the Spring '09 term at Georgia Institute of Technology.
- Spring '09