Optical Networks: A Practical Perspective - Part 36. This book describes a revolution within a revolution, the opening up of the capacity of the now-familiar optical fiber to carry more messages, handle a wider variety of transmission types, and provide improved reliabilities and ease of use. In many places where fiber has been installed simply as a better form of copper, even the gigabit capacities that result have not proved adequate to keep up with the demand. The inborn human voracity for more and more bandwidth, plus the growing realization that there are other flexibilities to be had by imaginative use of the fiber, have led people. | 320 Transmission System Engineering Polarization-Mode Dispersion PMD The origin of PMD lies in the fact that different polarizations travel with different group velocities because of the ellipticity of the fiber core we discussed this in Section . Moreover the distribution of signal energy over the different state of polarizations SOPs changes slowly with time for example because of changes in the ambient temperature. This causes the PMD penalty to vary with time as well. In addition to the fiber itself PMD can arise from individual components used in the network. The time-averaged differential time delay between the two orthogonal SOPs on a link is known to obey the relation KK97a Chapter 6 Ar DpmdV where Ar is called the differential group delay DGD L is the link length and Dpmd is the fiber PMD parameter measured in ps Vkm. The PMD for typical fiber lies between and 2 ps Vkm. However carefully constructed new links can have PMD as low as ps Vkm. In reality the SOPs vary slowly with time and the actual DGD Ar is a random variable. It is commonly assumed to have a Maxwellian probability density function see Appendix H . This means that the square of the DGD is modeled by a more familiar distribution the exponential distribution. The larger the DGD the larger is the power penalty due to PMD. Thus the power penalty due to PMD is also time varying and it turns out that it is proportional to Ar2 and thus obeys an exponential distribution see Problem . If the power penalty due to PMD is large it is termed a PMD outage and the link has effectively failed. For a DGD of where T is the bit duration the power penalty is approximately dB for a receiver limited by thermal noise and 1 dB for a receiver with signal-dependent noise ITU . Using the Maxwellian distribution the probability that the actual delay will be greater than three times the average delay is about 4 x 10-5 see Appendix H . Given our earlier reasoning this means that in .
