olarizations. When the optical wave is modulated by the carried informa- tion, the two waves corresponding to the two states of polarization are modulated. There- fore, two modes orthogonal to each other propagate in the waveguide. In systems where the receiver can detect both states of polarization, the receiver will function as expected if both modes arrive at the same time; that is, there is no differential delay between the two modes. However, the refractive index of the silica glass can be slightly different along the two orthogonal axes, and consequently there is differential delay. The average Differential Group Delay (DGD) is the PMD coefficient of the fiber. Chapter 5 Optical Fibers PMD = <DGD> (5.45) To compound the difficulties, the DGD is not the same along the length of the fiber but varies from section to section. Because of this feature, the PMD is not described by a single value but rather by a probability distribution function. The impact of PMD on pulse width is illustrated in Figure 5.17. The PMD is a complex effect, and its study and mitigation have attracted a very sub- stantial effort. Theoretical studies and experiments have demonstrated that the PMD is proportional to the square root of the fiber length-that is, DPMD=PMDcoeff pskm (5.46) Because of its importance for high-speed systems, the PMD fiber coefficient is part of the fiber specifications. For example, the Corning fiber STM-28e+ has a PMD coefficient of <0.06 ps//km . . The calculation of the amount of PMD a system can tolerate is an involved process and is part of the system design. An easily remembered guideline is that the PMD of the link satisfies the inequality width (5.47) The impact of PMD on high-speed, systems is quite severe, as can be seen from Table 5.6. From this table it is clear that long-haul high-capacity systems are very vulnerable to PMD, and the selection of the fiber type is a very important decision in system design. The ITU-T has issued standards for the conventional fiber that are described by the G.652 rec