idths. In that case we can assume that the birefringence they encounter is wavelength independent. The regime is called the "first-order PMD approximation," and its effect is seen in equation 6.33 for a short-span fiber in which we can ignore the dispersive term, d(An)/ In longer fiber spans we must perform statistical averaging and take care to account for the correct evolution of the state of polarization through a matrix-vector formulation (Haus & Phua, 2005). By this it can be shown that in the first-order approximation we can identify two orthogonal launch states of polarization that provide an undistorted output pulse. These are known as the fiber link's "Principal States of Polarization" (PSPs). Much of the extensive theoretical literature on polarization effects in optical fiber has been formulated in terms of PSPs. The first-order description of PMD becomes increasingly unrealistic in high-datarate systems, where the pulses are shorter and SO have broader bandwidths. In these cases the theory must be generalized to account for the dispersive effects that, as equation 6.33 shows, result from terms in d(An) / do. In a long fiber, as illustrated in Figure 6.27, An leads to terms that can be expanded as a Taylor series within the matrix-vector formulation. We can thus identify second- and higher-orders of the PMD. Second-order PMD has two components. The first, and normally the smaller of the two, is Polarization-Dependent Chromatic Dispersion (PCD), in which the DGD varies with wavelength, leading to polarization-dependent pulse compression or broadening. PCD causes the fiber's effective dispersion to change rapidly with wavelength. The other com- ponent of second-order PMD is called PSP depolarization. Commonly larger than PCD, it is a rotation of the PSPs with frequency. PSP depolarization causes spiking and the genera- tion of satellite pulses. Spun Fibers for Low PMD The optical fibers that were manufactured and deployed in the 1980s and early 1990s had numerous asymmetries that caused the intrinsic PMD