sed by twists, bends, splices, localized imperfections, and the like, and it normally varies randomly in time. A pulse of arbitrary polarization launched into the first section splits into two orthogo- nally polarized pulses that travel with different group velocities, as described earlier. There is then polarization mode coupling, and the second section decomposes the fast and slow modes from the first section according to the orientation of its axes. In this way, the pulses are passed from one short birefringent section to the next, experiencing randomly oriented fast and slow axes. Input After After After Section 1 Section 2 Section 3 Fig. 6.27 Accumulation of DGD over a long fiber link assumed to be the equivalent of many shorter birefringent fibers of randomly oriented axes. Chapter 6 Optical Fiber Transmission Equation 6.33 specifies a linear increase of DGD with length over one section. However, different behavior applies when considering many such sections because the changing birefringence can cause either addition or subtraction of the DGD. We must therefore account for the overall DGD accumulation in a stochastic manner by using the statistics of a "random walk." If this is done over a long link, it can be shown that At grows as the square root of length. For this reason, PMD is specified in units of picoseconds of differential group delay per square root kilometer distance traveled: ps/(km)¹/2. When considering an optical link, we can assign a "correlation length," L, which describes whether the DGD grows as the fiber length or as its square root. When L L, the variation is linear and when L >> L, it is as the square root and there is a gradual transi- tion between the two regimes. The correlation length depends on both the intrinsic and extrinsic factors that govern the PMD, but for a modern fiber it is typically 1 km if the fiber is cabled and as little as 1 meter if it is on a bobbin. Two consequences arise: Laboratory bench measurements do not necessarily represent the behavior of installed system