
Generally speaking, the birefringence changes with frequency at each point
along the fiber, as indicated in Figure 17.1, and so Eqn (17.5) must be integrated
anew at each frequency. This leads to changes in PMD (and hence DGD) with
optical frequency, and an experimental example of this is shown in Figure 17.2, for
which the DGD of a fiber is plotted at two different times. We will return to this
figure later, but note here that the DGD looks like a random function of frequency,
with large relative variations. The randomness can be appreciated in view of
Eqn (17.5), which imply that at different frequencies (or wavele ngths), while the
rotations in