Differential Group Delay, DGD
For a polarization maintaining fiber, this is a measure of the difference in transit time for light launched into the fast axis and light launched in the slow axis. This value is determined solely by birefringence B (or the beat length, Lp , at wavelength, λ).
For a PM fiber the interferogram is that of a weakly coupled system – weakly coupled since the waveguide parameters of the fast and slow axis are quite different and the two modes will propagate without significant interaction.
There is a peak at zero order where most of the interference takes place, and side lobe peaks at higher orders, symmetrically appearing either side of the peak, where the separation corresponds to the strength of the separation. In a weakly coupled system (where fast and slow axes are distinct and light tends to remain on one or the other) the interferogram has clearly defined peaks where the temporal separation corresponds to maxima in both axes.
If a non-PM fiber is measured then the interferogram is that associated with a strongly coupled system and the energy is distributed in the form of a Gaussian. If this measurement were made in the wavelength domain then the distribution is Maxwellian in nature. A strongly coupled system (where energy readily mixes across the axes) may yield an interferogram with some correlation peaks apparent either side of the zero-order, but the DGD can also simply be inferred from the width of the distribution.