In fiber optics, the differential group delay, which is often shorted as DGD is the time difference between the fractions of a pulse that are transmitted in the two principal states of polarization of an optical signal. Differential Group Delay in a long distance (several kilometers) optical fiber link can be statistically modeled as having Maxwellian distribution by assuming random polarization mode coupling.
The maximum differential group delay is defined to be the value of DGD that the system must tolerate with a maximum sensitivity degradation of approximately 1 dB. The relationship between maximum DGD and mean DGD can only be defined probabilistically. This is due to the statistical nature of polarization mode dispersion (PMD),
The probability of the instantaneous DGD exceeding any given value can be inferred from its Maxwellian statistics. Therefore, we can derive the equivalent mean DGD by dividing by the ratio of maximum to mean that corresponds to an acceptable probability provided we know the maximum DGD that the system can tolerate.