Polarization modal dispersion
Optical fibers for telecommunications

A single-mode fiber in the usual sense allows the propagation of a single mode (LP₀₁), but this mode is degenerate, i.e. it can be decomposed into two independent basic modes with orthogonal polarizations.

Ellipticity (a real fiber is never perfectly circular) and external stresses result in birefringence whose axes change randomly along the fiber. The result is coupling between the two basic modes, so that when a pulse is sent into the fiber, two pulses separated by a random delay are received at the end, giving rise to an echo phenomenon, generating interference between symbols (figure 17). The theory of this complex phenomenon will not be developed here, and we will confine ourselves to presenting the main results.