Solution:

Polarization mode dispersion is the polarization dependence of the propagation characteristics of light waves in optical fibers.

It can result from random birefringence and then needs to be described with statistical methods.

In single-mode fiber, we really have not one but two modes (traveling on physically the same path). This is because light can exist in two orthogonal polarisations. So we can send two possible signals without interference from one another on single-mode fiber if their polarisations are orthogonal.

In ordinary single-mode fiber, a sign consisting of both polarisations. However, polarisation states are not maintained in standard SM fiber.

During its journey light couples from one polarization to the other randomly.

Birefringence is the name given to the characters found in some materials and in some geometries where the ray path exhibits a different refractive index to the different polarisations.

This happens in normal single-mode fiber in that there is usually a very slight difference in RI for each polarization.

It can be a source of dispersion but this is usually less than .5 ps/nm/km (for most applications trivial). The effect is to cause a circular or elliptical polarisation to form as the signal travels along the fiber.

Dispersion resulting from the birefringent properties of fiber is called “Polarisation Mode Dispersion” (PMD).