A form of dispersion where the two polarization states of light travel at slightly different velocities causing the optical pulse to statistically spread and possibly get distorted in the time domain. When the broadening becomes too wide, the pulses overlap and produce inter-symbol interference, increasing the bit error rate. PMD is characterized by the frequency or wavelength and varies according to the index of refraction (i.e., index of propagation) of the medium. Since PMD is related to the propagation axes, it leads to birefringence.