Polarization mode dispersion, abbreviated as PMD is the broadening of pulse due to the difference in velocities of two orthogonally polarized light signals. In an optical fiber, the light signals undergoes polarization due to birefringence . Birefringence happens in optical fiber due to many factors including internal and external stresses and strains.
The two signals then start travel along the length of fiber. Depending on the obstacles on their way, they may not reach at the same time at the end of the fiber. The delay of the slowest signal compared to the fastest one is expressed in the unit of pico-seconds. The delay is due to the dispersion of polarized light signals and is called Polarization Mode Dispersion.