PMD (Polarization Mode Dispersion)

PMD is a polarization-dispersion phenomenon in which two perpendicular polarization components of light propagate through a fiber at slightly different speeds due to imperfect geometric symmetry of the fiber core.

What causes PMD?

A perfectly circular fiber core would guide both polarizations at identical speeds, but in practice mechanical stress, small geometric deviations, and cable bending introduce birefringence, causing a random, time-varying differential group delay (DGD) between the polarizations.

Applications

PMD analysis matters when designing long-haul, high-bit-rate links (40G, 100G, and beyond), where a DGD comparable to the duration of a single bit can significantly degrade BER — modern receivers use digital PMD compensation (DSP).

Related terms

Chromatic dispersion

Coherent detection

BER