In passive optical components, Loss that varies as the Polarization state of the propagating wave changes. Expressed as the difference between the maximum and minimum loss in decibels. It is abbreviated PDL.

Polarization dependent loss is a measure of the peak-to-peak difference in Transmission of an optical component or System with respect to all possible states of polarization. It is the ratio of the maximum and the minimum transmission of an optical device with respect to all polarization states.

The polarization dependence of the transmission properties of optical components has many sources. Some of the most common effects are:

• Dichroism
• Fiber bending
• Angled optical interfaces
• Oblique reflection.