Polarization-dependent loss (PDL) for a component or system is the maximum, peak-to-peak insertion loss (or gain) variation caused by a component when stimulated by all possible polarization states (see Figure 1). It is specified in dB units. Polarization-dependent loss may also be referred to as polarization sensitivity, polarization-dependent gain (PDG) or extinction ratio (for optical polarizers).
Some components are designed for maximum PDL. A linear optical polarizer, for example, must have high PDL in order to convert un-polarized light into linearly polarized light. Only one orientation of linearly polarized light passes through the polarizer un-attenuated. Misaligned orientations of polarized light are attenuated by the polarizer’s PDL.
In other situations, any amount of PDL is a liability. Long-haul telecommunication systems, for example, are more cost effective as transmission distances between amplification stages become longer. Transmission-distance calculations are partly based on guaranteed power levels. Large variations in system power occur as the PDL of individual system components randomly combine. This makes power-budget calculations more difficult, expands design margins and reduces guaranteed performance.