polarisation dependent loss (PDL)

When light passes through matter, power losses occur. One of the causes of this is polarization dependent loss, or PDL.

Since almost all optical materials affect the polarization of light, the PDL attenuation of optical fibers changes with the polarization state of the propagating light waves. The polarization-dependent attenuation results from the maximum difference in light transmission in an optical waveguide, under all possible polarization states. Polarization can result from strong or less strong absorption, from bending of the optical waveguide, from different optical density and from mode reflection.

To avoid polarization-dependent losses, there are polarization-maintaining fibers in which linearly polarized light retains the initial polarization as it passes through the optical fiber.