When light travels through matter it suffers power loss. One of the contributors to power loss is polarization. Virtually all optically transparent materials affect to some degree the polarization state of light. In general, optically transparent materials exhibit a spatial polarization distribution. As an optical signal passes through it, it suffers power reduction or optical power loss in selective directions due to spatial polarization interaction; this loss is wavelength dependent and is known as polarization dependent loss (PDL). It is measured in decibels (dB). PDL affects the signal quality and system performance. At low data rates, PDL is a minor contributor to loss. However, at 10 Gbit/s and above, PDL becomes comparable to insertion loss (IL).

Therefore, at high bit rates PDL needs careful examination. The surprising result here is that over a span with many connectors the total loss due to PDL is not an algebraic sum. To explain this, consider two cascaded PDL elements, A and B. Element A attenuates the optical signal due to a partial polarization rotation (PDLA). This partially distorted signal enters element B, which has a random polarization orientation and properties with element A. Therefore, the partially polarized light from element A undergoes another stage of polarization distortion and loss as it enters B (PDLB). As an example, if the two elements have parallel orientations and similar characteristics, the total PDL is LPDL,1–2 = (LPDL,1 + LPDL,2)/(1+ LPDL,1LPDL,2). The power loss due to polarization of the received signal may vary by ~0.5 dB. In worst-case optical transmission design, the maximum loss level (~ –0.5 dB) should be used, whereas in typical performance design an average level (~ –0.1 dB) is used. This value does not change with respect to the center wavelength of the received signal. However, asymmetric spectral polarization loss causes asymmetric amplitude signal distortions and the signal may appear with shifted center wavelength.