Measurement of Polarization-Dependent Loss for Plane Diffraction Gratings

Components in fiber-optic telecommunications networks must be insensitive to changes in the polarization state of the signal, a requirement that is particularly challenging for surface-relief diffraction gratings since those with metallic coatings tend to polarize the light incident on them. The measurable quantity used to evaluate the sensitivity of a network component to changes in polarization is called polarization-dependent loss (PDL).

There exists a class of gratings, however, for which the diffraction efficiency in the two polarization modes is nearly equal over a given wavelength range; these gratings are called low-PDL gratings.

The definition of PDL used by Richardson Gratings in the characterization of its diffraction gratings is

PDL = 10log₁₀(E_P/E_S)

where E_P and E_S are the measured diffraction efficiencies for P- and S-plane polarized incident light, respectively. PDL is expressed in decibels. Sometimes PDL is expressed as the absolute value of Eq. (1), but allowing PDL to be a signed (positive or negative) quantity provides more information regarding how the efficiencies vary (vs. wavelength) with respect to each other.

At Richardson Gratings, PDL is determined by separately measuring the P- and S-plane diffraction efficiencies and then substituting the relevant values into the above Equation. These independent measurements, each made with linearly polarized incident light, thus generate values of E_P and E_S. These measurements may be of absolute or relative efficiency (provided this is done consistently).