Measurement of Polarization-Dependent Loss for Plane Diffraction Gratings

Polarization-dependent loss (PDL) is an important characteristic of optical components used in telecommunications networks. This Technical Note provides the precise definition that Richardson Gratings uses for the PDL of its gratings, and explains the measurement procedure used to determine PDL values.

Polarization-Dependent Loss (PDL)

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.

Definition of PDL

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

PDL = 10log10(EP/ES)

where EP and ES 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.

Measurement of PDL

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 EP and ES. These measurements may be of absolute or relative efficiency (provided this is done consistently).