In this procedure, a modulated light source at a given wavelength is coupled into the component under test, and the phase of the modulated signal exiting the fiber at a first polarization state is compared with the phase at a second, orthogonal polarization state. Once the maximum phase difference has been determined, corresponding to the input states of polarization (SOP's) aligning to the two principal axes of the component, this is converted to a delay difference, and reported as the DGD for the component at that wavelength.

For the purposes of this document, the component is considered to be measured at a single wavelength, and therefore the result reported is stictly the DGD.

This FOTP does not apply to components that have in excess of 10 dB of polarization dependent loss (PDL).