Measurement methods and test procedures: Polarisation Mode Dispersion. This part of IEC 60793 establishes uniform requirements for measuring the PMD of singlemode optical fiber, thereby assisting in the inspection of fibers and cables for commercial purposes. PMD causes an optical pulse to spread in the time domain. This dispersion could impair the performance of a telecommunications system. The effect can be related to differential phase and group velocities and corresponding arrival times δτ of different polarization components of the signal. For a sufficiently narrow band source, the effect can be related to a differential group delay (DGD), Δτ, between pairs of orthogonally polarized principal states of polarization (PSP) at a given wavelength. For broadband transmission, the delays bifurcate and result in an output pulse that is spread out in the time domain. In this case, the spreading can be related to the average of DGD values.