Summary : In single-mode fibers with non-perfect geometry, two polarization modes exist due to mechanical stress, fiber bends, or twists. They experience different group velocity, which results in a broadening of the transmitted light pulses. This effect is called polarization mode dispersion (PMD). Field tests showed that the PMD and the resulting additional signal degradation varies from fiber to fiber. It is statistic in nature and depends on time, on wavelength, and on the input polarization of the signal to be transmitted. For network operators and equipment manufacturers it is important to test fiber transmission systems with respect to their vulnerability to PMD, especially if high bit rates (e.g. 10Gbit/s) are involved. However, when employing long fiber sections, the time variation of PMD-induced signal distortion makes testing difficult. The solution is to use of a time-invariant PMD emulator, which provides similar PMD properties as the real fiber, in combination with a special test routine..