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The performance degradation caused by polarization-dependent loss (PDL) in optical fiber transmission systems in both the absence and presence of polarization-mode dispersion (PMD) is studied. First a simple theory is given to show how PDL affects the system performance and then extensive simulation results are presented. We show that PDL causes a much larger fluctuation of optical-signal-to-orthogonal-noise ratio (OSNRort) than that of optical-signal-to-parallel-noise ratio (OSNRpar), but OSNRpar has a much larger impact on the system performance than OSNRort. We find that when there is no PMD, the system performance degradation induced by PDL can be effectively reduced by suppressing OSNRpar fluctuation. However, the presence of PMD in the system could significantly reduce the efficiency of the PDL mitigation technique, especially the mitigation technique that suppresses the OSNRpar fluctuation.