Polarization mode dispersion (PMD) is one of the most important limiting factors in high capacity wavelength-division-multiplexed (WDM) systems. The PMD-induced penalty depends on various factors such as the modulation format, the state of polarization (SOP) of the input signal, and the differential group delay (DGD) of the transmission link. Recently, it has also been reported that the PMD-induced penalty depends strongly on the receiver characteristics such as the bandwidth of electrical filter and the decision threshold. This is because the optimum decision point varies as the PMD value changes. Thus, it should be possible to avoid the excessive PMD-induced penalty by optimizing the decision threshold of receiver. However, most of conventional receivers are not optimized for PMD and designed to operate at the fixed decision threshold, while the polarization state of the signal varies randomly due to the environmental conditions such as temperature and pressure. Thus, for the reduction of the excessive PMD-induced penalty, it is necessary to develop a new receiver that can adjust its decision threshold adaptively.
In this thesis, the optimum decision threshold was analyzed as a function of the PMD values. Using this analysis, a simple decision-threshold tracking technique was developed to reduce the PMD-induced penalty. In addition, the performance of this technique was evaluated in the systems using return-to-zero (RZ) and nonreturn-to-zero (NRZ) signals. The results show that this technique could reduce the PMD-induced power penalty of RZ signal to less than 1 dB, even when the DGD were as large as 50 ps.
