A maximum-likelihood sequence estimation (MLSE) based equalizer with Viterbi algorithm is designed in this paper. It
can effectively compensate the intersymbol interference (ISI) caused by the polarization mode dispersion (PMD).The
simulation results prove that the electronic equalizer, in 40Gbit/s non-return-to-zero (NRZ) optical system with Gaussian
noise, can mitigate the polarization mode dispersion (PMD) and improve performance of the communication system. At
the last, we compared the MLSE equalizers and DFE(FFE),we can come to the conclusion that the MLSE equalizer is
the best way to combat polarization mode dispersion(PMD) and it can improve performance of system effectly.