Tolerance of Duobinary and NRZ-OOK Signals to Second-Order Polarization-Mode Dispersion

The sensitivity of tightly filtered duobinary and conventional nonreturn-to zero on-off-keyed (NRZ-OOK) signals to second-order polarization-mode dispersion (SO-PMD) is assessed through extensive numerical simulations. On average, duobinary signals are significantly more sensitive to SO-PMD than to ordinary fiber chromatic dispersion (CD), whereas NRZ-OOK signals are more tolerant to SO-PMD than to CD. This seemingly contradicting behavior is explained by the difference in the frequency dependence of SO-PMD and CD. In addition, duobinary signals suffer from residual differential group delay, even after first-order PMD compensation, which is introduced between signal components that are transmitted in the launched principal state of polarization (PSP) and those that are coupled from the launched to the orthogonal PSP.