Optical Solutions to Improve PDM-QPSK Resilience Against Cross-Channel Nonlinearities: A Comparison

We compare by simulation different optical methods to improve the resilience of coherent 112-Gb/s polarization-division multiplexing (PDM)-quadrature phase-shift keying (QPSK) wavelength-division-multiplexing (WDM) transmissions against cross-channel nonlinearities. Such methods consist of 1) increasing the line group velocity dispersion (GVD), or 2) the line polarization-mode dispersion (PMD), or 3) inserting in-line cross-phase modulation (XPM) suppressors. Such methods are tested using nonreturn-to-zero (NRZ), aligned return-to-zero (aRZ), and interleaved RZ (iRZ) pulse formats. We show that the nonlinearity-mitigating effect underlying all three methods is an increase of the interchannel decorrelation, obtained by either increasing walk-off (methods 1 and 3) or by depolarizing the WDM channels (method 2). Interchannel decorrelation improves performance and reduces the difference among the three pulse formats. We find that the best performance is achieved by iRZ-PDM-QPSK in a dispersion-managed link with an XPM suppressor at each span.