We report on the upgrade at 40 Gb/s and 100 Gb/s of deployed legacy optical networks originally designed for 10-Gb/s nonreturn to zero (NRZ) on–off keying (OOK) data. These networks operate on a 50 GHz grid and incorporate periodically modules to compensate for chromatic dispersion. We discuss here the insertion of coherent polarization-division-multiplexed (PDM)-quadrature phase shift keying (QPSK) and PDM-binary PSK (BPSK) channels in legacy optical systems based on low chromatic dispersion fibers. We first assess the nonlinear tolerance of PDM-QPSK paired with coherent detection as a function of the bit rate. We also investigate the effect of optimizing the power of the PDM-QPSK channels independently of the power of copropagating OOK channels as well as introducing guard bands in the multiplex. We particularly show that 40 Gb/s PDM-QPSK data suffer larger performance penalties from copropagating 10 Gb/s OOK channels than 100 Gb/s data. Finally, we demonstrate that PDM-BPSK is a more attractive solution than PDM-QPSK at the bit rate of 40 Gb/s. Our experiments reveal that the BPSK-based solution is about 7.5 dB more robust to cross nonlinearities than the QPSK-based solution when inserted in one wavelength slot with 50 GHz spacing, originally designed for NRZ-OOK channels at 10 Gb/s.
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