Insertion of 100Gb/s Coherent PDM-QPSK Channels over Legacy Optical Networks Relying on Low Chromatic Dispersion Fibres

We investigate the potential of 100 Gb/s polarization division multiplexed (PDM-) quadrature phase shift keying (QPSK) paired with coherent detection to enable the future capacity upgrade of current networks designed for 10 Gb/s nonreturn to zero (NRZ) on-off keying data. We experimentally assess the tolerance to nonlinear effects of such a solution over optical systems based on low chromatic dispersion fibres. We particularly study the performance penalties brought by copropagating channels (either PDM-QPSK or 10 Gb/s NRZ) onto 100 Gb/s PDM-QPSK data, in which both polarisation tributaries are over-modulated with return-to-zero and temporally interleaved by half a symbol period. Besides, we investigate one option to overcome these limitations: introducing band-gaps in the multiplex.