DGD monitoring issues in high-speed polarisation multiplexed coherent QPSK systems

Optical performance monitoring based on digital signal processing (DSP) of a polarisation diverse coherent receiver can allow the estimation of the differential group delay (DGD) of the link fibre. In this reported work, it was found that the DGD estimation can be affected by substantial errors, since the DSP can introduce or leave uncompensated any symbol-time delay between orthogonal polarisations and cannot discriminate the in-line DGD from any other link delay. These issues are investigated and a method is proposed to partially overcome the problems.