Patent ID: 8824886

Claim:
A method of monitoring a differential group delay of an optical communications signal having a polarisation multiplexed modulation format, the method comprising: receiving the optical communications signal in an optical receiver and performing analogue to digital conversion of the optical communications signal to generate a first digitised signal corresponding to a first polarisation of the optical communications signal and to generate a second digitised signal corresponding to a second polarisation of the optical communications signal; applying a polarisation mode dispersion compensation to each of the digitised signals; obtaining an indication of the channel transfer function of the optical communications signal; determining the differential group delay in dependence on the indication of the channel transfer function; determining a delay between a polarisation mode dispersion compensated first digitised signal and a polarisation mode dispersion compensated second digitised signal; subtracting the delay from the differential group delay to obtain a corrected differential group delay; and generating and transmitting a monitoring signal comprising an indication of the corrected differential group delay.