A method for dual polarisation optical transmission is disclosed. The method comprises splitting a continuous wave light source into first and second sub-channels, optically modulating each sub-channel with a data signal, and superimposing a first pilot tone onto the first optically modulated sub-channel and a second pilot tone, different from the first pilot tone, onto the second optically modulated sub-channel. The method further comprises polarisation multiplexing the first and second sub-channels to form a polarisation multiplexed signal, in which the first and second sub-channels have orthogonal states of polarisation, and transmitting the polarisation multiplexed signal.