“…One way to resolve these problems is to use self-heterodyne (SH) coherent detection. Previously, optical SH coherent system, where a continuous wave (CW) pilot-tone (PT) light was wavelength multiplexed with differential quadrature phase shift keying (DQPSK) signals before transmission and was finally chosen as the LO light at the receiver side, was proposed to simplify the optical coherent detection [19], and improve the tolerance of laser phase noise [20]. Secondly, in order to simultaneously detect all subcarrier channels and avoid image frequency interference (IFI) arising from the energy in the mirror spectrum of the LO, two LOs are indispensable for classical intradyne detection of both subcarriers [14].…”