Double-stage phase-diversity (DSPD) scheme for coherent optical fiber communications—optical circuit imbalance and its compensation

The double-stage phase-diversity (DSPD) scheme we have proposed features a phase-diversity-type optical homodyne detection followed by a phase-diversity-type electrical frequency up-conversion, giving a heterodyne-like intermediate frequency (IF) signal. It has the advantages of both heterodyning and homodyning. In this paper, we first calculate the sensitivity penalty due to the optical circuit (hybrid) imbalance in a DSPD receiver. It is shown that the penalty is as small as that in a conventional phase-diversity receiver and that the optical circuit requirement is not severe. The DSPD experiment with an adjustable imbalance is then performed to confirm the theory; experimental results are in good agreement with the theory. Finally, the possibility of compensation of the optical imbalance at the electronic stage of the receiver is shown.