Enhancing the coherent detection of optical QAM signals by exploiting the information of received optical power

In current optical coherent communication systems, balanced photo-detectors are typically employed, which preserve the product of the received optical signals and the local oscillators (LO) for detection but cancel out the power of both. Since only the product of the received optical signals and the LO is used to estimate the transmitted complex symbol, such detection scheme is suboptimal due to the loss of some useful information embedded in the transmitted signal power. Different from that, this paper proposes a new detection scheme which maintains full information obtained from the photo-detectors. Specifically, we first extract the transmitted signal power from the output of the photo-detector. After that, we use not only the product of the signals and the LO but also the power of signals to estimate the target signals. For 16-qudrature-amplitude-modulation (16-QAM) and 64-QAM signals, the simulation results show that our new scheme can outperform the conventional ones by more than 1 dB.