Subcarrier multiplexing based self-heterodyne coherent detection for PM-16QAM format

We propose and demonstrate subcarrier multiplexing (SCM) based self-heterodyne (SH) coherent detection for polarization-multiplexed 16 quadrature amplitude modulation (PM-16QAM) format. Three closely spaced subcarriers carrying pilot-tone (PT) and two 16QAM signals, respectively, are generated via subcarrier modulation technique. An effective digital signal processing (DSP) scheme is proposed to eliminate image frequency interference (IFI) arising in such SH coherent detection. Meanwhile, a closed-form analytical BER formula is verified by numerical simulations with a deviation less than 0.3 dB at BER=10−3. The concept of optimum modulation index (MI) ratio between data and PT signal is derived and verified, in order to reduce the distortion of 16QAM constellation. Meanwhile, we also explore the effects of optical filter bandwidth and subcarrier frequency spacing on the quality of PT signal. Finally, we are able to obtain several optimum parameters for the proposed SH coherent detection.