We report a multi–dimensional quadrature amplitude modulation with M-ary differential phase shift keying (QAM-MDPSK) modulation for optical access network transmission systems employing a Mach–Zehnder modulator (MZM) and a phase modulator (PM) in a direct detection system. Even though several problems that occur in a QAM-DPSK employing a single MZM were solved, inter-signal distortion between two modulations remains when the optical phase signal is converted into an optical intensity signal for detection. To maximize the transmission capacity of QAM-MDPSK, the effect of inter-signal distortion in QAM-MDPSK need to be analyzed for optimization. Mathematical analysis and simulations were performed to evaluate inter-signal distortion. Furthermore, experimental transmission on the proposed QAM-MDPSK modulation has been performed to verify the inter-signal distortion analysis.
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