3D QAM-DPSK Optical Transmission Employing a Single Mach–Zehnder Modulator and Optical Direct Detection

We propose a novel optical transmission technique, 3D QAM-DPSK, that employs a single Mach-Zehnder modulator based on a direct detection system. Through 3D QAM-DPSK, the optical signal is simultaneously transmitted via the intensity and phase of the optical source. The QAM signal is transmitted via the intensity of the optical source, similar to conventional optical direct detection systems. The modified DPSK signal is transmitted via the phase of the optical source. As a QAM signal has two axes, and a DPSK signal has a single axis, the optical modulation technique can be treated as 3D QAM-DPSK. In the proposed optical transmission technique, signal transmission capacity and signal performance were enhanced in terms of bit error rate, as compared with ASK-DPSK. The signal transmission performance of the proposed modulation technique was numerically and experimentally demonstrated.