Quadrature amplitude modulation (QAM) is an excellent modulation format for realizing optical communication systems with a high spectral efficiency of much greater than 1 bit/s/Hz. We describe QAM coherent optical communication that we achieved by using heterodyne detection with a frequency- stabilized fiber laser and an optical phase-locked loop (OPLL) technique. The phase error variance of the intermediate frequency signal of the OPLL was 6.1 × 10-3 rad. A 1-Gsymbol/s 64-QAM coherent signal was successfully transmitted over 150 km.
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