Abstract
In recent years, with the aim of keeping up with the rapid increase in transmission capacity, coherent
optical transmission employing multi-level modulation formats has been intensively studied. In
particular, quadrature amplitude modulation (QAM), in which information is encoded on both the
amplitude and phase of the optical carrier, has attracted a lot of attention because of its ability to achieve
higher spectral efficiency (SE) than other formats. Indispensable to the realization of ultra-multilevel
QAM coherent optical transmission with an extremely high SE are narrow linewidth lasers and a precise
optical carrier phase synchronization scheme between the transmitted data and the local oscillator. In this
paper, we summarize our recent progress on ultra-multilevel QAM coherent transmission using a
homodyne detection method by employing a frequency-stabilized fiber laser as a coherent light source,
an optical phase-locked loop circuit, and an injection locking scheme for carrier phase synchronization
at the receiver.
