Challenges to FDM-QAM coherent transmission with ultrahigh spectral efficiency

Quadrature amplitude modulation (QAM) is an excellent modulation format for realizing optical communication systems with a high spectral efficiency of much greater that 1 bit/s/Hz. I describe recent progress on 1 Gsymbol/s, FDM (Frequency Division Multiplexed)-QAM coherent transmission experiments by using heterodyne detection with a frequency-stabilized laser and an optical phase-locked loop technique.