Abstract
To meet the increasing demand to expand wavelength division multiplexing (WDM) transmission capacity, ultrahigh spectral density coherent optical transmission employing multi-level modulation formats has attracted a lot of attention. In particular, ultrahigh multi-level quadrature amplitude modulation (QAM) has an enormous advantage as regards expanding the spectral efficiency to 10 bit/s/Hz and even approaching the Shannon limit. This chapter describes fundamental technologies for ultrahigh spectral density coherent QAM transmission and presents experimental results on 128 QAM and 64 QAM-orthogonal frequency division multiplexing (OFDM) transmissions using heterodyne detection with a frequency-stabilized laser and an optical phase-locked loop technique.
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