256 QAM digital coherent optical transmission using Raman amplifiers

Masato Yoshida, Seiji Okamoto, Tatsunori Omiya, Keisuke Kasai, Masataka Nakazawa

Research output: Contribution to journalArticlepeer-review

3 Citations (Scopus)


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. We describe fundamental technologies for ultrahigh spectral density coherent QAM transmission and present experimental results on polarization-multiplexed 256 QAM coherent optical transmission using heterodyne and homodyne detection with a frequency-stabilized laser and an optical phase-locked loop technique. In this experiment, Raman amplifiers are newly adopted to decrease the signal power, which can reduce the fiber nonlinearity. As a result, the power penalty was reduced from 5.3 to 2.0 dB. A 64Gbit/s data signal is successfully transmitted over 160 km with an optical bandwidth of 5.4 GHz.

Original languageEnglish
Pages (from-to)417-424
Number of pages8
JournalIEICE Transactions on Communications
Issue number2
Publication statusPublished - 2011 Feb


  • Coherent transmission
  • Frequency-stabilized laser
  • Optical phase-locked loop
  • Quadrature amplitude modulation
  • Spectral efficiency


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