256 QAM Digital Coherent Optical Transmission Using Raman Amplifiers

Masato YOSHIDA  Seiji OKAMOTO  Tatsunori OMIYA  Keisuke KASAI  Masataka NAKAZAWA  

IEICE TRANSACTIONS on Communications   Vol.E94-B   No.2   pp.417-424
Publication Date: 2011/02/01
Online ISSN: 1745-1345
DOI: 10.1587/transcom.E94.B.417
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on Extremely Advanced Optical Transmission Technologies and Transmission Optical Fiber Technologies towards Exabit Era)
coherent transmission,  quadrature amplitude modulation,  spectral efficiency,  frequency-stabilized laser,  optical phase-locked loop,  

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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 64 Gbit/s data signal is successfully transmitted over 160 km with an optical bandwidth of 5.4 GHz.