The Best Paper Award
256 QAM digital coherent optical transmission using Raman amplifiers
Masato Yoshida　・　Seiji Okamoto　・　 Tatsunori Omiya　・　Keisuke Kasai
Masataka Nakazawa
（英文論文誌B　平成23年2月号掲載）
　Due to the popularization of broadband services such as high-resolution video streaming, the amount of internet traffic is growing at an annual rate of 40 %. To meet the increasing demand to expand transmission capacity, ultrahigh spectral efficient coherent optical transmission, employing multi-level quadrature amplitude modulation (QAM) formats, has attracted a lot of attention. This paper reports experimental results for an ultra-multi-level coherent optical transmission with a 256 QAM format by using the authors’ pioneering hardware and software technologies.
　To increase the spectral efficiency, it is important to increase the multiplicity of the QAM signal. However, more precise control of the signal amplitude and phase are needed to distinguish the symbols with higher multiplicity. In recent work on digital coherent transmission, signal phase error compensation is provided by digital signal processing (DSP). In this system, the error correction algorithm becomes complex with increasing multiplicity, and the phase error is residual. On the other hand, in this paper the phase error is suppressed to within 0.5 degrees by using CW fiber lasers with a linewidth of 4 kHz and an optical phase-locked loop circuit. The phase error is less than the tolerable phase noise of 2 degrees for 256 QAM, which is determined by the phase difference between the nearest symbols. It is also important to note that Raman amplifiers are used to increase an OSNR in the transmission system. As a result, the power penalty after a 160 km transmission of a polarization-multiplexed 4 Gsymbol/s, 256 QAM (64 Gbit/s) signal is successfully reduced from 5.3 to 2.0 dB. In this system, a 64 Gbit/s data signal is transmitted with an optical bandwidth of 5.4 GHz. These results indicate the possibility of achieving a spectral efficiency of 11 bit/s/Hz for multi-channel operation when taking account of the 7% FEC overhead.
　As mentioned above, this paper reports an ultra-multi-level coherent optical transmission with a 256 QAM format realized by using narrow linewidth lasers and OPLL technique, and it achieves excellent performance.

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