| Development of a 10-GHz Digital Coherent Optical Transmission System |
| Masahito Tomizawa , Hiroshi Onaka , Kazuro Kikuchi |
 Masahito Tomizawa |
 Hiroshi Onaka |
 Kazuro Kikuchi |
In 2004, Professor Kazuro Kikuchi proposed a digital coherent transmission scheme that is characterized with a hybrid technology with coherent transmission and digital signal processing. The electrical complex amplitude, real and imaginary parts, of the received optical signal is converted into digital data by analog-to-digital converters and the signal distortion was compensated by a digital signal processor; ultimately, the original signal was recovered. In the 1980s, considerable research was carried out on coherent optical transmission techniques using the phase and the amplitude of the light signal for transmitting information. However, this research once lost its substantial benefits by the development of IM/DD and optical fiber amplifier technologies in the 1990s. In the concept of digital coherent optical transmission system, since the polarization fluctuation and phase noise of the signal light are compensated by using dramatically advanced very large scale CMOS technology in the digital domain, stability of the system is dramatically improved in principle, as compared to the legacy coherent system. Chromatic dispersion compensation in the electrical domain is possible in the system; therefore, the chromatic dispersion compensation fibers that were used in long-distance transmission systems become unnecessary. Multi-level modulation and demodulation, and polarization multiplexing and de-multiplexing, are also possible, and transmission capacity can be expanded significantly. While these features mean that realization of the economical transmission system with large capacity can be expected, there has been an issue concerning how to realize the key part, a large digital signal processing circuit and an A-D converter with high-speed sampling above the baud rate of the optical signal.
Dr. Masahito Tomizawa and Mr. Hiroshi Onaka formed a consortium framework for open innovation in conjunction with several enterprises (NTT, NEC, Fujitsu and Mitsubishi Electronics) each of which has excellent optical transmission technologies to develop the digital signal processing LSI that enables long-distance transmission of 100Gps/lambda. In a short period of just three years, commencing in 2009, they successfully conducted technical development of the LSI under the advice of Prof. K. Kikuchi, et al. The developed digital signal processing LSI integrates outstanding functions, such as a polarization-multiplexed QPSK (Quadrature Phase Shift Keying) modulation scheme of 127 G bps, high-speed estimation of and compensation for chromatic dispersion of single-mode fiber transmission line of 2000 km or longer, fast polarization control for separating the signal component of horizontal and vertical polarization while following the polarization fast variations which could occur in the field optical fiber, polarization mode dispersion compensation, highly efficient soft decision forward error correction, etc. This LSI is the enabler of economical 100G-WDM systems of the world's leading vendors, including, of course, Japanese companies, under the evaluation that "the advanced functionalities of the LSI are very effective for practical systems". These contributions by the award winners have that the trend of high-capacity optical transmission system is transferred to the digital coherent system.
As described above, the award winners proposed the world’s first digital coherent optical transmission system. Then, they built a consortium framework that goes beyond organizational boundaries, and they succeeded in mobilizing and implementing intellectual property of consortium members in Japan. And, finally, they were the first to develop, promote and standardize in de-fact a highest-performance 100Gbps optical transmission technology, which is widely used in all-over-the-world.
Figure 1 Digital coherent transmitter and receiver configuration (5)
(２) Tsukamoto, S., Ly-Gagnon, D., Katoh, K., Kikuchi, K., "Coherent demodulation of 40-Gbit/s polarization-multiplexed QPSK signals with 16-GHz spacing after 200-km transmission," OFC/NFOEC2005, PDP29, 2005
(３) E. Yamazaki, S. Yamanaka, Y. Kisaka, T. Nakagawa, K. Murata, E. Yoshida, T. Sakano, M. Tomizawa, Y. Miyamoto, S. Matsuoka, J. Matsui, A. Shibayama, J. Abe, Y. Nakamura, H. Noguchi, K. Fukuchi, H. Onaka, K. Fukumitsu, K. Komaki, O. Takeuchi, Y. Sakamoto, H. Nakashima, T. Mizuochi, K. Kubo, Y. Miyata, H. Nishimoto, S. Hirano, and K. Onohara, “Fast optical channel recovery in field demonstration of 100-Gbit/s Ethernet over OTN using real-time DSP,” Optics Express, vol. 19, pp. 13179-13184, 2011
(４) Masahito Tomizawa, “DSP aspects for deployment of 100G-DWDM systems in carrier networks,” OFC/NFOEC, 2012
(５) S. Suzuki, Y. Miyamoto, M. Tomizawa, T. Sakano, K. Murata, S. Mino, A. Shibayama, M. Shibutani, K. Fukuchi, H. Onaka, T. Hoshida, K. Komaki, T. Mizuochi, K. Kubo, Y. Miyata, Y. Kamio, “R&D of digital coherent signal processing technologies for a large-capacity optical communication network," Journal of the IEICE, vol. 95, No.12, pp. 1100-1116, 2012 (in Japanese)
