NTT has successfully tested 400 Gbps/channel-class digital coherent optical transmissions technology in conjunction with NEC and Fujitsu.
The tests multiplexed up to 62 channels of 400 Gbps. This demonstrated fiber capacity of up to 24.8 Tbps. The tests covered various modulation methods over distances ranging from several thousand kilometers up to 10,000 km.
The companies said these tests accelerate the effort to commercialize 400 Gbps-class optical transmission technology. Japan’s Ministry of Internal Affairs and Communications (MIC) sponsored the testing as part of its “Research and Development Project for the Ultra-high Speed and Green Photonic Networks” program.
Key technologies demonstrated included:
- Extremely flexible 400 Gbps-class adaptive modulation/demodulation. In addition to Quadrature Phase Shift Keying (QPSK), which is used in existing 100 Gbps transmissions and which superimposes information on the phase of the light, an 8 Quadrature Amplitude Modulation (QAM) and a 16 QAM were used, superimposing information on both the phase and amplitude of the optical waves to expand data volume, and combined with sub-carrier multiplexing enabled by spectral compression technology called Nyquist filtering. The companies cited clear advantages by selecting a modulation format appropriate for the quality of the link. This required the development of an algorithm that can be implemented in an electronic circuit including an 8 QAM. Transmission ranges of 500 km to 1500km for capacities of 10 — 20 Tbps per each core of optical fiber were successfully covered, which was not possible up until now, even with QPSK and 16 QAM.
- Compensation function using digital backward propagation signal processing, enabling long-distance transmissions. To overcome complex waveform distortions caused by nonlinear optical effects, the companies developed digital backward propagation signal processing, which, through refinements to the algorithm and circuit designs that dramatically reduced the volume of calculations, enabled circuit implementation and compensation of the nonlinear optical effects. They also developed chromatic dispersion estimation technology enabling estimations, for 10,000km of optical fiber, of the values of chromatic dispersion, which is a phenomenon in which the propagation lag times differ for each wavelength in an optical fiber.
More online.
