Fujitsu Ltd, NTT Corp., and NEC Corp. say they have made a significant step towards commercialization of 400G optical transmission technologies. As part of research funded by the Japanese government, the companies say they have successfully tested and verified coherent transmission of 400-Gbps signals over distances up to 10,000 km.
The 400G channels were wavelength division multiplexed using up to 62 channels to achieve total capacity between 12.4 and 24.8 Tbps, depending on the distance and modulation method.
This technology, which has been built into an optical transceiver, will make it possible to quadruple optical transmission capacity versus the current use of 100-Gbps wavelengths over existing optical fiber, the companies claim.
A number of new technologies were developed to make this possible. The Japanese partners developed advanced quadrature amplitude modulation (QAM) schemes – 8-QAM and a 16-QAM were used – as well as sub-carrier multiplexing enabled by Nyquist filtering spectral compression technology.
Adaptive modulation also was used, which selects a modulation format in response to the characteristics of the optical transmission links. Therefore, the same hardware can support various modulation/demodulation formats in response to the conditions of the transmission line, such as transmission distance. This technology enables a highly adaptable and flexible network, the partners say, echoing vendors who also have added this capability to their systems.
Compensation using digital signal processing was necessary to achieve such long-distance transmissions. It is necessary to compensate for complex waveform distortions caused by nonlinear optical effects, which are generated by high-power optical signals that modulate the optical fiber's refractive index in accordance with the intensity of the optical signal. These distortions would otherwise limit the power of incoming optical signals into the optical fiber, which would in turn limit the transmission distance that can be achieved.
Up until now, however, compensating for the nonlinear optical effects of multi-level modulation signals within the optical fiber was difficult because the complexity of the electronic circuit was greater than could be implemented. This has been the primary limiting factor standing in the way of extending the distance of transmissions, the companies assert.
To overcome this problem, the companies developed "backward propagation signal processing" that, through refinements to the algorithm and circuit designs that dramatically reduced the volume of calculations, enabled the digital signal processing circuit to be implemented, and thus compensate for the nonlinear optical effects. They also developed technology to estimate the values of chromatic dispersion for 10,000 km of optical fiber
Moreover, a high-performance MSSC-LDPC (multiple-structured spatially coupled type low-density parity-check) error-correction code was used to further extend transmission distances. As a result of these technologies, the amount of equipment needed for long-haul transmission can be reduced, leading to expectations that the network would also consume less electricity, the partners say.
By combining the various technologies, Fujitsu, NTT, and NEC successfully performed straight-line (not recirculating) transmission tests for optically repeatered transmissions of up to 10,000 km over a setup emulating a submarine cable transmission link and optically repeatered transmissions of up to 3,000 km over a setup emulating a terrestrial transmission link.
The transmission tests were based on joint research with Japan's National Institute of Information and Communications Technology (NICT), and were performed using NICT's test equipment.
Based on these results, the companies hope to move forward quickly with development and commercialization of 400-Gbps optical transmission technology with the goal of creating a world-leading optical network that delivers flexibility along with ultra-high speeds and low power consumption. In addition, they will collaborate with institutions inside and outside Japan to deploy their achievements on a global scale.
The work was commissioned and sponsored by Japan's Ministry of Internal Affairs and Communications (MIC) as part of its "Research and Development Project for the Ultra-high Speed and Green Photonic Networks" program. It builds on previous work carried out under the MIC's "Research and Development on High Speed Optical Transport System Technologies" program (fiscal 2009) and "Research and Development on Ultra-high Speed Optical Edge Node Technologies" program (fiscal 2010-2011).
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