TOPICS / Ultrahigh-Speed / Spectrally-Efficient Coherent Optical Transmission

According to a report from the Japanese Ministry of Internal Affairs and Communications, the volume of data traffic transmitted in Japan reached as high as 1 Tbit/s in 2009, and it is still increasing by 40% annually. The growing demand for a larger transmission capacity in backbone optical networks has made it important to increase the bit rate per channel and spectral efficiency simultaneously to take full advantage of the available bandwidth resources.

Figure 1 Single-channel 1.92Tbit/s, 64QAM ultrahigh-speed multi-level transmission experiment.

Figure 1 Single-channel 1.92Tbit/s, 64QAM ultrahigh-speed multi-level transmission experiment.

To meet this goal, we have developed an ultrahigh-speed and highly spectrally efficient transmission system, in which data are encoded on the amplitude and phase of coherent optical pulses and then time-division multiplexed in the optical domain. Figure 1 shows an overview of the transmission system. This scheme features an ultrahigh bit rate beyond the limit of electronics as well as a large increase in spectral efficiency due to the adoption of multi-level quadrature amplitude modulation (QAM), which results in highly efficient bandwidth utilization and reduced power consumption. By employing 64 QAM modulation in 10 GHz coherent Gaussian pulses and multiplexing them to 160 Gsymbol/s, we achieved a single-channel bit rate of 1.92 Tbit/s and successfully transmitted a signal over 150 km. Despite such an ultrahigh bit rate, the system involves only electronic devices operating at 10 GHz and passive optical devices, and therefore the power consumption can be greatly reduced.

Figure 2 Comparison between (a) conventional TDM and (b) proposed Nyquist TDM transmission.

Figure 2 Comparison between (a) conventional TDM and (b) proposed Nyquist TDM transmission.

It has been well known that with conventional Gaussian or sech short optical pulses, it is difficult to improve the spectral efficiency even with higher-order QAM. This is because these short pulses inevitably occupy a large bandwidth. To overcome this bottleneck, we proposed a novel high-speed and highly spectrally efficient transmission scheme using an "optical Nyquist pulse." An optical Nyquist pulse and its comparison with conventional pulses are shown in Fig. 2. Unlike conventional pulses, it has a periodically oscillating tail in the time domain and does not decay rapidly. This means that the bandwidth of the Nyquist pulses is much smaller than that of Gaussian or sech short pulses. It is important to note that, by setting the bit interval equal to the oscillation period, the multiplexed pulses do not interfere with each other even when they strongly overlap. As a result, the signal bandwidth can be greatly reduced. Non-coherent Nyquist pulses have already been applied to single-channel 1.28 Tbit/s-525 km transmission, for which tolerances to chromatic dispersion and polarization-mode dispersion have been greatly improved. As a future prospect, the combination of high-order QAM and high-speed coherent Nyquist pulses is expected to constitute the ultimate goal as regards maximum transmission capacity with the available bandwidth resource.

This research was awarded a grant by the "Funding Program for Next Generation World-Leading Researchers (NEXT Program)" launched by the Council for Science and Technology Policy (CSTP). I would like to thank Prof. Masataka Nakazawa and the members of the laboratory for their help and contribution to this work.

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