One Petabit per Second Fiber Transmission over a Record Distance of 200 km - Paving the Way to Realizing the 1000 km Inline Optical Amplified Transmission Systems within C band only -

1.Background of the research

Along with the rapid advancement of broadband and cloud services resulting from the increased use of smartphones in recent years, communication traffic has been increasing at an annual rate of over 1.4 times (about 1000 times in 20 years), according to the statistics of the Ministry of Internal Affairs and Communications [1]. To cope with this rapid increase in communication traffic, we have so far been economically realizing high capacity optical networks by increasing the capacity of optical communication system equipment without changing the basic structure of the optical fiber already deployed. However, the physical capacity limit of the optical fiber used in current large capacity optical networks is estimated to be around 100 Tbit/s, which is only about ten times the current commercial capacity. If communication traffic is to increase at the same rate as it is now, we will reach the capacity limit with the existing optical fiber (Capacity Crunch) by the mid-2020s.
To realize a large capacity optical network capable of accommodating the expected increase in communication traffic long into the future, research and development of space-division multiplexing (SDM) optical communication technology^*2 [2] using an optical fiber with new spatial structures such as a multi-core fiber (MCF) having multiple cores in one optical fiber is underway, attracting worldwide attention. NTT, Fujikura, Hokkaido University, DTU, UOS, and COR have been bringing together the technologies we each possess under joint academic and industrial collaboration, with the aim of realizing a large capacity optical network using SDM technology. We have been conducting research and development in the design and manufacturing technology of MCFs, and pushing the performance of the new fibers to the extreme limit.

2.Experiment and experimental results

By using the 32-core MCF, which we have successfully prototyped for a long length of over 50 km [3], fan-in/fan-out (FI/FO) device to couple light into the MCF, and by using a new digital coherent optical transmission technology, we have realized a high-capacity optical transmission rate of 1 Pbit/s. We achieved this by exploiting dense space and wavelength division multiplexing (DSDM and DWDM) over long-distance. The 32-core MCF used in the experiment utilized a new arrangement of cores that greatly reduces inter-core light leakage that otherwise degrades performance [3]. In addition, using the wave properties of light (phase^*3 and polarization^*4), we applied multi-dimensional coding^*6 to polarization division multiplexed 16-quadrature amplitude modulation (PDM-16QAM) digital coherent technology^*5 to improve the long-distance transmission performance in each core.
Figure 2 shows the experimental results. We realized 31.3 Tbit/s capacity per core (=680 Gbit/s per wavelength x 46 wavelength channels), and using the 32-core MCF, we recirculated and amplified the signals over four spans of the 51.4 km fiber, and showed that the signal transmission of an aggregate 1.00 Pbit/s capacity is possible over 205.6 km. Since the Q-factor, which shows the transmission quality of the PDM-16QAM signals, were uniform, it shows that high quality transmission with small variations between cores and low error is possible. Compared to the world-first 1 Pbit/s capacity experiment over 52.4 km that we reported in 2012 [4], we demonstrated about four times longer distance of 205.6 km, and is the world longest distance for over Pbit/s capacity transmission.
Moreover, by applying a digital signal processing technique called multi-dimensional coded modulation, the capacity per wavelength will be reduced by 25% to 510 Gbit/s, but we demonstrated that the transmission distance can be increased to over 1000 km. As a result, with one optical fiber, we showed that there is a possibility of the ultra-high capacity equivalent to 0.75 Pbit/s using just the 5 THz bandwidth of the C-band^*7, and 1.5 Pbit/s using the 10 THz bandwidth provided by the combined C and L bands^*7, with a potential transmission distance over 1000 km (Figure 4).

3.Technological features and roles

(1)32-core MCF transmission line (Figure 3)

The MCF we used in this experiment was jointly designed and prototyped by DTU, Fujikura, and Hokkaido University. The fiber has a new structure (single-mode heterogeneous-core MCF) with 32 cores incorporating several types of cores, each with different properties [3]. The characteristic of this fiber is that two kinds of cores with slightly different refractive indices are arranged in a square lattice pattern. With this structure, even if the number of cores is increased to 30 or more, the crosstalk between adjacent cores can be greatly reduced [3], and thus realize long-distance DSDM transmission [5]. NTT and COR have evaluated the long distance characteristics of the 51.4 km MCF transmission line with the 32-core MCF and FI/FO devices prototyped by Fujikura, UOS, and NTT. As a result, we confirmed that all cores had low crosstalk and low loss characteristics over the whole C-band, which is a requirement for a 32-core MCF transmission line suitable for transmission over 1000 km distance.

(2)Multi-dimensional coded 16 QAM technique (Figure 4)

In recent large-capacity optical communications, instead of the intensity modulation signal transmitted using binary states of either ON or OFF, a highly efficient PDM-QAM digital coherent signal^*5 has been used that realizes a large number of signal states created by using the wave properties (phase and polarization) of the light. Such multi-level QAM signals can realize a highly efficient ultra-high speed optical signal by associating a plurality of bits of digital signals with a plurality of optical signal states encoded using the phase and polarization of light. However, the drawback is that when we increase transmission efficiency by increasing the number of multi-levels, the transmission distance sharply decreases. In addition, signal quality degrades by the crosstalk that arises in MCF transmission.
This time, NTT reduced the multi-level number of the QAM signal from 32 in the conventional report [4] to 16, and applied a wideband digital-analog conversion technique [6] to the digital coherent signal using a highly efficient error correction coding. As a result, we successfully transmitted a capacity of 680 Gbit/s per wavelength (1 Pbit/s per fiber) over a 205.6 km distance; the longest distance for a Pbit/s capacity transmission. Furthermore, by applying the 8-dimensional encoded 16-QAM technique [7] and by improving the allocation method of the digital signal and the optical signal state, the transmission quality can improve compared with the normal QAM code. With the same 16-level QAM, the transmission rate will be reduced to 510 Gbit/s per wavelength, but by doing this, we showed that it has the potential to extend the transmission distance to possibly over 1000 km.

4.Future Developments

The transmission distance of 1000 km, the possibility of which was proven in these experiments, corresponds to the distance needed to construct optical networks in Japan and in Europe, connecting major cities. In the future, we will promote cooperative experiments using multicore optical amplification technology [8] and network control techniques [9] in a testbed to be constructed at COR in Europe, and we will continue to support the development of broadband services for the realization of future long distance high capacity optical networks.

References

[1] http://www.soumu.go.jp/main_content/000462459.pdf
[2.12MB] (Japanese)
[2]T. Morioka, "New generation optical infrastructure technologies: "EXAT initiative" towards 2020 and beyond," 14th OptoElectronics and Communications Conference (OECC) 2009, paper FT4, 2009.
[3]Y. Sasaki et al., "Crosstalk-Managed Heterogeneous Single-Mode 32-Core Fibre," European Conference on Optical Communication (ECOC) 2016, Paper W.2.B.2, 2016.
[4]http://www.ntt.co.jp/news2012/1209e/120920a.html
[5]T. Mizuno et al., "Long-Haul Dense Space-Division Multiplexed Transmission Over Low-Crosstalk Heterogeneous 32-Core Transmission Line Using a Partial Recirculating Loop System," J. Lightwave Technol., Special Issue on the Optical Fiber Communication Conference (OFC) 2016, vol. 35, no. 3, pp. 488-498, Feb. 1, 2017.
[6]http://www.ntt.co.jp/news2016/1609e/160923b.html
[7]M. Nakamura et al., "Coded 8-Dimensional QAM Technique Using Iterative Soft-Output Decoding and Its Demonstration in High Baud-Rate Transmission," J. Lightwave Technol., Special Issue on the European Conference on Optical Communication (ECOC) 2016, vol. 35, no.8, to be published in April 2017.
[8]S. Jain, et al., "32-core Inline Multicore Fiber Amplifier for Dense Space Division Multiplexed Transmission Systems," European Conference on Optical Communication (ECOC) 2016, post deadline paper, Th.3.A.1, 2016.
[9]Coriant Transcend™ SDN Solution, http://www.coriant.com/products/transcend-sdn.asp .

Fig.1 The achievement in Petabit/s-class transmission