March 23, 2017
Nippon Telegraph and Telephone Corporation (NTT, Chiyoda-ku, Tokyo, President and CEO Hiroo Unoura) demonstrated ultra-large capacity inline optical amplified transmission of 1 petabit (1000 terabit) per second over a 205.6 km length of 32-core (light paths) optical fiber in collaboration with the Technical University of Denmark (DTU, Lyngby, Denmark, President Anders Overgaard Bjarklev), Fujikura Ltd. (Fujikura, Koto-ku, Tokyo, President and CEO Masahiko Ito), Hokkaido University (Hokkaido Univ., Sapporo, Hokkaido, President Keizo Yamaguchi), the University of Southampton (UOS, Southampton, UK, President and Vice-Chancellor Professor Sir Chris Snowden), and Coriant GmbH (COR, Munich, Germany, CEO Shaygan Kheradpir).
This sets a new world record for the transmission distance of one Pbit/s capacity over a single strand of optical fiber within a single optical amplifier bandwidth (C-band), which is half the bandwidth used in the previous experiment (Figure 1). The present achievement indicates that the transmission of one Pbit/s, a capacity equivalent to sending 5,000 HDTV videos of two hours in a single second, is potentially possible over 1000 km, which is approximately the distance between major cities both in Japan and in Europe. This remarkable achievement was reported as a post deadline paper*1 on March 23, 2017 at the Optical Fiber Communication Conference and Exhibition (OFC 2017), the largest annual conference on optical communication, which was held in Los Angeles, USA, from March 19 - 23, 2017.
Part of this research utilized results from the EU-Japan coordinated R&D project on “Scalable And Flexible optical Architecture for Reconfigurable Infrastructure (SAFARI)” http://www.ict-safari.eu/ commissioned by the Ministry of Internal Affairs and Communications (MIC) of Japan and EC Horizon 2020.
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 . 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  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.
By using the 32-core MCF, which we have successfully prototyped for a long length of over 50 km , 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 . 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 , 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).
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 . 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 , and thus realize long-distance DSDM transmission . 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.
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  to 16, and applied a wideband digital-analog conversion technique  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  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.
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  and network control techniques  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.
Nippon Telegraph and Telephone Corporation
Science and Core Technology Laboratory Group, Public Relations
NTT Has Instituted a Logo to Represent R&D Activities.
Information is current as of the date of issue of the individual press release.
Please be advised that information may be outdated after that point.
PDF Files require Adobe Reader software which is a free download from