By using a connected 12-core multicore fiber – a standard outer diameter optical fiber (0.125 mm) with 12 optical signal transmission paths – NEC and NTT have successfully carried out a first-of-its-kind transoceanic-class 7,280km transmission experiment. With this accomplishment, a new generation of transmission infrastructure technology is anticipated to be realized, one that will aid in the development of large-capacity optical networks and prospective optical undersea cables.
International Internet traffic surged at an average annual rate of 30% from 2018 to 2022, and this trend is projected to continue due to the worldwide rollout of 5G and increasing connectivity between data centers. There would be a rising need to both expand the number of optical undersea cables and the transmission capacity per cable system in order to fulfill the high demand for communication.
Single-core fiber, which contains a single optical transmission route termed a core inside a single fiber, is used in the optical underwater cables that are now in use. On the other hand, multicore fiber, which contains numerous cores to boost transmission capacity without modifying the standard outer diameter of the fiber, is the subject of research and development efforts worldwide in an effort to increase cable capacity.
Using two-core multicore fiber with two optical transmission pathways, NEC is now working on a project to establish a long-haul optical undersea cable system.
Research Results
The quality of mutual communications is lowered when additional cores are added to an optical fiber with a standard outer diameter because crosstalk occurs when optical signals leaking from one core interact with optical signals in neighboring cores. Not only is crosstalk a severe problem in long-distance transmission, but the non-uniformity of delay and loss between optical signals would make it harder to receive sent data precisely.
NEC Develops MIMO Signal Demodulation Algorithm
The scope of MIMO signal processing that has been practically used in current optical communications is restricted to two-polarization multiplexed signals, despite the fact that MIMO technology is often utilized to separate numerous interfering radio signals.
Additionally, since the optical data are further multiplexed in multicore fiber with several cores, more sophisticated signal processing is needed. Furthermore, one problem that has to be handled in long-distance transmission is the random occurrence of crosstalk. With the development of an algorithm for long-distance transmission, NEC is now able to accurately separate and demodulate high-speed received signals from 24 × 24 MIMO (12 cores x 2 polarizations).
NTT Develops 12-Core Fiber Optic Line
Implementation and execution of MIMO signal processing during reception become more challenging in long-haul optical communications employing multicore fiber when non-uniform delays and losses arise in the transmission between multiplexed optical signals. Furthermore, the transmission distance is significantly restricted by non-uniformity in propagation loss.
In this study, NTT has developed optical transmission line design evaluation technologies for long-haul applications, coupled multicore fiber design technologies, and optical input/output devices (connecting fan-in/fan-out) that can lessen the effects of non-uniformity among signal delay and loss.
The Combined Technologies and Future Developments
By combining these technologies, NEC and NTT were able to successfully demodulate 12-spatially multiplexed optical signals offline for the first time in history during long-distance transmission trials over 7,280 km, assuming a transoceanic-class optical undersea cable.
With a view to commercializing them as a long-haul, high-capacity optical submarine cable system and terrestrial core network system that will help realize a high-capacity optical transmission infrastructure in the IOWN concept and Beyond 5G/6G era in the 2030s, NEC and NTT will jointly continue to advance the research and development of these technologies.
