Progress toward increasing capacity of transoceanic optical submarine cables — NEC and NTT successfully conducted world's first-long-distance transmission experiment

On March 21, NEC and NTT announced that they have successfully conducted the world's first transoceanic long-distance transmission experiment over a distance of 7280 km using a 12-core coupled multicore fiber with 12 optical signal transmission paths in a standard outer diameter (0.125 mm) optical fiber. Both companies expect that this will be the next-generation transmission infrastructure technology that will help realize future large-capacity optical networks using optical submarine cables.

With the global spread of 5G and increased communication between data centers, international Internet traffic has grown at an average annual rate of 30% from 2018 to 2022 and this trend is expected to continue. To meet such increased communication demand, transmission capacities using optical submarine cable systems must be increased in addition to expanding the number of optical submarine cables. Existing optical submarine cables use single-core fiber, which has one optical transmission line called a core within one fiber.

In response, research and development is being conducted around the world to improve cable capacity by using multi-core fiber, which has higher communication capacity due to multiple cores without changing the standard outer diameter of the fiber. NEC is currently working on a project to install a long-distance optical submarine cable system using, as the aforementioned multi-core fiber, a two-core fiber with two optical transmission lines. When more cores are added to an optical fiber with a standard outer diameter, optical signals leaking from individual cores interfere with optical signals in adjacent cores, causing signal mixture and deterioration of communication quality. This problem is called crosstalk. In addition to this crosstalk which becomes more severe especially in long-distance transmission, non-uniformity due to delays and losses between optical signals can cause a situation when transmitted signals cannot be received accurately. The two companies have developed technologies to address these issues.

The first developed technology is an algorithm for demodulation of received signals using the Multiple Input Multiple Output (MIMO) signal processing method. Although the MIMO method is commonly used to separate a large number of mixed radio signals, using it to separate signals in optical communications which are two orders of magnitude faster than 5G wireless communications requires faster processing. Additionally, multicore fiber with a large number of cores requires more extensive signal processing because the optical signals are further multiplexed. The random occurrence of crosstalk, especially in long-distance transmission, is another problem that must be addressed.

To address these issues, NEC has developed a new algorithm for long-distance transmission and applied it to 24 x 24 MIMO (12 cores x 2 polarizations), enabling accurate separation and demodulation of high-speed received signals. The second developed technology is a 12-core coupled multicore fiber optical transmission line. In long-distance optical communications using multicore fiber, when propagation delays occur between multiplexed optical signals and non-uniformity takes place, the circuit resources required for MIMO signal processing during reception increase, making implementation and realization difficult. In addition, non-uniformity caused by propagation loss greatly limits the transmission distance.

To solve these problems, NTT has developed new design technology for coupled multicore fiber and input/output devices (connected fan-in fan-out) that can reduce the effect of non-uniformity of signals caused by delay and loss, and optical transmission line design and evaluation technology for long-distance applications. Combining these developed technologies, both companies conducted a long-distance transmission experiment over 7,280 kilometers, assuming a transoceanic optical submarine cable, and succeeded for the first time in the world in accurate offline demodulation of 12 spatially multiplexed optical signals.

In the future, both companies will further advance the research and development of these technologies with the aim of putting them to practical use as long-distance, high-capacity optical submarine cable systems and land core network systems that will realize the high-capacity optical transmission infrastructure in the IOWN concept and Beyond 5G/6G era in the 2030s. The two companies also presented their results as a high-scoring paper at the Technical Conference of OFC 2024, the world's largest event on optical communications, held March 24 to March 28 in San Diego, California, in the USA.