NEC, NTT claim success in first-of-its-kind subsea optical fibre cable trial

NEC and NTT have conducted a first-of-its-kind transoceanic-class 7,280km transmission experiment using coupled multicore fibre.

The two Japanese firms expect the work they have been carrying out will result in a next-generation transmission infrastructure technology that will contribute to the realisation of large-capacity optical networks, including future optical submarine cables.

Existing optical submarine cables use single-core fibre, which has a single optical transmission path. In contrast, research and development is being conducted to increase cable capacity by using multicore fibre, which can increase transmission capacity without changing the standard outer diameter of the fibre.

As more cores are added to an optical fibre with a standard outer diameter, crosstalk can occur when optical signals leaking from a core interfere with optical signals in adjacent cores, resulting in interference, which deteriorates the quality of mutual communications. Especially in long-distance transmission, in addition to the seriousness of crosstalk, it can in some circumstances become difficult to receive transmitted signals accurately due to the non-uniformity of delay and loss between optical signals.

NEC is currently engaged in a project to install a long-haul optical submarine cable system using two-core multicore fibre with two optical transmission paths. Working with NTT to address these issues, NEC has been working on demodulation of received signals using multiple input multiple output (MIMO) signalling processing technology for long-distance transmission. The new NTT cable consists of 12 optical signal transmission paths in a standard outer diameter optical fibre (0.125 mm). It has also designed evaluation technology for long-haul optical transmission paths.

The firms stressed that even though MIMO technology is commonly used to separate multiple interfering radio signals, the scale of MIMO signal processing that has been put into practical use in existing optical communications is limited to two-polarisation multiplexed signals. In addition, multicore fibre with many cores requires more extensive signal processing because the optical signals are further multiplexed. In addition, the random occurrence of crosstalk in long-distance transmission is an issue that the firms say must be addressed.

Furthermore, within long-haul optical communications using multicore fibre, when non-uniform delays and losses occur in the propagation between multiplexed optical signals, the circuit resources required for MIMO signal processing during reception increase, making implementation and realisation difficult. Non-uniformity in propagation loss can greatly limit the distance that can be transmitted.

NTT has developed design technologies for coupled multicore fibre and optical input/output devices, connecting fan-in/fan-out, that can reduce the effects of non-uniformity among signal delay and loss, as well as optical transmission line design evaluation technologies for long-haul applications. NEC has now developed an algorithm for long-distance transmission and applied it to 24 x 24 MIMO (12 cores x 2 polarisations), enabling accurate separation and demodulation of high-speed received signals.

Combining these technologies, NEC and NTT conducted long-distance transmission experiments over 7,280km, assuming a transoceanic-class optical submarine cable, and claim to have succeeded for the first time in accurately demodulating 12-spatially multiplexed optical signals offline.

Going forward, the two companies will further advance the research and development of these technologies with the aim of commercialising them as a long-haul, high-capacity optical submarine cable system and terrestrial core network system that will contribute to the realisation of a high-capacity optical transmission infrastructure in the IOWN concept and beyond the 5G/6G era in the 2030s.