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OFC 2024: Coupled multi-core fibre opens opportunities for higher submarine cable capacity

Coupled multi-core fibre opens opportunities for higher submarine cable capacity

Coupled multi-core fibre opens opportunities for higher submarine cable capacity (Vismar UK/

A team of researchers from NEC Corporation and NTT Corporation in Japan have achieved wavelength/space-division multiplexed (WDM/SDM) transmission across a transoceanic distance of 7280km with 12 spatial channels using a coupled multi-core fibre with a standard cladding diameter. This accomplishment opens new possibilities for increasing the capacity of current submarine cabling technology using fibre technology that doesn’t take up more space.

The research findings will be presented at the OFC conference and exhibition, taking place from 24-28 March 2024 in San Diego.

WDM and SDM are leveraged to boost the capacity and efficiency of optical fibre communication systems. WDM transmits multiple signals simultaneously over a single fibre by assigning each channel a unique wavelength of light, while SDM uses separate spatial paths or fibre cores to transmit multiple data streams within a single fibre or across different fibres.

Multi-core fibres for submarine cables

When it comes to submarine cables, achieving more than 10 spatial channels has only previously been demonstrated for transmission distances of up to 1,001km with a 15-mode fibre or 1,560km with a 10-mode fibre. One of the biggest challenges in achieving high spatial count transmission over transoceanic distances is finding a way to reduce the spatial mode dispersion (SMD) and mode-dependent loss (MDL) of the transmission line. Multi-core fibres offer a solution as their multiple individual cores can each carry an optical communication channel, therefore allowing parallel data transmission. Compared to the uncoupled version, coupled multi-core fibres can accommodate many more cores in a standard 125-µm diameter cladding.

In the latest research from NEC Corporation and NTT Corporation, the team carried out WDM/SDM transmission using a 32-GBd PDM-QPSK modulation format over a recirculating loop consisting of a single 52-km length of coupled 12-core fibre (C12CF) with a standard cladding diameter. After determining the optimum span input power, they evaluated transmission performance at three wavelength ranges in the C-band.

The research findings

The team observed error-free transmission after forward error correction for wavelengths up to 7,280km (140 loops) for 1,536.6nm and up to 9,360km (180 loops) for 1,550.9nm and 1,560.6nm in a single-span loop configuration. They also demonstrated a spatial mode dispersion of 0.1 ns and mode-dependent loss of 0.3 dB per 52-km C12CF span, together with relatively low wavelength dependence.

Manabu Arikawa, Principal Researcher with System Platform Research Laboratories at NEC Corporation will present the research findings at OFC. He says: “Submarine cable systems are vital infrastructure for our lives, connecting the world across oceans; future cables require more and more capacity because of the exponentially growing global traffic demand. This research result can lead to higher capacity submarine cables, reduced cost per transmitted bit, and more efficient connectivity by significantly increasing the number of spatial channels for the same amount of optical fibres in the cables. One of the following important steps is the evaluation of large-scale multiple-input multiple-output (MIMO) processing in real-time implementation from the point of view of the future realisation of a MIMO transceiver for optical communication,” said Arikawa. Another important topic is the impact and scalability of the MDL of the fibres with the number of spatial channels to characterise and overcome this capacity limitation in the future.

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