NEC, Sumitomo trial uncoupled multi-core submarine fiber

The NEC OCC SC500 multi-core fiber cable was used in a test to show the advantages of uncoupled multi-core fiber for submarine networks. (NEC)

NEC and its subsidiary OCC Corporation teamed up with Sumitomo Electric Industries, Ltd. to complete the first trial of an uncoupled four-core submarine fiber cable, and verified its transmission performance for use in telecom networks. Such multi-core cables could help submarine network operators maximize the number of available channels for transmission, while uncoupling is believed to reduce signal interference and complexity.

Leading submarine cable research firm Telegeography has stated that international data usage is expected to have a compound annual growth rate of 30% to 40% from 2020 to 2026, driven by trends like 5G data usage and distributed data center networking. 

Submarine network operators and their vendor partners are addressing these trends by adopting new technology, such as space division multiplexing (SDM), which increases the number of independent spatial channels to maximize total system capacity, reduce power consumption and optimize cost per bit. The sector also is innovating to increase the number of fibers that can be installed per cable, with NEC earlier this year qualifying a cable that could contain up to 48 fibers.

Using multi-core fiber instead of conventional single-mode fiber with one core per fiber can further help increase capacity without increasing the physical size and structure of submarine cables. That means a four-core fiber like the one used in this trial represents a four-fold enlargement in the number of spatial channels for the same amount of optical fibers and with the same fiber structure (each fiber being 250µm diameter (0.25mm) after coating), NEC said. Japan’s KDDI and others also have conducted research using four-core fibers.

A comparison of single-core and multi-sore fiber. (NEC)

Such multi-core fibers could be used in coupled or uncoupled states, but as NEC noted in a statement, “In coupled multicore fibers, the optical signals propagating in the respective cores tend to interfere with each other, requiring special signal processing at the optical receiver. In contrast, uncoupled multicore fibers minimize the interference between the cores, allowing conventional (lower complexity) transmitters/receivers to be used.”

The company added that uncoupled 4-core fiber from Sumitomo Electric is being deployed within the OCC SC500 series Lightweight cable, which can be deployed at water depth down to 8,000 meters, has a 17mm outer diameter and can contain up to 32 fibers, with exponentially more spatial channels. Ultimately, that will result in lower cost per bit for the entire cable system.

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The trial allowed NEC and OCC to demonstrate that the cable's optical transmission performance in the water could meet the requirements of modern long-haul submarine cables, while also proving that the process of cabling Sumitomo's multicore fiber had no effect on its optical characteristics and attenuation properties, the companies said. The testing was supported by the Ministry of Internal Affairs and Communications, Japan, under the initiative "Research and Development of Innovative Optical Network Technology for a Novel Social Infrastructure.”