NTT and NEC eye a huge increase in subsea cable capacity

Japan's NTT and NEC have announced that they have conducted a transmission experiment using a new fiber optic technology that they say could substantially increase the capacity of submarine cables.

The companies said Thursday that they have successfully tested a coupled 12-core multicore fiber over 7,280km in an experiment that simulated the conditions of submarine cables on the ocean floor.

NTT and NEC pointed out that existing optical submarine cables use single-core fiber, which has a single optical transmission path called a core within a single fiber. They added that researchers worldwide have been working on multicore fibers that add more paths without increasing the size of a standard 0.125mm diameter optical fiber. NTT and NEC are working on fibers that pack 12 cores in that tiny space.

"This achievement is expected to be a next-generation transmission infrastructure technology that will contribute to the realization of large-capacity optical networks, including future optical submarine cables," the partners said in a statement.

Addressing the interference issue

With more cores packed into an optical fiber, crosstalk occurs between cores and degrades communications.

"Especially in long-distance transmission, it becomes difficult to receive transmitted signals accurately due to non-uniformity of delay and loss between optical signals," the companies said.

To fix this issue, NEC and NTT developed an algorithm to bring Multiple Input Multiple Output (MIMO) technology to optical networks. MIMO technology is widely used in wireless communications to separate multiple interfering radio signals.

Moreover, the companies have developed design technologies for coupled multicore fiber and optical input/output devices (connecting fan-in/fan-out) that can reduce the effects of non-uniformity among signal delay and loss.

The two companies are aiming to have their new fiber optic technology ready for commercial deployment by the 2030s, just in time for the launch of 6G.

[Editor's note: An earlier version of this story mistakenly stated that NTT and NEC were looking for a 12-fold increase in subsea capacity. That's theoretically possible, but after checking with NEC, we felt it best not to guess how much capacity a 12-core fiber would deliver in a live network.]