While COVID-19 and its associated economic dislocations have complicated 5G technology's 2022 rollout, changes wrought by the pandemic may boost the case for the generation mobile network in the long run.

The world is increasingly more connected, and data consumption continues to multiply, driving a constant need for better communication infrastructure. These rapid changes are transforming the landscape for telecom services, equipment and information.

In this age of wireless networking and smart devices connected across the global internet, undersea cables are a vital artery for data transmission. More than 400 submarine cables are used to connect the world across oceans, and these cables need constant upgrades to ensure they can support the increasing bandwidth demanded of them.

OFC 2021

Hitoshi Takeshita is part of a team of researchers developing the first uncoupled 4-core multicore fiber in a submarine cable. He will present his prototype at the 2022 Optical Fiber Communication Conference and Exhibition (OFC) on Monday, 7 March, being held at the San Diego Convention Center.

"We have demonstrated that the optical transmission performance through cabled uncoupled 4-core multicore fiber fully meets the requirements of modern long-haul submarine cables," said Hitoshi Takeshita, System Platform Research Laboratories, NEC Corporation.

The researchers built a prototype cable using SC520-type submarine cable, which is capable of withstanding water pressures at a depth of 8,000 meters, and can hold 16 fiber pairs. The team used four pairs of multicore fiber and 12 single-core pairs in their prototype.

Most significantly, the team built their 4-core fiber design to the same dimensions as a single-core fiber, managing to dramatically increase the capacity of the cable without increasing its size or reducing the number of fiber pairs.

"Multicore fiber is now expected to further increase the number of parallel optical fiber cores without increasing the submarine cable size and structure, enabling the second generation of submarine SDM (space division multiplexing) systems," said Takeshita.

With the cores so close together, and with the cable undergoing additional stresses due to the submarine environment, extensive effort is required to ensure minimal interference between cores. In their trial, tests showed intercore crosstalk was negligible.

Equally important is reducing signal attenuation along the length of the cable. For a submarine cable that would one day be expected to span hundreds or possibly thousands of miles, ensuring the signal remains strong across the cable's length is critical. In particular, the team needed to guarantee the multicore fiber would not experience additional attenuation when cabled.

"It was never shown how multicore fiber would behave when cabled inside a submarine cable, with different bending and tension as it is usually evaluated in laboratory experiment," said Takeshita. "As submarine transmission is an ultra-long haul, even slight variations of properties of cabled fiber have significant impact on the system performance as it accumulates with distance."

To accomplish a reduction in intercore crosstalk and improved signal attenuation, the team integrated a multicore fiber amplifier into the cable, splicing the amplifier with the fiber directly. This reduced bulk in the cable and improved performance.

The researchers hope their multicore fiber bundled in a submarine cable will be used to increase data transmission rates as demand increases over the next few years.

"International data usage is expected to expand, driven by factors such as the growth of 5G mobile data and the need to share ever more content between data centers distributed around the world," Takeshita concluded. "To meet this demand, submarine networks are expected to adopt space division multiplexing technology using multicore fiber."