As our world becomes increasingly connected, communication infrastructure must scale to meet the growing demand. Critical parts of that infrastructure are submarine cable connecting networks between continents.
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These submarine cables operate in an environment where space is a luxury. One of the biggest challenges in designing next-generation submarine cables is how to increase data capacity without increasing the cable’s size. Even a simple change, like adding more fiber pairs, comes with additional power requirements that complicate the design.
Pierre Mertz and Siddharth Varughese are part of a team behind a submarine cable field trial that succeeded in transmitting data at a record 307.2 terabits per second without any required size increase. They will present their results next week at the Optical Fiber Communication Conference and Exhibition (OFC), San Diego, California.
The team achieved this record on the transatlantic cable Dunant and employed a nascent technology called space-division multiplexing (SDM). With SDM technology, each fiber pair operates at a lower optical power and signal-to-noise ratio.
“At first thought, it would seem counterintuitive to reduce the optical power as this would reduce the capacity of the individual optical fiber,” said Varughese. “However, reducing the power on individual fiber allows one to introduce additional fiber in the submarine cable, which can thereby increase the total capacity of the cable.”
The researchers compared the Dunant cable to a previous cable, Marea, that they worked on in the past. Marea has eight fiber pairs, which support 27.8 Tbps each. Dunant, in contrast, has 12 fiber pairs that can support 25.6 Tbps each. Each fiber pair in Dunant is less efficient than the fiber pairs in Marea, but the cable, as a whole, supports nearly 40 percent more capacity.
“Through our field trial, we demonstrated how the simplest SDM cable consisting of 12 fiber pairs can improve capacities by as much as 38%,” continued Varughese. “SDM cables are the way forward if we are to meet the exponentially scaling global data rate requirements.”
Implementing SDM architecture in Dunant was no easy feat due to the lower signal-to-noise ratio involved. The researchers paid close attention to their modulation formats, operating baud rates, and digital signal processing algorithms to make the most of their limited optical power.
The result is a successful proof of concept for SDM technology. The Dunant cable achieved a capacity of over 300 Tbps, which approaches or exceeds the limits of traditional cables. The team expects future SDM cables should have an even greater capacity and hopes their demonstration will provide a motivation to build these future cables.