“The rollout of multicore fiber as part of 5G is possible, but not for a long time. Short term, this technology seems more suitable for data centers or submarine cables.”
With the explosion of digital traffic—driven by more people working remotely, increased use of e-commerce, photo and video sharing, streaming and so on—and the ever-increasing number of applications constantly consuming more bandwidth, optical transport networks are under intense pressure. This underlying trend, amplified by the pandemic, is forcing network carriers to explore new avenues to increase optical fiber transmission capacity. “Until now, the use of increasingly advanced WDM technologies enabled greater throughput per channel, which, consequently, mitigated the high pressure on networks,” states Erwan Pincemin, Research and Development Engineer at Orange. “But with capacity growth of 25%–30% per year, and even 40% in some network carriers, limits are quickly reached.”
Fiber: A Vast Field of Research
For decades, researchers have been working on new generations of fiber optic to increase transmission throughput. Following on from traditional fibers made up of a single silica core (single mode or multimode), research has become focused once again on multicore fibers — technology that emerged decades ago. The aim? A fiber optic cable with several cores, enabling more information to be transmitted at the same time, results in increased transmission capacity. “One of the first examples of this dates back to the 1990s,” points out Noëlla Evanno, who is in charge of studies on optical fibers and cables at Orange. “It had four cores and was in the shape of a four-leaf clover. It had been designed by CNET in Lannion. This fiber allowed transmission throughput to be increased. Since then, technology has progressed with publications now referencing fiber cables with up to 19 cores.”
A Great Leap Forward
Last August, Orange Poland conducted tests on a seven-core photonic crystal fiber (with air voids running along the length of the fiber) that was 1.5 km in length. It was developed by InPhoTech using ICE6 800G optical data transmission technology from Infinera. By using two channels to send data at speeds of 1.6 TB/s through each of the seven cores simultaneously, these tests revealed several interesting results, including a total transmission of 11.2 TB/s versus 1.6 TB/s in a single core. This is therefore a real step forward not only for customers, who could benefit from greater throughput and more services, but also for network carriers. It remains to be proven whether these results, obtained under laboratory conditions, can be achieved in reality.
Feasibility Yet to Be Proven
There are still many steps to be taken between laboratory testing and rollout. “This type of experiment is necessary to understand the performance of a new technology,” explains Evanno. “But to make it a real technological breakthrough, we need to be sure of its technical-economic feasibility.” We are only in the early stages, with the Polish experiment having been carried out over a distance of just 0.93 miles. One of the main challenges is to extend the reach of this multicore fiber across hundreds of miles. For Pincemin, “An interesting study would start off by doping multicore fiber with rare-earth elements (erbium/ytterbium). This would make the fiber active and therefore produce multicore optical amplifiers with reduced energy consumption, making it very useful for future submarine optical cables.”
Promising, But Not for All Uses
Researchers and carriers must now continue their work to determine the most relevant application areas for multicore fiber. “Single-core fibers are continuing to advance and are still very efficient for long-distance, land-based networks,” says Evanno. “The use of this new technology as part of 5G is still possible, but not for years. Short term, multicore fiber seems to be more suitable for rollout in data centers, as the distances involved are short, which reduces interference problems.” Submarine cables, the backbone of the global Internet, comprise another area of application offering an interesting outlook for multicore fiber. “These submarine cables have limited capacity due to energy constraints,” explains Pincemin. A multicore fiber cable with amplifiers that consume much less energy would address these concerns and would also address the space constraints of these cables.”
