March 21, 2024

Internet traffic is rapidly increasing across the world. Once 5G, autonomous driving, and metaverse become commonplace, the capacity of current optical fiber networks is expected to reach its limit. What is garnering attention amid such circumstances is the concept of multicore optical fiber. Conventional optical fiber has a core that goes through the center for transmitting light. Multicore optical fiber, on the other hand, has multiple cores passing through a single optical fiber, which drastically increases traffic while maintaining the diameter of the optical fiber.

Among its possible applications, submarine cables are considered to be the most effective use of multicore optical fiber. Rather than loading many cables that are thousands of kilometers long on a ship to be laid out on the seafloor, it is overwhelmingly more efficient to install a single cable. NEC, as one of the top three enterprises in the submarine cable market, succeeded in prototyping the world’s first four-core optical fiber submarine cable in July 2022. In this press release, we announce the success of our transoceanic long-distance transmission experiment over 7,280 km using 12-core optical fiber. We spoke with the researchers about the details on what purpose and meaning this success has and what technologies were used to achieve this success.

Larger-capacity optical submarine cables are coming into sight

――What does the success of a long-distance transmission experiment using 12-core optical fiber mean?

de Gabory: NEC has successfully prototyped a four-core optical fiber in the past. However, there is a definite difference between the previous four-core and this 12-core optical fiber. Four-core optical fiber can be created in uncoupled form, which is relieved from serious interference or crosstalk between cores. This also means that it can use conventional transponders as-is. However, multicore optical fiber that uses more than four cores must be designed with crosstalk inevitably. This design is the coupled spatial multiplexing. Since inter-core interference and crosstalk will occur, the final received signals must be separated for each core for demodulation. Therefore, new terminal equipment is required. A 12-core optical fiber calls for new transmission and demodulation technologies.

Among NICT’s BU projects spearheaded by NTT as the project leader, this research was collaboratively conducted by NEC and NTT. In this project, NEC and NTT, among others, propose and promote technologies that materialize a future large-capacity optical transmission framework. The success of the experiment is the product of NEC’s efforts in spatial multiplexing technology applied to submarine systems, which is part of the scope of this project. This success also owes to NTT’s development of a coupled multicore optical fiber, including fan-in/fan-out devices, that mitigates the impact of non-uniform propagation delays and losses to a level that makes possible transoceanic-scale transmission of 12-core multiplexed optical signals. On the other hand, NEC developed innovative transmission and demodulation technologies for using this optical fiber transmission line. The collaboration of technologies from both companies resulted in the successful experiment.


Arikawa: On an academic research level, studies on spatial multiplexing using more than 10 cores have been presented, but none of them supported long-distance transmission, the longest being somewhere around 1,000 km. Under such circumstances, the success of this transoceanic-scale 7,280-kilometer transmission experiment using 12-core optical fiber that opens up possibilities for submarine cable has great significance.

There are already various approaches in the world attempting to expand the capacity of optical transmission systems. Our experiment drew a road map for research and development with the multicore fiber approach, presenting a realistic possibility of submarine cable installation using multicore optical fiber.

Drawing global attention at the largest international convention in the optical communications field

――What breakthrough technology led the experiment to success?

de Gabory: We developed an algorithm that demodulates multiple interfering signals using Multiple Input Multiple Output (MIMO) technology. MIMO is a technology that enables transmission with split traffic using multiple antennas and is drawing attention in areas such as 5G wireless communication.


Arikawa: MIMO processing has also been commonly used for demodulation in optical fiber communications. This is called polarization multiplexing, where signals are transmitted by dual polarization and demultiplexed to original state after they interfere with one another during reception. It should also be noted that this is used for single-core optical fiber―which means that a 12-core optical fiber requires 24 x 24 MIMO. In such a scale, you may be able to say that MIMO is playing a role similar to wireless communication in this application. Having said that, the estimated signal speed in optical fiber communication outclasses that over 5G wireless communication. While wireless communication coverage is generally several kilometers, we are dealing with a transoceanic-class transmission line―we need to additionally consider the impact of distance. So, we developed an algorithm that is completely different from that used for 5G wireless MIMO.


de Gabory: This is the part that leverages Mr. Arikawa’s over a decade worth of expertise in the study.


Arikawa: I have been studying algorithms that reliably work with large-scale communications. The years of study fruited in this successful experiment.


de Gabory: The long-distance transmission experiment that we have demonstrated is also well-recognized in academia. OFC 2024, the world’s largest optical communications conference, is to be held starting March 24, where we plan to give an oral presentation as a high-score research paper in the very top tier. In addition, a summary of our research paper is already announced from OFC as press release even before the conference. This is such a rare occasion that only occurs several times a year. We are feeling the global attention that we are getting for the achievement.

Accelerating research toward the implementation of real-time processing

――How do you envision the future progress of research of this technology?

de Gabory: This experiment only shows the offline result. Specifically, we acquired and demodulated the received waveform using a computer. Since real-time processing using LSI is required to work an actual system, we have more to accomplish. Therefore, our next step will be to design and build a circuit that can be used to develop a viable system. Here is the part where Mr. Nakanishi’s competence comes into full play. Mr. Nakanishi is a new powerful member that joined NEC last May and is an expert who has long worked on circuit design and LSI design for hard discs.


Nakanishi: Right. This technology involves advanced signal processing, which can result in large-scale circuits and a huge power consumption. However, that will bloat overall costs and make commercialization difficult, so we are aiming to develop an application-specific integrated circuit (ASIC) that keeps down the size of the circuit as well as power consumption for suitability for commercialization. We will start with implementing MIMO on field-programmable gate array (FPGA) and find issues, which we plan to apply to the development of the ASIC.


de Gabory: We now have a stronger team with the addition of new employees that joined NEC last April. In fact, Mr. Odagawa is working on ways to streamline MIMO processing in the research of this technology.

Odagawa: Yes. There is a preparatory process before MIMO processing, and I am working on signal processing that may be able to contribute to better efficiency. I studied particle experiments during my school years, so signal processing was a completely new challenge for me; however, there were quite a few common things like considering algorithms and actually developing systems. I have been working on research with the hope to use some of that experience. Communications is an industry that is closely related to our life. It’s exciting to be able to research and work on what is related to familiar things in life.


de Gabory: Each member of the team has his own competence, which contributed to finally getting the research and development of coupled multicore optical fiber off the ground. Our next task is to further accelerate research toward future demonstration.

Go back to Featured Technologies