World's first demonstration of a new structural design for multi-core and multi-mode optical fiber
More than 10 spatial multiplexing with less than 10 cores and reduced propagation delay difference between multimode

News Highlights:

Research Background

NTT is conducting R&D on a multi-core optical fiber, which is one of the elemental technologies for realizing IOWN's high-capacity optical transmission infrastructure. NTT has been conducting R&D on a four-core MCF² that multiplexes four optical paths in a glass of the same thickness as current optical fibers. In addition, IOWN aims to realize 125 times the current transmission capacity in the future, and to increase the capacity of optical transmission lines further, it is required to search for a new realization technology to expand the optical multiplicity (Four in the case of 4-core MCF) to 10 or more. As one of the options for achieving a multiplicity of 10 or more, research and reports have been conducted on multimode optical fiber (MMF), in which multiple types of light (modes) propagate within a single core, and coupled-type MCF, which is designed to reduce the distance between cores and couple optical signals between them³. In these optical fibers, if the propagation delay difference between optical signals is large, the transmission characteristics deteriorate. However, coupled MCF is known to be easier to reduce the propagation delay difference than MMF. Therefore, if MMF and coupled MCF can be fused, it is expected that optical multiplicity of more than 10 or more than several 10 can be realized while maintaining the outer diameter of optical fiber. However, until now, when multiple modes propagate in one core, optical signals with different modes between adjacent cores could not be combined.
　In this research, we have devised and demonstrated for the first time in the world a new structural design that realizes complete optical coupling between cores and modes.

Figure 1 Outline of Newly Proposed and Demonstrated Core-to-Core and Mode-to-Mode Optical Coupling Type Optical Fiber

Result of research

① Structural design of perfect optical coupling

Coupling of the same mode of optical signals between cores is achieved by properly spacing the cores, and the coupling is accelerated by the natural bending and twisting of the optical fibers. Optical fibers implemented in actual optical cables are subjected to random bending and twisting, and it has been demonstrated that coupled-type MCF cables can be realized by actively controlling and utilizing it⁴. However, when each core has multiple optical signals, to achieve coupling between adjacent cores of different optical signals, it is necessary to apply extreme bending (small radius) to the optical fiber, and the optical signal itself leaks out of the optical fiber, preventing optical communication (see Figure 2, left). In this research, we have discovered a new optical fiber design technology that extends the optical coupling model from two adjacent cores to three adjacent cores, and for the first time in the world, we have proposed a method that can realize complete coupling between cores and optical signals by greatly relaxing the bending condition (bending radius) required to control coupling characteristics.

Figure 2 Relationship Between Optical Coupling Between Two Adjacent Cores and Bending Condition (bending radius) (left: conventional design model) and Relationship Between Optical Coupling Between Three Adjacent Cores and Bending Condition (bending radius) (right: proposed design model)

② Demonstration of perfect optical coupling

As an example, we fabricated a three-mode seven-core optical fiber by arranging seven hexagonal close-packed cores capable of propagating three-mode optical signals with one core (Figure 3), and evaluated the relationship between the optical coupling characteristics and the bending condition (bending radius). The evaluation results in Figure 3 show the deviation of the arrival time of a three-mode optical signal after 1 km propagation in the fiber under bending conditions (bending radius R) observed on the time axis. When R is 140 mm, there is a large deviation in the arrival time between the blue signal and the yellow or orange signal. However, the deviation in the arrival time also decreases as R is reduced. At R =50 mm, which is the optimum condition for this design example, the arrival time distributions of the optical signals of the three modes coincide, confirming that perfect coupling of all optical signals has been realized. As a result, the proposed model demonstrated for the first time in the world complete optical coupling between all cores and all optical signals. In this example, a total of 21 optical signal couplings can be realized in seven cores × three modes.

Figure 3 Example of Experiment in Which Optical Signals of Three Modes Were Coupled Between Cores in a Prototype Optical Fiber (the delay distributions of the three types of optical signals coincided on the time axis under the designed bending condition (bending radius R =50 mm))

Role of each party

NTT: Fabrication of prototype of three-mode seven-core MCF and demonstration of fully optical coupling.
　Hokkaido University: Analysis of coupling characteristics between all cores and modes and its dependence on bending radius.

Outlook

This research result represents a new alternative to the existing technology³, which realizes spatial multiplexing of 10 or more within a limited cross section of optical fiber, and has the potential to dramatically improve spatial multiplexing while taking advantage of the thinness of optical fiber. In preparation for a further increase in transmission capacity demand in the 2030s and beyond, we will continue to promote basic research on MCF with a multiplicity of 10 or more and the elemental technologies necessary for system construction such as connection and amplification.

[Glossary]

¹Innovative Optical and Wireless Network (IOWN) Concept
https://www.rd.ntt/e/iown/index.html

²Example of Four-core MCF
https://group.ntt/en/newsrelease/2017/08/08/170808b.html

³Example of optical fiber that realizes spatial multiplexing of 10 or more
https://group.ntt/en/newsrelease/2024/03/21/240321a.html
https://doi.org/10.1109/JLT.2023.3275074

⁴World's First Attempt at Optical Transmission Property Control by Optimizing Optical Cable Structure
https://group.ntt/en/newsrelease/2020/03/09/200309a.html

About NTT

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