Group Delay Spread Analysis of Coupled-Multicore Fibers

Significance Statement

Recently, mode-division-multiplexing transmission technique has attracted considerable attention due to its promising potential as a technology for increasing network capacity. Various few-mode fiber designs have been proposed as a result of the comprehensive studies of their application in mode-division-multiplexing transmission. The most popular few-mode fiber studied are the coupled-multicore fibers, since their measured group delay spread is considerably lower than that predicted by differential modal group delay of the Eigen modes of the fiber. The reduced group delay spread contributes to easier configuration of MIMO receiver and consequently a longer system reach is achieved. In recent experimental studies, macro-bending with twisting has been observed to be a dominant mechanism for random modal coupling. In such studies, results for short length fiber indicate that the correlation length is much shorter for strongly bent conditions. Thus for the ultimate performance of coupled-multicore fibers installed in various conditions to be extracted, general theoretical model studies for estimating group delay spread of coupled-multicore fibers have been seen necessary.

Takeshi Fujisawa and Kunimasa Saitoh at Hokkaido University in Japan investigated group delay spread of coupled three-core fiber as applied based on coupled-wave theory with super-mode or discrete core mode basis. The researchers hoped to unearth the differences between super-mode and discrete core mode models and reveal applicability for both models for specific fiber bending condition. Their work is now published in the peer-reviewed journal, Optics Communications.

Fujisawa and Saitoh begun by studying the fiber structure and coupled wave theory where they used coupled-wave theory to estimate group delay spread while fully considering the field amplitude and phase amplitude, since they were essential for treating the changes of modal group delay due to perturbations. Supermode models for gentle bending conditions were presented by applying the finite element method during the computation of the coupling coefficient. Discrete core mode model for tight bending conditions were also presented. In general, they thoroughly investigated the variances between supermodes and discrete core mode models so as to reveal applicability of both models in a precise fiber bending condition.

The authors also observed that for the weakly bent condition, both discrete core mode models and supermode model were applicable. Conversely, for strongly bent condition, the discrete core mode model was seen applicable for use to account for the increased differential modal group delay for the fiber without twisting and short correlation length as recently observed from other experimental studies.

Fujisawa and Saitoh developed an innovative technique for investigating the group delay spread of coupled three-core fiber based on coupled wave theory. The discrete core mode model has been observed to be superior to the supermode model for the analysis of coupled-multicore fibers for various bent conditions. Therefore, we conclude by stating that in estimating group delay spread of coupled-multi-core fiber, it is critically important to consider the fiber bending conditions.

About the author

Takeshi Fujisawa was born in Sapporo, Japan, on January 12, 1979. He received the B.E., M.E., and Ph.D. degrees in electronic engineering from Hokkaido University, Sapporo, Japan, in 2001, 2003, and 2005, respectively. He was a Research Fellow of the Japan Society for the Promotion of Science from 2003 to 2006. In 2006, he joined the Nippon Telegraph and Telephone (NTT) Photonics Laboratories, NTT Corporation, Atsugi, Japan. In 2014, he became an Associate Professor in the Graduate School of Information Science and Technology, Hokkaido University. He is the Author or Coauthor of more than 90 papers in international refereed journals.

His research interests include active optical devices for optical communication systems, such as semiconductor lasers and modulators, and theoretical modeling of optical fibers and devices. He has been serving as an Associate Editor for Optics Express since 2014 and is a member of IEEE and the Institute of Electronics, Information and Communication Engineers of Japan.

About the author

Kunimasa Saitoh received the B.S., M.S., and Ph.D. degrees in electronic engineering from Hokkaido University, Sapporo, Japan, in 1997, 1999, and 2001, respectively. From 1999 to 2001, he was a Research Fellow of the Japan Society for the Promotion of Science. From 2001 to 2005, he was a Research Associate with the Graduate School of Engineering, Hokkaido University. From 2005 to 2013, he was an Associate Professor at Graduate School of Information Science and Technology, Hokkaido University, and in 2013, he became a Professor there. He has been involved in research on fiber optics, nano-photonics, integrated optical devices, and computer-aided design and modeling of guided-wave devices. He is the author of more than 190 research papers in refereed international journals and more than 240 refereed international conference presentations.

He is a Member of the Optical Society of America (OSA), and the Institute of Electronics, Information and Communication Engineers (IEICE). He was a Chair of Subcommittee D4 of Optical Fiber Communication Conference (OFC) in 2016. He is currently a Technical Program Committee of European Conference on Optical Communication (ECOC). He received the Excellent Paper Award and the Young Scientist Award from the IEICE, in 1999 and 2002, respectively, the Young Scientists’ Prize of the Commendation for Science and Technology from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Government of Japan in 2008, the JSPS Prize from the Japan Society for the Promotion of Science in 2015, and the Distinguished Lecturers Award from the IEEE Photonics Society in 2017.


Takeshi Fujisawa, Kunimasa Saitoh. Group delay spread analysis of coupled-multicore fibers: A comparison between weak and tight bending conditions. Optics Communications, volume 393 (2017) pages 232–237.

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