Group Delay Spread Analysis of Coupled-Multicore Fibers

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.