Few-Mode Multi-Core Fiber for Random Coupling Across all Propagation Modes: Design and Demonstration

Few-mode multi-core fibers (FM-MCFs) offer a promising solution for increasing spatial channel counts in optical fiber communication systems, achieving high spatial utilization efficiency without expanding the cladding diameter. However, while coupling within individual mode group has been demonstrated, random mode coupling across all linearly polarized (LP) mode groups in FM-MCFs remains a challenge. In this study, we propose and experimentally demonstrate a novel design for coupled few-mode multi-core fibers that enables random mode coupling across all propagation modes, including inter- and intra-mode group coupling. A hexagonal 3-mode 7-core fiber was fabricated, thereby achieving random coupling among 21 spatial modes. This was realized through inter-mode group coupling between non-adjacent cores via an intermediate core, with the coupling controlled by optimizing the bending radius of the fiber.