HHON: a hierarchical hybrid optical network for data centers with Non-planar FSO shortcuts

As artificial intelligence (AI) and high-performance computing (HPC) clusters scale, interconnect performance increasingly becomes a primary bottleneck. Small-world networks offer logarithmic average shortest-path length (ASPL), yet realizing random long-range shortcuts in physical data centers is constrained by structured cabling and hard per-node limits. This paper presents the hierarchical hybrid optical network (HHON), a hierarchical hybrid optical network that overlays a regular fiber skeleton with non-planar free-space optical (FSO) shortcuts. A dual-track architecture is introduced to decouple logical-topology stability from physical shortcut placement, together with a global-aware greedy placement rule that combines hop-based gain with a stress-centrality signal approximated via k-Pivots. On hexagonal grids up to 8827 nodes, HHON exhibits small-world-like behavior: ASPL maintains logarithmic scaling with network size and stays within 11.7%–18.7% above an Erdős–Rényi baseline, while the Watts–Strogatz small-worldness metric σ increases from 6.23 to 171.36. Discrete-event simulations under stylized all-to-all and ring all-reduce workloads further suggest stable end-to-end delay within the evaluated scales and settings.