Hollow-core fiber (HCF)
Fiber that guides light in an air core surrounded by thin antiresonant glass membranes, now below the attenuation of solid silica, with one-third lower latency and far weaker nonlinearity.
A hollow-core fiber guides light in air rather than glass. Modern designs abandon the photonic-bandgap lattice for antiresonant guidance: a ring of thin nested glass tubes surrounds the hollow core, and their membranes act as antiresonant reflectors that confine the mode with only a tiny overlap, parts in ten thousand, between the field and the glass. Because almost none of the light travels in silica, the mechanisms that define conventional fiber weaken together: Rayleigh scattering, Kerr nonlinearity, and much of the thermal and radiation sensitivity.
Two numbers explain the current attention. Light in air travels about 31% faster than in silica, roughly 1.54 µs/km against 4.9 µs/km of one-way latency per kilometer of glass, which finance links paid for years and AI datacenter interconnects now value. And since 2024 the loss story flipped: nested antiresonant designs from the Southampton and Microsoft lineage reported attenuation at and below the ~0.14 dB/km floor of the best solid silica, with records near 0.1 dB/km, removing the historical penalty. Microsoft has deployed thousands of kilometers in the Azure backbone and in 2026 moved production to outsourced manufacturing scale, which is the clearest sign the technology has left the pilot phase. Low nonlinearity also raises the launch-power ceiling, and near-vacuum dispersion simplifies some coherent links.
The bench differences deserve respect before the first splice. Mode-field matching to standard fiber needs interposers or tapered interconnects, and a poor joint excites surface and higher-order modes that show up later as multipath interference. OTDR traces read differently in an air core, cleaving and contamination control matter more, and connectorization remains a specialist task, one reason deployments still favor engineered cable assemblies over field termination.
References: microstructured and nested antiresonant fiber loss results, Nature and Nature Photonics reports from the University of Southampton and Microsoft Azure Fiber (2024–2025); Microsoft Azure networking technical blog on HCF deployment and manufacturing scale-up (2025–2026).