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 an air core arrives in about 3.34 µs/km against 4.90 µs/km in glass, 47% faster, a saving of roughly 1.56 µs per kilometer that 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 a record below 0.1 dB/km, removing the historical penalty. Microsoft carries Azure traffic on hollow-core fiber and has contracted Corning and Heraeus to manufacture it at scale. Low nonlinearity also raises the launch-power ceiling, and low chromatic 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.
The full argument, from the Rayleigh floor of solid fiber through the antiresonant designs that got below it, with the latency and span arithmetic worked, is in Hollow-core fiber: below the Rayleigh floor. The fiber latency calculator compares hollow-core and solid fiber delays for any route length.
References: M. N. Petrovich et al., "Broadband optical fibre with an attenuation lower than 0.1 decibel per kilometre," Nature Photonics (2025), doi:10.1038/s41566-025-01747-5; Microsoft Azure Networking Blog, "Microsoft Azure scales Hollow Core Fiber (HCF) production through outsourced manufacturing."