Hollow-core optical fibre: sending light through air instead of glass

Almost all optical fibre is a thin strand of solid glass that carries data as pulses of light. Hollow-core optical fibre keeps the light in an air-filled channel at its centre, surrounded by very thin glass walls. Microsoft says the fibre can carry data with less delay, and that a signal can travel further before it needs amplifying (Microsoft Source). That matters because data centre links and long-distance networks depend on fibre, and both delay and the number of amplifier sites can affect cost and energy use.

How it works

The design in the recent results is called a double nested antiresonant nodeless hollow-core fibre, or DNANF. Rings of thin glass tubes, nested inside one another, sit around a central core of air. The tube walls reflect the signal back into the core, which keeps the light confined to the air. Microsoft's Francesco Poletti told IEEE Spectrum that over 99.995 percent of the light travels through air (IEEE Spectrum).

Loss is measured in decibels per kilometre (dB/km). A smaller number means less light is lost per kilometre.

What has been demonstrated

The clearest lab result is a peer-reviewed paper by Petrovich, Numkam Fokoua, Poletti and colleagues at Microsoft Azure Fiber and the University of Southampton, published in Nature Photonics on 1 September 2025 (Nature Photonics). It reports a measured loss of 0.091 dB/km at 1550 nm, and loss below 0.2 dB/km across a 66 THz wide window. The authors compare this with 0.14 dB/km and 26 THz for the best solid glass fibres. An earlier version appeared as a preprint in March 2025 (arXiv preprint).

Conditions matter when reading that figure. It was measured on a single 15 km length of fibre, wound on a bobbin in the laboratory, using repeated cutback measurements. It is not a figure for installed cable, and it does not include losses at joints or connectors.

What is only reported

Several figures come from companies or the press, not from published measurements on installed networks.

• Latency. Microsoft says hollow-core fibre has about 33% lower latency than conventional single-mode fibre, citing published research. The article does not give the route length, test conditions or number of tests behind the figure (Microsoft Source). The Nature Photonics paper describes a 30% reduction in latency as typical of these fibres (Nature Photonics).
• Speed. The gain in the speed at which light travels along the fibre is reported differently by each source. The peer-reviewed paper says 45% (Nature Photonics), while its preprint said 50% (arXiv preprint). IEEE Spectrum says up to 45 percent (IEEE Spectrum), and Microsoft says up to 47% (Microsoft Source). No single figure should be treated as settled.
• Reach. Trade news site optics.org reports that optical power in the new fibre halves after 33 km, compared with 15 km for standard telecom fibre with a typical loss of around 0.2 dB/km. It adds that the listed benefits apply "if and when" the fibre is deployed at scale (optics.org).
• Deployment. Microsoft said in March 2026 that the technology is already in use in some Azure regions and is being deployed in more (Microsoft Source). In September 2025, IEEE Spectrum quoted Poletti saying 1,280 km of hollow-core fibre was deployed and carrying live traffic (IEEE Spectrum). The sources we checked include no independent figures for that network.
• Test installation. IEEE Spectrum describes an earlier generation of the design that links two Azure data centres in Europe over two routes, each more than 20 km long, in cables that combine 32 hollow-core fibres with 48 standard fibres (IEEE Spectrum).
• Competing makers. IEEE Spectrum reports that Yangtze Optical Fibre and Cable and Linfiber Technology, both in China, have reported comparable results, and that Linfiber says it has drawn a continuous 47.5 km length with a loss of 0.1 dB/km (IEEE Spectrum).
• Compatibility and origin. Microsoft says the fibre is designed to work with existing single-mode equipment. It says the technology was developed at the University of Southampton, further developed in a spinoff called Lumenisity, and acquired by Microsoft in 2022 (Microsoft Source).

What is speculative

Microsoft says the fibre could extend the area that one data centre and Azure region can serve, and could eventually play a role inside data centres as well as between them (Microsoft Source). Neither has been shown in the sources we checked.

Limits and open problems

• Joining losses. The 0.091 dB/km figure describes the fibre alone. IEEE Spectrum notes that the fibre needs its own connectors, splicers and amplifiers (IEEE Spectrum), but the sources we checked do not quantify the loss at those points, and that loss can decide whether a link works in practice.
• Manufacturing. IEEE Spectrum quotes CNRS optics researcher Francesco Tani saying that drawing tens or hundreds of kilometres of this fibre is harder than drawing solid fibre, and that a significant part of fabrication is still manual (IEEE Spectrum). Microsoft says it has agreed manufacturing collaborations to increase production (Microsoft Source).
• Cost. None of the sources we checked gives a production cost against conventional fibre.
• Bending. The Nature Photonics paper models bend behaviour and notes that versions with larger cores would be stiffer and less bendable (Nature Photonics). The sources we checked give no measured figures for loss in installed ducts and cabinets.
• Lab to network. The best loss figure comes from a 15 km laboratory sample, the speed and latency figures for installed routes come from the company deploying the fibre, and the sources we checked include no independent audit of those claims.

What to watch next

• Whether other labs reproduce the 0.091 dB/km result, and whether similar loss is reached in production lengths and installed cable.
• Published latency and loss measurements from the Azure routes, with route lengths and test conditions.
• Connector, splice and amplifier products designed for hollow-core fibre, and their loss figures.
• Production volumes and cost from the manufacturing partnerships Microsoft has announced.