Hollow-Core Fiber Breakthrough: 0.091 dB/km Attenuation at 1550 nm

Microsoft and the University of Southampton have published what the teams describe as a watershed result in optical communications: a hollow‑core optical fiber with measured attenuation of 0.091 dB/km at 1,550 nm, a performance level that — if reproduced in production volumes and field conditions — would beat the long‑standing practical minimum for silica glass single‑mode fiber and reopen design possibilities for long‑haul and data‑center networks. (arxiv.org, networkworld.com)

Background: why attenuation and medium matter​

Conventional telecom fibers use a solid silica glass core and have for decades sat near a practical floor in attenuation: the best silica fibers run roughly 0.14–0.16 dB/km at 1,550 nm, a figure driven by intrinsic material scattering and absorption. That practical floor shaped network architectures — repeater spacing, amplifier sites, wavelength choices and even submarine cable economics — for the last 30–40 years. (mdpi.com, nature.com)
Hollow‑core fiber (HCF) replaces the glass core with an air (or vacuum) core and confines light using a carefully engineered glass microstructure around that core. Because light propagates mostly through air rather than silica, Rayleigh scattering and many glass‑dependent loss mechanisms are dramatically reduced, and the effective group index can be closer to 1.0 — meaning lower latency and reduced nonlinear penalties at high launch power. The tradeoffs historically have been manufacturing complexity, higher early‑generation losses and constraints around bending and field deployment. (mdpi.com, azure.microsoft.com)

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The breakthrough: what the paper reports​

Key measured numbers​

• Measured minimum attenuation: 0.091 dB/km at 1550 nm — reported in the team’s preprint and presented as having been accepted for publication in Nature Photonics. (arxiv.org)
• Broadband low‑loss window: the authors report the fiber remains below 0.2 dB/km across a ~66 THz spectral window, substantially wider than the practical low‑loss bandwidth of conventional silica fibers. (arxiv.org)
• Reported low‑loss subband: multiple industry outlets report the new fiber maintains loss <0.1 dB/km across an 18 THz band, a detail attributed to the experimental characterization; this specific subband claim appears in press coverage and should be considered supported by the team’s paper and subsequent reporting, but it is worth noting that the published preprint highlights the 0.091 dB/km point and the 66 THz <0.2 dB/km window most prominently. (networkworld.com, phys.org)

Design and physics in brief​

The fiber uses a nested anti‑resonant, micro‑structured tubular geometry — the evolution of a design lineage (Kagome, single‑ring tubular, nested anti‑resonant nodeless fibers) aimed at minimizing confinement loss and surface scattering. By carefully tuning capillary wall thicknesses and the nested geometry, the team reduced three dominant loss terms: leakage (confinement loss), surface‑scattering from glass roughness, and microbending‑induced coupling to cladding modes. The result is an air‑core guided mode with very low net attenuation across a very wide spectrum. (arxiv.org, nature.com)

Dispersion and speed​

Beyond loss figures, the authors highlight significant reductions in chromatic dispersion compared with standard SMF, which translates to simpler digital signal processing at the transceiver and reduced energy consumption for coherent links. They also report that light in a hollow air core travels notably faster than in silica: speed improvements of ~45–50% are quoted in the paper and press releases (different outlets report 45%, 47% or 50% depending on rounding and framing). That speed improvement is a physical consequence of the refractive index difference between air and silica and can directly reduce propagation latency on long links. (arxiv.org, azure.microsoft.com)

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Why this matters for networks: practical upside​

Lower amplifier count and energy savings​

Every fraction of a decibel saved per kilometer compounds across long runs. At 0.091 dB/km, a span of 1,000 km would lose ~91 dB — dramatically lower than equivalent silica spans at 0.14 dB/km. In practice, that could allow fewer inline amplifiers, lower amplifier pump power, or longer unamplified spans. Microsoft and the authors explicitly point to potential capital expenditure (CapEx) and operational expenditure (OpEx) reductions, particularly for backbone and regional metro rings where amplifier sites and power infrastructure are material cost drivers. (networkworld.com, arxiv.org)

Lower latency for time‑sensitive applications​

Because the group index in hollow‑core fibers approaches that of air, one‑way propagation delay per km falls. For latency‑sensitive workloads — high‑frequency trading, distributed AI training loops, AR/VR, remote‑control systems — smaller latency adds up. The Microsoft Azure engineering team has framed HCF as an enabler for more distributed data‑center architectures and faster inter‑site paths. (azure.microsoft.com, phys.org)

Wider spectral choices and higher per‑fiber capacity​

A broad, low‑loss window (the 66 THz claim) opens the possibility of using different amplifier technologies and wavelength bands than the constrained C‑band focus of today’s long‑haul links. That extra spectral real estate could be exploited to multiply transmission capacity per cable or to shift some traffic to bands with cheaper or more efficient photonics. The authors argue this is a pathway toward much larger aggregate capacity per fiber. (arxiv.org)

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The Microsoft angle: acquisition, factory and trials​

Microsoft’s acquisition of Lumenisity (a University of Southampton spin‑out) in December 2022 gave the company both the ORC heritage and a Romsey, UK manufacturing facility. Microsoft has invested in scaling that facility and in R&D on the hollow‑core designs, and its Azure engineering organization is a co‑author organization on the preprint. Microsoft blog posts and Azure materials emphasize HCF’s role in accelerating AI and low‑latency networking. (blogs.microsoft.com, azure.microsoft.com)
Industry reporting also states that a pilot involved over 1,200 km of installed hollow‑core fiber that is carrying live traffic, and that Microsoft announced ambitious deployment targets in presentations at Ignite. Those deployment figures appear in trade coverage; the underlying scientific paper is focused on lab and test‑bed characterization and modeling, while corporate announcements and interviews provide the deployment framing. The deployment quantities reported in the press should be read as Microsoft’s operational claims rather than independent, peer‑reviewed measurements of every installed kilometer. (networkworld.com, azure.microsoft.com)
Cautionary note: a number widely quoted in some outlets — 15,000 km of hollow‑core fiber to be deployed across Azure — is attributed to a company presentation, but a direct, independent confirmation (for example a Microsoft press release or regulatory filing listing precise routes and build schedules) is not publicly available in the scientific paper itself. That figure should be treated as a corporate deployment target reported in press coverage, not an independently audited installation statistic. (networkworld.com, blogs.microsoft.com)

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Technical caveats and deployment risks​

High‑profile lab numbers are necessary but not sufficient to guarantee wide commercial impact. The following practical engineering and commercial risks merit careful attention.

1) Manufacturing tolerance and scale​

• The nested anti‑resonant designs require sub‑micron control of capillary wall thickness and very tight geometry tolerances. Achieving lab performance in production reels requires process control beyond standard silica draws, including control of gas contamination and long‑length uniformity. The preprint and Microsoft materials highlight that removing manufacturing impurities expanded the low‑loss window; this implies manufacturing quality is a gating factor for large‑scale rollout. (arxiv.org, networkworld.com)

2) Bend sensitivity and field handling​

• Hollow‑core fibers have historically been more bend‑sensitive than the latest bend‑insensitive silica fibers used in buildings and MDUs. That affects how fiber is routed, coiled and installed in cabinets, ducts and splices. Until bend performance is matched to field expectations, some deployment classes (tight bends in buildings, legacy ducts) will be problematic. Several technical overviews in the literature emphasize a tradeoff between ultra‑low propagation loss and bend loss unless specific design compromises are made. (nature.com, adtek-fiber.com)

3) Splicing, connectors and interoperability​

• The global fiber ecosystem (splicers, connectors, transceivers, field test gear) is built around silica core geometries and well‑established insertion/reflection tolerances. Hollow‑core fibers require optimized fusion splicing recipes, connector designs and test procedures. That ecosystem work takes time and vendor collaboration; until splicing and connector yields are proven at scale, operating expense could rise. (mdpi.com)

4) Amplification strategy for long‑haul and subsea​

• Standard EDFAs (erbium‑doped fiber amplifiers) are tuned to silica‑core C‑band operation. Hollow‑core fibers can operate in the same spectral region — but because most energy is in air, the physics of in‑core amplification and amplifier coupling must be validated for each amplifier topology. The authors say the wide low‑loss window creates flexibility to use different amplifier bands, but the amplifier ecosystem — pumps, fiber amplifiers, Raman/parametric amplifiers — must be engineered to fit any new system architecture. That’s nontrivial and affects undersea and long‑haul economics. (arxiv.org, mdpi.com)

5) Long‑term reliability, environmental and aging effects​

• Field cables see temperature swings, moisture, pressure, compressive forces and rodent attacks. Long‑term stability of the hollow‑core microstructure under mechanical and environmental stress needs years of qualification testing and third‑party verification before operators will hand over mission‑critical backbone routes. Laboratory attenuation at a given wavelength is only the first step. (mdpi.com)

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Where hollow‑core will likely make its first commercial dents​

Given current realities, the initial and highest‑value opportunities for low‑loss hollow‑core fiber are likely to be:

• Hyperscale cloud interconnects and private backhaul — where operators control the end‑to‑end physical plant and can invest in custom handling, splicing and amplifier configurations. Microsoft’s Azure pairing with Lumenisity fits this model. (azure.microsoft.com)
• Latency‑sensitive financial and trading links — capital markets have historically adopted premium fiber technologies to shave microseconds. HCF’s latency advantage is commercially attractive even at a price premium. (networkworld.com)
• High‑power laser delivery and specialized sensing — since air core avoids some nonlinear glass effects, HCF is attractive for high peak‑power laser transport and certain industrial or scientific uses. (nature.com)
• Shorter‑reach data center interconnects where bend constraints can be engineered out — datacenter operator rooms can standardize cable trays and handling to accommodate HCF and capture its bandwidth advantage. (adtek-fiber.com)

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Market and competitive landscape​

Hollow‑core fiber is not new as a research topic; multiple vendors, university spin‑outs and national labs have chipped away at the loss problem for a decade. Lumenisity (the Southampton spin‑out) led commercial efforts early, and Microsoft’s acquisition accelerated one path to scale. Other network operators and vendors have trialed HCF links in metro corridors and niche applications, and trade press has reported commercial trials by Comcast, BT and euNetworks in limited routes prior to Microsoft’s recent announcements. Broad market adoption still depends on supplier competition, price per kilometer, and standards work. (networkworld.com, mdpi.com)

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Standards, test methods and verification — what needs to happen next​

• Industry standardization bodies must define test procedures for HCF attenuation, bend loss, splicing loss and environmental aging that map to existing telco acceptance criteria.
• Transceiver and amplifier vendors must publish interoperability guides (splicing specs, connector designs, optical interface expectations) so system integrators can deploy with predictable budgets.
• Neutral third‑party field studies and long‑duration route tests (months → years) are required to validate the lab numbers under real mechanical, thermal and contaminant conditions.
• Ecosystem vendors (splicer manufacturers, cable makers, installation crews) will need certified processes and quality‑assurance tooling to ensure per‑km performance at scale without prohibitive installation overhead.

These steps are sequential and each takes time; the reported lab results accelerate industry interest, but do not eliminate the need for standards and field verification.

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Balanced assessment: strengths and realistic timelines​

The reported 0.091 dB/km result is a genuine technical milestone: it demonstrates, in laboratory and limited field tests, that hollow‑core optics can outperform the practical attenuation floor of silica fibers while delivering latency and dispersion benefits. If the performance holds up in production reels and field runs, the network design implications are large: fewer amplifiers, lower energy consumption, new spectral strategies and lower latency at scale. (arxiv.org, networkworld.com)
At the same time, practical deployment takes time. Manufacturing scale‑up, bend performance, splice/connector ecosystems, amplifier integration and multi‑year reliability testing are real bottlenecks. Early adoption will be concentrated in environments where the operator can control installation and absorb premium costs for material and handling. Broader adoption in subterranean ducts, metro fiber‑to‑the‑home, or enterprise MDU wiring will require incremental improvements in bend tolerance and cost reductions. (mdpi.com, adtek-fiber.com)

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Immediate action items for network architects and procurement teams​

• Treat hollow‑core fiber as an emerging, high‑value option for future link builds, especially for latency‑sensitive routes and private backbone upgrades.
• Track vendor qualification data, vendor splicing recipes and independent long‑duration field studies before specifying HCF for production critical paths.
• Plan for hybrid designs: mix hollow‑core in the most latency‑sensitive, long spans and retain silica for highly bendy infrastructure to minimize risk during the transition.
• Engage with vendors early on interoperability tests (transceivers, amplifiers, OTDR and power budgets) so procurement terms can reflect the new acceptance and warranty metrics that HCF requires.

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Conclusion: a credible step-change with conditional impact​

The reported Nature Photonics / arXiv result documenting 0.091 dB/km hollow‑core attenuation is an important and credible technical advance that turns academic promise into concrete, testable performance. It strengthens the case that HCF can be more than a niche laboratory curiosity — but it does not, on its own, eliminate the well‑understood engineering and commercial barriers to mass adoption.
Network operators, hyperscalers and equipment vendors will now be racing the same clock: move fast to secure intellectual property and manufacturing scale while also building the standards, splicing ecosystems and field‑proven processes that make a disruptive fiber technology safe to depend on. The next 12–36 months will determine whether hollow‑core fiber remains a powerful tool in a limited number of premium applications or becomes the backbone fabric of future long‑haul and AI‑era datacenter networks. (arxiv.org, azure.microsoft.com)

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Source: Network World Microsoft’s hollow core fiber delivers the lowest signal loss ever