Lowest

A Microsoft-backed research team from Lumenisity (a spinout of the University of Southampton Optoelectronics Research Center) has built a hollow-core cable with what it claims is the lowest signal loss ever recorded in any optical fiber. Published in Nature Photonics on Sept. 1, the team claims to have achieved 0.091 dB/km at 1,550nm using a design known as double nested antiresonant nodeless fiber (DNANF).

This figure undercuts the ~0.14 dB/km floor of today’s best silica fibers — and that number hasn’t meaningfully improved since the 1980s. According to the researcherrs, this is “one of the most noteworthy improvements in waveguide technology for the past 40 years.”

Conventional single-mode fiber guides light through glass, slowing signals to roughly 200 million meters per second, which is two-thirds of their potential vacuum speed. In contrast, hollow-core fibers route photons mostly through air — reducing latency by nearly half and minimizing nonlinear effects. This, however, comes with a steep penalty: hollow-core designs leak energy at rates exceeding 1 dB/km, effectively confining them to short, specialist links.

The DNANF design solves this by using concentric glass tubes that are just microns thick. These act like tiny mirrors, bouncing the light back into the air core and suppressing higher-order modes. Testing this on 15km spools using multiple measurement methods, including optical time-domain reflectometry, confirmed attenuation under 0.1 dB/km.

Equally important: loss remained below 0.2 dB/km across a 66THz spectral band. That’s far broader than the narrow telecom windows where silica performs best. Also, chromatic dispersion — which is where different wavelengths of light travel at different speeds through a medium — is reported to be seven times lower than it is in legacy fiber, which could simplify transceiver design and reduce energy use in network gear.

Microsoft’s 2022 acquisition of Lumenisity was a clear sign that it intended to push hollow-core tech out of the lab and into production. At the time, the company’s hollow-core fibers were achieving around 2.5 dB/km loss, which was still a far cry from the traditional ~0.14 dB/km floor of glass fiber.

Following a pilot project conducted alongside the team’s research, Microsoft told Network World that some 1,200 km of the new fiber is actively carrying live traffic. During 2024’s Microsoft Ignite conference, CEO Satya Nadella announced that the company plans to deploy 15,000 km of fiber across the Azure network over the next two years to support AI connectivity.

Francesco Poletti, who co-invented the design, said that the low levels of loss achieved could let operators “skip one in every two or three amplifier sites, resulting in significant reductions in both capital and operational expenditure.”

Of course, there are still some challenges. Scaling production will require new tooling, and standards have yet to be written. But for the first time ever, a fiber that carries light through air is both faster and less lossy than the glass it’s trying to replace.

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