Hollow core fiber marks a turning point in fiber technology. But first, there are obstacles to overcome
In Sept, 2025, Microsoft Azure announced it is scaling its hollow core fiber (HCF) production by outsourcing manufacturing to Corning and Heraeus. It made clear that the tech giant is taking its commitment seriously, despite being deep in the AI waters — or especially because of it.
Microsoft’s HCF play
If you have followed the developments in HCF, you’d know that Microsoft has been noodling with it for quite some time now. At Ignite 2024, CEO Satya Nadella himself praised it generously for its “absolute breakthroughs” in speed, bandwidth, and power efficiency.
The noise around HCF is not all marketing fluff. Physicists at the University of Southampton in U.K. have shown that HCF exhibits 35% less attenuation and 45% faster signal transmission than standard glass fibers.
Recently, the Azure team tested 1,200 km of Microsoft’s homegrown HCF fiber, achieving 0.091 dB per kilometer of transmission loss, the lowest ever recorded — and a far cry from fiber’s 0.14 dB/km.
“This low fiber loss is absolutely critical for data center to data center connectivity, and we now have production routes of hollow fiber running. In fact, we’re going to add 15,000 additional kilometers planned over the next 24 months,” Nadella declared.
The planned 15,000 km HCF route will lay the ground for wholesale deployment across Azure’s global network, the company said, an effort to provide higher speeds and lower latency to power-hungry cloud and AI workloads in data centers.
Breaking a 40 year deadlock
Optical fiber cables that carry Internet around the world have a solid glass core that guides light signals through with the help of its high refractive index. Hollow core fiber brings a simple architecture modification. It swaps out the solid glass core for an air-filled tube. The walls of this tube are made of ultra-thin glass membranes for easier passage of signals.
As air is more transparent than glass, by default photons travel faster through this hollow center. Additionally, the surrounding glass membranes move the signals through without scattering. This translates as faster transfers — and most importantly, way lower data loss which is critical for AI and high-performance computing (HPC).
This allows fiber players to sidestep signal loss, attenuation, and performance plateau in optical fibers, problems that have existed for four decades. Even the best fibers in the market today need amplification every 12 kms or so.
The high transmission and reduced loss of HCF can prove transformative for next-generation applications, like AI and remote data centers. And the performance gain can uplift other technologies as well, including 5G/6G networks, quantum computing, defense communications, IoT smart cities, and high-frequency trading.
Challenges and barriers
Scala Data Centers, Lightera, and Nokia tag-teamed in November to conduct an HCF test in Sao Paulo. With optical testing and certification equipment from Viavi, the trio tested AccuCore HCF, Lightera’s hollow core fiber optic cable solution, demonstrating 32% reduction in latency. The data transmission speeds are close to “the speed of light”, the companies claimed.
“A 32% reduction in latency time, verified using commercially available 400G high end measurement equipment such as those from Viavi solutions, adds significant weight to this test,” said Andre Champavere, fiber optic measurement and sensing expert, in a LinkedIn comment. As for the OTDR [Optical Time Domain Reflectometer] characterization of HCF fibers, it is somewhat specific (very low backscatter level, high reflective peaks at SMF/HCF hybrid connections), but it can work.”
But before HCF becomes a medium of long-haul connectivity, an obstacle course of challenges lie ahead for HCF players. Manufacturing and installation of HCF need to be heavily controlled. It requires very tight thermal conditions, sub-micron alignment, and precise humidity management. When a cable breaks in the field, repairing becomes tricky as the fibers need to spliced back together within a limited window.
“You cannot leave it open for weeks or months,” Mario Simard, product line manager at Viavi said in a webinar. “If there’s pressure differential, you can get some contaminant inside the fiber,” he explained.
Making matters worse, HCF cables have brittle microstructures. The thin walls are prone to breakage, making the cables sensitive to bending. These factors make handling and splicing a nightmare.
Currently, the solid core and hollow core infrastructures are not directly compatible. “You have to have an adapter from glass to air to connect the HCF to a system or to a test instrument,” Simard noted. Even so, adapter loss can lead to a poor-quality link.
And as with any new technology, HCF also currently faces a lack of standardized Method of Procedure (MoP) — and a shortage of skilled professionals further complicates test and monitoring.
The condition demands specialized testing techniques and parameters, including non-standard (OTDR) settings, additional test wavelengths, and new post-processing analysis algorithms. Best practices, like testing the attenuation profile and chromatic dispersion are also critical to avoid surprises in the field.
Hollow core fiber is still in its early stages, but one thing is clear: hyperscalers and internet service providers (ISPs) are both taking interest in upgrading bits and parts of their infrastructure with HCF to augment performance where there is demand. Side by side, test suppliers are also working on challenges behind the scenes so that when HCF becomes commercially available, companies have a purpose-built test kit at their disposal. That might still be some years away, but for now testing vendors can lean into their decades of fiber testing expertise to understand this new kind of fiber.
