Key Highlights
- Single-core fiber has delivered dramatic speed gains through decades of upgrades, and remains essential to global networks.
- Four-core multicore fiber increases capacity within existing cable dimensions, supporting AI infrastructure and high-capacity subsea links.
- Hollow-core fiber can reduce latency and signal loss while enabling higher transmission rates over long distances.
- Microsoft and industry partners are scaling hollow-core production, deployment, and supporting installation ecosystems.
- Hollow-core fiber may also improve quantum networking and allow quantum and classical data to share fiber infrastructure.
For decades, the fiber optic ecosystem has been standardized and optimized around one medium: single-core fiber (SCF), with steady refinements and improvements rolled out over time through electronics upgrades without modifying the cable. SCF fiber cable was placed in the ground in the 1990s for a metro-area T3 telecommunications circuit delivering a whopping 45 Mbps. Today, it delivers 400 Gbps and faster speeds. Undersea circuits deployed in the early 2000s to connect continents at gigabit speeds are now operating at terabit speeds.
Over the years, SCF’s refinements have included improvements to packaging and deployment, such as moving from loose tube fiber into ribbon packaging, reducing the diameter of fiber claddings, and improving the bend radius for tight spaces. We can now pack 864 fibers into a 0.9-inch cable diameter, and you can buy your own DIY Fiber to the Room (FTTR) installation kit on Amazon.
Other innovations have included pre-connectorized and “cut to length” fiber cable to speed installations by making fiber more plug-and-play than stretch-and-splice, reducing both time and the need for skilled technicians. Over the past decade, there has been discussion about manufacturing low-defect ZBLAN optical fibers in space to further increase fiber efficiency, with several demonstrations on board the International Space Station. Still, the economics of producing such media at scale have not been demonstrated.
SCF is and will remain the cornerstone of the fiber world because it is a proven commodity. Still, AI and quantum are demanding more broadband capacity at higher densities, lower latency, and ultra-low transmission loss. Multicore fiber (MCF) and hollow-core fiber (HCF) are moving from specialty to mainstream use to support AI factories and quantum applications, where users are willing to pay a premium for the specialized materials and labor needed to implement them.
Multicore: One fiber, multiple roads
AI scale-out requirements to connect tens of thousands of GPUs have increased the need for more data-carrying capacity in the same space, straining existing conduits and even the weight-bearing capacity of existing cable trays. Multicore fiber packs four separate cores into a single cladding, providing four times the data-carrying capacity in the same diameter.
This spring, AFL, Corning, Lightera, and TeraHOP announced the SDM4 Multicore Fiber Multi-Source Agreement (MSA) to define the technical requirements and operating procedures for four-core fiber use and help the hyperscaler community adopt the technology as they race to build AI factories for next-generation models.
But multicore’s capacity improvement is being applied beyond connecting servers and racks. Last month, Meta announced Petal, the first subsea cable system to utilize multicore fiber technology at scale. The two-core fiber connecting France and the United States is about 4,300 miles long. According to Meta's press release, it will be the first subsea cable to deliver petabit-class speeds over transoceanic distances. It will carry twice the capacity of today’s best-in-class cables without a proportional increase in power or physical infrastructure.
Using multicore technology means Meta can pull the same-sized cable instead of doubling the number of strands, which would double the weight; this saves material, resources, and carbon footprint while delivering the same capacity. The challenges in building Petal were ensuring low attenuation while maintaining the outer fiber's physical dimensions, minimizing crosstalk between cores to maximize optical performance and capacity, and building amplifiers that could handle the 96 total cores while keeping power compatible with existing subsea infrastructure.
Hollow Core = High performance
Hollow core fiber uses a simple concept: a pathway for laser light. Instead of a solid glass core, hollow core is an empty glass tube. Since light travels up to 40% faster through air than glass, HCF provides faster speeds, lower latency, less distortion over distance, and lower signal loss than its SCF counterpart. In addition, air makes additional light frequencies available, adding broadband capacity.
But building hollow core fiber and supporting the ecosystem around it isn’t easy. Fiber manufacturers and their customers have been reluctant to share details on how they are using HCF, given the substantial performance benefits the medium is already delivering. But data center hyperscalers such as Amazon are already deploying the technology, with Microsoft the largest supporter.
On its Azure blog, Microsoft has discussed how its HCF design delivered 33% lower latency, enhanced signal performance for improved data transmission quality, and improved optical efficiency resulting in 47% faster data transmission rates. Since 2023, Microsoft has deployed thousands of kilometers of HCF within its Azure optical network, connecting data centers at metro distances in multiple Azure regions alongside single-mode fiber.
But Microsoft isn’t just using hollow core; it’s an investor. In 2022, it bought UK-based Lumenisity and now collaborates with Corning and Heraeus Covantics to industrialize HCF production to meet its needs. Its efforts include building a larger ecosystem to include new cabling, field splicing, installation, and testing. Microsoft’s longer-term roadmap includes smaller cables with more fibers and enhanced system components to further increase capacity, standardize and simplify deployment and operations, and extend the deployable distance of HCF long-haul solutions.
Microsoft is also working with others to find out exactly how far hollow core can go. At the 2026 European Conference on Optical Communications (ECOC), Microsoft made almost 50 contributions across technical sessions, dedicated workshops, and five post-deadline papers. Building on its work deploying the first bidirectional transmission system using HCF on a 25-kilometer route, Microsoft has demonstrated more than 850 Tbps of aggregate bidirectional transmission, including same-wavelength transmission across the O, E, S, C, and L bands. Keep in mind that today’s SCF is operating at rates up to 1.6 Tbps, while last year’s “hero experiment” announced by CableLabs demonstrated 50 Tbps over existing fiber.
Papers at ECOC discussed how additional wavelengths can be carried over longer distances, with O-band delivering over 61 Tbps over 1,000 kilometers and over 31 Tbps over 3,000 kilometers using Lightera wideband amplifiers. Combining both O- and C-bands on HCF delivered aggregate rates exceeding 105 Tb/s across 2,000 kilometers of HCF.
Microsoft and its partners aren’t the only ones building hollow core. Relativity Networks has successfully manufactured hollow core fiber at Prysmian’s plant in Claremont, North Carolina, with 24 hollow core fibers put into a 10mm diameter cable for data center applications. Prysmian is scaling production of Relativity hollow core with investments at its facility in Eindhoven, Netherlands.
Hollow core fiber is also getting significant attention in the quantum world. Individual photons, or qubits as they are called in the quantum world, can be sent much longer distances over hollow core than SCF before more complex equipment is needed to preserve their quantum properties. CableLabs has been evaluating and measuring how quantum key distribution (QKD) can coexist with traditional data services on operational fiber networks. Its recent work includes measured coexistence performance over both standard single-mode fiber (SMF) and HCF. Businesses can use their existing fiber strand for both, rather than using separate strands for quantum and classical data, resulting in lower operational costs.
Two different techniques have been demonstrated to transmit quantum and classical data along the same fiber. The first method uses two different bands, with QKD information transmitted on the O-band and classical data delivered in the C-band, while the second uses time-division multiplexing, transmitting QKD information at specific time slots down the fiber. Both techniques work better on hollow-core fiber.
As fiber broadband moves toward forty years of operation, the industry is entering an era where multiple media options will meet the needs of next-generation services such as AI and quantum, while single-core fiber will still provide the quality, speed, and robustness households and businesses have come to expect. The next five years will bring fiber-enabled advances we are only just beginning to see today.
About the Author
Gary BoltonRelated
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