What Is Hollow Core Fiber?

In conventional optical fiber, information is transmitted by modulating light and guiding that light through a solid silica-glass core.

The refractive index of the core is slightly higher than that of the surrounding cladding. This difference allows the fiber to confine light through total internal reflection.

Hollow core fiber takes a fundamentally different approach.

Instead of propagating primarily through a solid glass core, the optical field is concentrated in a hollow, air-filled central region.

This difference has an important consequence for propagation velocity.

where c is the speed of light in vacuum and n is the refractive index of the medium.

The speed of light in vacuum is approximately 299,792 km/s. In conventional silica fiber, light propagates at roughly two-thirds of this speed because of the refractive index of glass, giving a propagation velocity of approximately 200,000 km/s.

In hollow core fiber, most of the optical field propagates through air. The effective refractive index experienced by the propagating mode can therefore be much closer to 1, allowing the signal to propagate substantially faster.

For a broader discussion of propagation delay, route length, and low-latency network architecture, see Low-Latency Optical Networks.

How Does Light Propagate Through Hollow Core Fiber?

The most important question is: If the center of the fiber is air, how does the fiber keep the light from simply escaping into the surrounding glass?

The answer is the highly engineered microstructure surrounding the hollow core.

Conventional Fiber: Total Internal Reflection

In conventional single-mode fiber, the structure can be simplified as:

When light reaches the core-cladding interface at an appropriate angle, total internal reflection keeps the optical field confined to the core.

Hollow-core fiber cannot use this mechanism because the refractive index relationship is reversed.

Because the core has a lower refractive index than the surrounding glass, conventional total internal reflection does not provide the required confinement.

Instead, modern HCF designs use antiresonant optical guidance.

Antiresonant Guidance

Modern hollow-core fibers are generally based on the principles of an antiresonant reflecting optical waveguide (ARROW).

Rather than using a conventional solid cladding, the structure surrounding the hollow core consists of carefully engineered glass elements.

These elements act as wavelength-selective optical reflectors. Their thickness and geometry are carefully designed so that, at the operating wavelength, the structure operates in an antiresonant condition.

Under these conditions, optical energy is strongly discouraged from propagating outward through the surrounding glass structure. Instead, the optical field is effectively reflected back toward the hollow core.

The result is optical confinement without relying on total internal reflection.

This is the fundamental physical principle behind modern antiresonant hollow-core fiber.

Nested Antiresonant Nodeless Fiber — NANF

One of the most important developments in hollow-core technology is Nested Antiresonant Nodeless Fiber (NANF).

The NANF concept improves upon earlier antiresonant fiber structures by using nested glass tubes around the central hollow core.

Imagine several thin-walled glass tubes surrounding the central air core. Inside each tube is another smaller glass element.

This nested structure creates additional antiresonant interfaces that help prevent optical energy from escaping from the core.

Why "Nodeless"?

The term nodeless is important. In some earlier microstructured fiber designs, neighboring glass elements could touch each other, creating junctions or "nodes."

These regions can increase optical leakage and scattering.

NANF avoids these connections by keeping the antiresonant elements separated. The result is a cleaner optical structure with improved confinement.

How NANF Confines Light

The operation of NANF can be understood in three basic steps.

• Light propagates primarily through the air core: The fundamental optical mode is concentrated in the central hollow region, with only a relatively small fraction of the optical field interacting with the surrounding glass.
• Thin glass membranes provide antiresonant reflection: The wall thickness of the glass elements is carefully controlled so that, at the operating wavelength, the membranes strongly inhibit transmission of the optical field into the surrounding structure.
• Nested elements provide additional confinement: The inner and outer glass structures create additional optical barriers that reduce leakage and help keep the fundamental mode concentrated in the hollow core.

The result is a fiber in which the majority of the optical power travels through air rather than glass.

From NANF to Double-Nested NANF

The development of NANF was an important milestone, but hollow-core technology has continued to evolve.

Microsoft's current hollow-core technology is based on Double Nested Antiresonant Nodeless Fiber (DNANF).

As the name implies, DNANF adds another level of nested antiresonant structure.

The actual geometry is considerably more sophisticated than this simplified representation, but the basic principle is straightforward: additional carefully engineered antiresonant structure can improve optical confinement and reduce leakage.

The distinction between NANF and DNANF is important because NANF describes a broader class of antiresonant fiber architecture, while DNANF represents a further development of the nested concept.

Why Keeping Light Out of the Glass Matters

The fact that the optical field spends most of its time in air has consequences beyond latency.

In conventional silica fiber, the optical field interacts continuously with the glass. That interaction contributes to several important physical effects, including:

• Rayleigh scattering
• Chromatic dispersion
• Kerr nonlinearity
• Self-phase modulation
• Cross-phase modulation
• Four-wave mixing

Hollow-core fiber dramatically reduces the interaction between the optical field and the glass.

This creates the possibility of reducing several of the physical limitations encountered in conventional optical transmission.

Reduced Kerr Nonlinearity

One of the most interesting advantages of HCF is the potential reduction in Kerr nonlinearities.

In conventional optical fiber, the refractive index depends slightly on optical intensity:

where n₀ is the linear refractive index, n₂ is the nonlinear refractive-index coefficient, and I is optical intensity.

At sufficiently high optical powers, this nonlinear response can produce effects such as:

• Self-phase modulation (SPM)
• Cross-phase modulation (XPM)
• Four-wave mixing (FWM)
• Nonlinear phase shifts

These effects become increasingly important as engineers push transmission systems toward higher optical powers, higher baud rates, and greater spectral efficiency.

Because most of the optical field in HCF propagates through air, its interaction with silica is substantially reduced. This can dramatically lower the effective nonlinear coefficient of the fiber.

For a deeper explanation of Kerr nonlinearity, self-phase modulation, cross-phase modulation, and four-wave mixing, see the FiberGuide article on Nonlinear Optics.

Dispersion in Hollow Core Fiber

Chromatic dispersion occurs because different wavelengths propagate at slightly different velocities.

In conventional silica fiber, both material dispersion and waveguide dispersion contribute to the overall dispersion experienced by the signal.

Hollow-core fiber changes this relationship.

Because the optical field propagates primarily through air, the contribution from the material properties of silica is greatly reduced.

The resulting dispersion characteristics can therefore be substantially different from those of conventional single-mode fiber.

However, HCF should not simply be described as "zero dispersion." The geometry of the hollow core and surrounding structures still produces waveguide dispersion, and total dispersion depends on the specific fiber design and operating wavelength.

For high-speed coherent systems, the possibility of very low dispersion can reduce the burden placed on digital signal processing and simplify transmission-system design.

Hollow Core Fiber and Coherent Optical Transmission

The advantages of HCF become particularly interesting when combined with modern coherent optical technology.

Coherent systems already use sophisticated digital signal processing to compensate for impairments including:

• Chromatic dispersion
• Polarization effects
• Frequency offset
• Phase noise
• Nonlinear distortion

Hollow-core fiber can potentially reduce some of these physical impairments before the signal even reaches the receiver.

This creates an interesting combination:

Together, these technologies can address several challenges associated with next-generation optical networks.

This is also where HCF connects naturally to advanced optical-network engineering. FiberGuide's Certified Optical Network Engineer (CONE) program covers coherent transmission, advanced modulation, DSP, optical impairments, nonlinearities, and high-capacity optical systems.

Why HCF Is Attractive for Data Centers

The first major commercial opportunity for HCF has been latency-sensitive connectivity.

Consider two data centers separated by 100 km.

In conventional silica fiber, propagation delay is approximately 5 microseconds per kilometer, resulting in roughly:

This represents only propagation through the fiber. Equipment, routing, switching, transceivers, and other network components add additional latency.

Because HCF can substantially reduce propagation delay, the potential savings become significant as route length increases.

The benefit is particularly relevant to:

• High-frequency trading
• Data-center interconnects
• Cloud computing
• AI infrastructure
• Real-time analytics
• Financial exchanges
• High-performance computing
• Synchronization applications

For an ordinary internet application, a few microseconds may have little practical significance. For latency-sensitive infrastructure, however, the difference can matter.

For a broader discussion of how fiber propagation, route length, network equipment, and architecture affect latency, see Low-Latency Optical Networks.

Commercial Deployment of Hollow Core Fiber

Hollow-core fiber is no longer purely a laboratory technology.

One of the earliest visible commercial applications was in London's financial network infrastructure, where Lumenisity's NANF technology was deployed by euNetworks on a live route connecting a London data center with the London Stock Exchange.

The technology subsequently attracted significant interest from hyperscale cloud providers.

Microsoft's acquisition of Lumenisity was an important milestone in the commercialization of HCF. Microsoft has since described its current HCF technology as DNANF and has announced collaborations with Corning and Heraeus to scale manufacturing.

This shift is important because the challenge is no longer simply demonstrating that light can propagate through an air core.

The challenge is manufacturing large quantities of high-performance fiber with:

• Consistent geometry
• Low attenuation
• Reliable mechanical characteristics
• Low splice loss
• Acceptable bending performance
• Consistent optical performance

Manufacturing scale may ultimately be just as important as the underlying optical technology.

Remaining Challenges

Despite its promise, HCF still faces important engineering and economic challenges.

Attenuation

Modern HCF attenuation has improved dramatically, but conventional single-mode fiber benefits from decades of manufacturing optimization. For long-haul networks, loss remains an important consideration.

Bending

The microstructured geometry of HCF can make it more sensitive to bending than conventional fiber. Cable construction and route engineering therefore become particularly important.

Splicing and Connectorization

Connecting HCF to another HCF fiber—or to conventional single-mode fiber—is more complicated than a conventional fusion splice.

Differences in mode-field characteristics, geometry, and microstructured cladding must be carefully managed.

Manufacturing Complexity

NANF and DNANF structures contain extremely small geometric features that must be controlled over long lengths of fiber.

Small variations in tube dimensions, wall thickness, or spacing can affect optical performance.

This makes manufacturing technology a critical part of the commercial equation.

Network Integration

HCF is unlikely to replace every kilometer of installed optical fiber.

Instead, its initial deployment is likely to concentrate on routes where the value of lower latency and other performance benefits justify the additional engineering and deployment costs.

Potential applications include:

• Financial trading routes
• Data-center interconnects
• Cloud-provider networks
• Metropolitan low-latency networks
• High-performance computing
• Selected long-haul routes

The Future of Hollow Core Fiber

Hollow-core fiber represents a fundamental change in the way optical transmission can be engineered.

For more than half a century, telecommunications has relied on a remarkably successful concept:

HCF introduces a different paradigm:

The evolution from early antiresonant fibers to NANF and DNANF has brought this concept considerably closer to practical large-scale deployment.

The potential benefits include:

• Lower propagation latency
• Reduced light-glass interaction
• Lower nonlinear effects
• Low chromatic dispersion
• Reduced scattering-related limitations
• Greater optical-power handling potential
• New opportunities for high-capacity coherent transmission

The technology is particularly interesting at a time when AI, cloud computing, data-center expansion, and financial applications are simultaneously increasing demand for both capacity and latency performance.

The ultimate challenge is no longer whether light can be guided through an air core.

It can.

The question is whether HCF can achieve the combination of loss, reliability, manufacturing scale, mechanical robustness, network compatibility, and cost necessary for widespread deployment.

The continuing development of NANF and DNANF suggests that hollow-core fiber is becoming an increasingly important technology to watch.

Learn More About Optical Networking

Understanding hollow-core fiber requires more than understanding the fiber itself.

Engineers working with modern optical networks need to understand how fiber characteristics, dispersion, nonlinearities, optical amplification, DWDM, coherent detection, DSP, and network architecture interact.

FiberGuide provides professional fiber optic training covering the technologies used in modern optical networks.