Traditional optical fibers are solid-core fibers, with both the cladding and core generally made of silica. In hollow-core fiber (HCF), the core is air (or vacuum).

Nested Antiresonant Nodeless Fiber (NANF) leverages the anti-resonance effect generated by nested thin-walled glass capillaries in the cladding to efficiently confine and reflect light back into the air core.

Since NANF has been deployed for experimental commercial applications, this paper only focuses on NANF in the following sections.

The outermost layer is the cladding glass, with nested glass tubes arranged immediately adjacent to it to form an anti-resonant structure. The corrugated petal-shaped pattern in the middle illustrates the effect of light being reflected by the capillary glass tubes.

NANF: Nested Anti-resonance Nodeless Fiber

• Nested: It refers to the nesting structure, namely the nested arrangement of glass tubes shown in the figure above.
• Anti-resonance: The glass sleeve can achieve anti-resonance (also termed resonant suppression), which reflects almost all light back and confines laser propagation within the central air core.
• Nodeless: There are no contact nodes between adjacent glass sleeves. If such nodes exist, light will leak out from the nodes and cause a significant increase in transmission attenuation.

To enhance the light reflection performance, multiple layers of nested glass tubes can be arranged in a tube-inside-tube nested configuration.

Anti-Resonance Principle

Simply put, for light of a specific wavelength, the nested tube wall acts as a perfect total reflection mirror. Any light attempting to enter the glass is efficiently reflected back into the central air core, enabling low-loss light guidance.

In depth, the wall thickness t of the glass tube must satisfy a specific condition. When light waves of wavelength \(\lambda\) try to pass through the tube wall, they undergo multiple reflections inside the glass wall and produce destructive interference, so that optical energy cannot transmit through the glass tube.

The glass tube wall thickness requires special design, typically set to \(1/4\lambda\), \(3/4\lambda\), \(5/4\lambda\) and other odd multiples of a quarter wavelength. If the principle is not easy to understand, you can first review the fundamentals of thin-film interference.

Advantages of Hollow-Core Fiber

Low Latency

The speed of light in air is approximately 300,000 km/s, while in solid silica glass it is about 200,000 km/s (\(v=c/n\)). The light velocity in hollow-core fiber is roughly 31% faster, which significantly reduces transmission latency.

Low Loss

The theoretical loss of conventional G.652 solid-core fiber is 0.14 dB/km, with practical deployment around 0.2 dB/km. By contrast, state-of-the-art laboratory NANF achieves a loss of 0.04 dB/km, and field deployment can reach 0.1 dB/km, with further improvement potential in the future.

High Power Handling

In conventional solid-core fibers, strong light–glass interaction under high optical power induces nonlinear effects that cause signal distortion, and may even burn the fiber end face. In the air core, nonlinear effects are negligible, the laser damage threshold is much higher, and there is no photodarkening effect, enabling high-power laser transmission.

Broad Transmission Bandwidth

Conventional silica glass absorbs light in specific wavelength bands such as ultraviolet and mid-infrared, making these bands undetectable and untransmissible. Air exhibits extremely low absorption at these wavelengths, facilitating transmission band extension toward the ultraviolet and mid-infrared regions.