The Game-Changing Potential of Hollow-Core Optical Fibres

In the rapidly evolving field of optical fibres, a groundbreaking innovation is drawing significant attention: hollow-core optical fibres. Unlike traditional optical fibres, which rely on solid glass or plastic cores, this revolutionary design uses a hollow core filled with air or a vacuum. Light signals are guided and transmitted through the structured cladding, utilising mechanisms such as photonic bandgaps or anti-resonance. This unique design minimizes signal loss and dispersion, enabling faster and more efficient data transmission.

Figure 1 – Electron Micrograph of the End Face of an HC-PCF

Image Source: researchgate.net

Structure of Hollow-Core Optical Fibres

The unique design of hollow-core optical fibres consists of a core, cladding, and protective layer. The core’s defining feature is its hollow nature, which can be filled with air, vacuum, or other gases, depending on the specific application and design requirements. Surrounding the hollow core is the cladding structure, which guides the light signals through mechanisms such as photonic bandgap effects or anti-resonance reflection. The cladding is typically made of high-purity quartz and features a periodic structure to achieve the desired optical properties. Similar to traditional optical fibres, hollow-core fibres also have a protective coating and buffer layer to safeguard the delicate internal structure from environmental damage, mechanical stress, and wear during operation, ensuring the fibre’s durability and lifespan in real-world applications.

Light Guiding Mechanism of Hollow-Core Optical Fibres

In traditional solid-core optical fibres, light is confined to the core by total internal reflection at the interface between the high-refractive-index glass core and the low-refractive-index cladding. This relies on the following conditions:

Refractive Index Difference: The core must have a higher refractive index than the cladding.

Incidence Angle: The light must strike the core-cladding interface at an angle greater than the critical angle.

This mechanism ensures efficient propagation of light within the core. However, in hollow-core optical fibres, since the core is made of air (which has a lower refractive index), the traditional total internal reflection conditions cannot be met. Therefore, alternative light guiding mechanisms are required:

A.Photonic Bandgap Effect (Figure 2):

The cladding surrounding a hollow-core optical fibre is made of a photonic crystal composed of periodically arranged micro-holes. When light enters the photonic crystal, the periodic structure causes the light waves to diffract and interfere with each other. This interference creates a series of energy bands, within which some light waves are unable to propagate through the crystal, forming a photonic bandgap. When a light signal enters the hollow-core optical fibre, only light with frequencies that fall within the allowed energy bands can propagate through the hollow core, while other wavelengths are confined to the cladding. This phenomenon is similar to the energy band structure of electrons in a crystal. By designing the structure of the photonic crystal, the propagation characteristics of light in the hollow-core fibre can be precisely controlled, enabling various functionalities, such as single-mode transmission, low-loss transmission, and high-power transmission.

Figure 2 – Photonic Bandgap

Image Source: wikipedia.org

B. Anti-Resonant Optical Fibres (Anti-Resonant Reflection Mechanism, ARROW)

By designing the microstructure of the cladding, specific wavelengths of light undergo multiple reflections within the core without leaking into the cladding. The principle behind this includes the cladding typically being composed of multiple layers of microstructures made from different materials or sizes, which create anti-resonant conditions. The design ensures that light is reflected at the core-cladding interface, and interference effects from the multi-layered structure further suppress light leakage. This design effectively confines the light within the hollow core, enabling low-loss and high-bandwidth optical transmission.

Since 2011, various types of hollow-core anti-resonant fibres have been developed worldwide, including ice-cream structure, single-ring structure, multi-layer structure, nested tube structure, and connecting tube structure, among others. As structural parameters have been continuously optimized, the fibre loss has steadily decreased. For example, the loss of the five-hole nested anti-resonant fibre has dropped to 0.22 dB/km, which is now close to the loss of conventional communication fibres, opening new opportunities for applications such as gas lasers and high-power light transmission.

Figure 3 – LadonX™ Hollow-Core
Anti-Resonant Optical Fibre (HCARF)

Figure 4 – Light Guiding Principle of Hollow-Core Anti-Resonant Optical Fibre

(A) Anti-Resonance; (B) Resonance Image Source: wulixb.iphy.ac.cn

The LadonX™ series low-loss hollow-core anti-resonant optical fibre is constructed with seamless cladding tubes, offering stable single-mode characteristics, ultra-low transmission loss, and a broad transmission bandwidth. Additionally, this fibre exhibits excellent beam quality control, making it an ideal choice for short-distance high-power laser transmission.

Thanks to the unique light-guiding mechanism of hollow-core anti-resonant fibres, where light signals propagate through air, it also possesses low-dispersion transmission properties, holding immense potential in high-speed optical communication.

Hermesys has collaborated with equipment manufacturers to conduct experiments validating the performance of anti-resonant hollow-core fibres under real-world conditions, such as tension, compression, exposure to moisture, and outdoor splicing. The results achieved critical technical benchmarks, including maintaining fibre optic cable losses at 1 dB per kilometre after installation.

C. Other Light-Guiding Mechanisms

Evanescent Field Suppression: The evanescent field in the cladding can couple with the light field in the hollow core, enabling effective confinement.

Inhibited Coupling: By designing the geometry of the cladding, the likelihood of light coupling from the core to the cladding is minimised, achieving low-loss transmission. Optimising the cladding’s microstructure further suppresses unwanted light mode leakage.

Hybrid Mechanisms: Hollow-core optical fibres often combine multiple guiding mechanisms, such as anti-resonance and photonic bandgap effects. Through refined design, they achieve lower transmission loss, broader bandwidth, and higher power tolerance.

These light-guiding mechanisms enable hollow-core optical fibres to confine light efficiently within the hollow core, significantly reducing losses. This has expanded their application scope, especially in fields requiring high power, wide bandwidth, and low nonlinearity. By designing diverse cladding structures, precise control of the light field can be achieved to meet the requirements of different scenarios.

Hermesys offers a comprehensive product portfolio in high-precision optical cables and communication technologies, delivering high-performance fibre and optoelectronic connectivity solutions to users.

Classification and Applications of Hollow-Core Fibre Cladding

Figure 5 – Various Types of Photonic Crystal Fibres

Image Source: researchgate.net

Hollow-Core Photonic Bandgap Fibre (HCPBF) [Figures 5 – 6]:This specialised optical fibre leverages a photonic crystal structure, where its periodic micro structured cladding creates photonic bandgaps to enable efficient light transmission through the hollow core. The photonic bandgap structure confines light energy within the hollow core by restricting specific wavelengths from entering the cladding, significantly reducing transmission losses.

Figure 6 – Hollow-Core

Photonic Bandgap Fibre (HCPBF)

Figure 7 – LadonX™ Large Mode Area Polarisation-Maintaining Ytterbium-Doped Photonic Crystal Fibre (PMYBPCF)

The LadonX™ Series Large Mode Area Polarisation-Maintaining Ytterbium-Doped Photonic Crystal Fibres [Figure 7] offer a significantly expanded mode field area, minimising nonlinear effects to facilitate high-peak-power and large-pulse-capacity laser transmission. With adjustable fibre parameters, these fibres can be customised in various sizes and shapes to cater to diverse application needs.

They find extensive use in high-performance optical communication systems, especially in polarisation multiplexing setups; polarisation-sensitive sensors, such as Faraday rotation sensors and Sagnac interferometers; and high-power laser applications, including fibre lasers.

Figure 8 – Design, Simulation, and Fabrication of NANF Structure

Image Source: nature.com

Nested Antiresonant Nodeless Fiber (NANF) [Figure 8]:The cladding of NANF contains multiple concentric air holes, with light predominantly propagating in the air core. These air holes are precisely designed in terms of diameter and spacing to effectively suppress light leakage into the cladding, resulting in low-loss transmission. Notably, the air holes do not form “nodes” as they are not in contact with each other, allowing for efficient control of light propagation and reduced fiber loss. As of 2020, the loss of NANF fibers has been reduced to 0.28 dB/km.

NANF is highly suitable for high-speed, long-distance fiber-optic communication systems, supporting ultra-large bandwidth and low-loss light transmission, especially for long-distance connections between Fiber to the Home (FTTH) and data centers. Additionally, it plays a crucial role in high-power laser transmission, quantum communication, quantum computing, and sensor technology, particularly in applications with stringent transmission quality requirements. NANF also holds extensive potential in nonlinear optics, including fiber amplifiers, laser modulation, ultrashort pulse lasers, and various medical laser applications, such as laser therapy and laser surgery.

Hollow-core optical fibers offer a new alternative to traditional solid-core fibers. With their unique properties and performance advantages, hollow-core fibers are expected to play a pivotal role in improving internet infrastructure, innovating medical technologies, and optimizing industrial processes. Hermesys has deeply optimized the FC connector design for hollow-core fibers, integrating their distinctive characteristics to precisely align both ends of the fiber, minimizing loss while coupling the emitted light energy into the receiving fiber. This design minimizes the impact on the optical system and supports large-scale deployment, meeting the future needs for high-bandwidth, low-loss, and high-reliability optical communication networks.

References:

“Preparation and mode conversion application of narrowband hollow-core anti-resonant fiber,” Acta Physica Sinica, 71, 134207 (2022).
Yuan, Jinhui et al.: “Study on Hollow-Core Photonic Bandgap Fibers for Visible Light Transmission,” Acta Physica Sinica.