An In-Depth Analysis of Hollow-Core Fiber: Principles, Developments and Applications
2026-04-01
In the field of optical communication and photonics, hollow-core fiber (HCF) has emerged as a revolutionary technology that challenges the dominance of traditional solid-core optical fibers. Unlike conventional fibers which rely on a solid silica core for light transmission, hollow-core fibers feature a hollow central region—filled with air, inert gas, or even vacuum—surrounded by a specially designed cladding structure. This unique design overcomes many inherent limitations of solid-core fibers, opening up new possibilities in high-speed communication, high-power laser transmission, optical sensing, and quantum technology. This article provides a comprehensive analysis of hollow-core fiber, covering its basic principles, historical development, key types, advantages, practical applications, current challenges, and future prospects.
To understand hollow-core fiber, it is essential to first compare it with traditional solid-core fibers and clarify the core differences in their light-guiding mechanisms. Traditional solid-core fibers consist of three main components: a solid silica core with a high refractive index, a cladding with a lower refractive index, and a protective coating. Their light transmission relies on the principle of total internal reflection (TIR): when light enters the core at an angle greater than the critical angle, it is continuously reflected between the core and cladding interfaces, thus propagating along the fiber. However, this TIR mechanism is not applicable to hollow-core fibers, because the refractive index of air (or vacuum) in the hollow core is lower than that of the cladding material. Therefore, hollow-core fibers must adopt innovative light-guiding principles to "confine" light within the hollow core, which has driven the continuous evolution of their structural design over decades.
The development of hollow-core fiber can be traced back to the 1960s, when the concept of hollow-core fiber was first proposed shortly after Charles Kao published his groundbreaking paper on fiber optics. However, due to the immaturity of material science and microfabrication technology at that time, this idea could not be realized. The breakthrough came in 1987, when American applied physicists Eli Yablonovitch and Sajeev John proposed the concept of photonic crystals—artificial microstructures composed of periodic arrangements of materials with different refractive indices. Photonic crystals have the ability to selectively transmit light of specific wavelengths while blocking others, a property that laid the foundation for the development of hollow-core fiber.
In 1991, Philip St.J. Russel from the University of Southampton proposed the concept of photonic crystal fiber (PCF), which marked a key milestone in the development of hollow-core fiber. In 1996, his colleagues Jonathan Knight and Tim Birks successfully developed the first solid-core photonic crystal fiber sample, verifying the feasibility of light transmission in such microstructured fibers. Two years later, Knight’s team discovered the "photonic bandgap guiding effect" in fibers and fabricated the world’s first photonic bandgap photonic crystal fiber (PBG-PCF), which is the earliest practical hollow-core fiber. In 1999, Russel’s team published a paper in Science proposing the hollow-core single-mode photonic bandgap photonic crystal fiber (HC-SM-PBG-PCF), and samples were soon developed by R.F. Cregan and others. This type of fiber has a honeycomb-like cross-section, with a hollow core surrounded by periodically arranged air holes in the cladding, which form a photonic crystal structure to confine light in the core.
However, PBG-PCF faced significant limitations, particularly high transmission loss (at the level of dB/km) and complex fabrication processes, which hindered its practical application. This led researchers to explore new structural designs, with Kagome-type hollow-core fiber and anti-resonant hollow-core fiber becoming the main research directions. A major breakthrough occurred in 2019, when Francesco Poletti’s team from the University of Southampton invented the nested anti-resonant nodeless fiber (NANF), reducing the transmission loss of hollow-core fiber to 1.3 dB/km. Just one year later, Lumenisity, a spin-off company of the university, further reduced the loss to 0.28 dB/km, causing a sensation in the industry. NANF features a hollow core filled with gas, surrounded by parallel glass capillaries, each of which is nested with another glass tube. This nested structure forms a resonant cavity, and the "nodeless" design (no contact between capillaries) further reduces loss by avoiding energy leakage at contact points. The anti-resonant principle of NANF ensures that light of specific wavelengths is efficiently reflected back to the core, significantly reducing leakage loss.
Currently, hollow-core fibers are mainly classified into two types based on their light-guiding mechanisms: photonic bandgap hollow-core fibers (PBG-HCF) and anti-resonant hollow-core fibers (AR-HCF). PBG-HCF uses the photonic bandgap effect of the cladding’s periodic air hole structure to block light from escaping the core, while AR-HCF relies on the anti-resonant reflection of the cladding’s capillary structure to confine light. Among them, AR-HCF, especially NANF, has become the mainstream due to its lower loss, wider bandwidth, and simpler fabrication process. In addition, there are Kagome-type hollow-core fibers, which have a unique lattice structure and are suitable for some special application scenarios such as high-power laser transmission.
The unique structure of hollow-core fiber endows it with a series of irreplaceable advantages compared to traditional solid-core fibers. Firstly, it has ultra-low transmission latency. Since light travels in air (or vacuum) at a speed close to the speed of light in vacuum (about 3.46 µs/km), while the speed in solid silica is only about two-thirds of the speed of light (about 5 µs/km), hollow-core fiber can reduce transmission latency by more than 30%. This is crucial for latency-sensitive scenarios such as high-frequency financial transactions, autonomous vehicle networking, and remote precision medical treatment. Secondly, it has ultra-low nonlinear effects, which are 3-4 orders of magnitude lower than those of solid-core fibers. This allows hollow-core fiber to support high-power laser transmission (up to kilowatt level) without signal distortion, making it suitable for industrial laser processing, medical surgery, and other fields. Thirdly, it has a wider bandwidth and higher capacity. The spectral bandwidth of hollow-core fiber can exceed 1000 nm, and the theoretical single-fiber capacity exceeds 270 Tbit/s, which can meet the growing bandwidth demand of future 5G/6G and AI computing networks. Additionally, hollow-core fiber has lower Rayleigh scattering, resulting in higher signal transmission quality and better performance in long-distance transmission.
The unique structure of hollow-core fiber endows it with a series of irreplaceable advantages compared to traditional solid-core fibers. Firstly, it has ultra-low transmission latency. Since light travels in air (or vacuum) at a speed close to the speed of light in vacuum (about 3.46 µs/km), while the speed in solid silica is only about two-thirds of the speed of light (about 5 µs/km), hollow-core fiber can reduce transmission latency by more than 30%. This is crucial for latency-sensitive scenarios such as high-frequency financial transactions, autonomous vehicle networking, and remote precision medical treatment. Secondly, it has ultra-low nonlinear effects, which are 3-4 orders of magnitude lower than those of solid-core fibers. This allows hollow-core fiber to support high-power laser transmission (up to kilowatt level) without signal distortion, making it suitable for industrial laser processing, medical surgery, and other fields. Thirdly, it has a wider bandwidth and higher capacity. The spectral bandwidth of hollow-core fiber can exceed 1000 nm, and the theoretical single-fiber capacity exceeds 270 Tbit/s, which can meet the growing bandwidth demand of future 5G/6G and AI computing networks. Additionally, hollow-core fiber has lower Rayleigh scattering, resulting in higher signal transmission quality and better performance in long-distance transmission.
In addition, hollow-core fiber has important applications in optical sensing and high-power laser transmission. In sensing, its large aperture and high flexibility make it suitable for measuring temperature, pressure, flow, and chemical components, and it can even be used in extreme environments. For example, the European Organization for Nuclear Research (CERN) has used gas-filled hollow-core fibers to make radiation sensors for beam monitoring in particle accelerators. In high-power laser applications, hollow-core fiber’s high laser damage threshold enables it to transmit high-power laser beams for industrial cutting, etching, and medical imaging and treatment of deep lesions in the human body.
Despite its remarkable advantages and rapid development, hollow-core fiber still faces several challenges in achieving large-scale commercialization. Firstly, the manufacturing process is complex and costly. The precise control of the cladding’s microstructures (such as the size and spacing of air holes in PBG-HCF, and the thickness and nesting structure of capillaries in NANF) requires advanced fabrication technologies, resulting in a cost thousands of times higher than that of traditional single-mode fibers. Secondly, although the laboratory loss has been reduced to 0.05-0.091 dB/km (achieved by Microsoft and the University of Southampton in September 2025), achieving stable low-loss transmission over long distances and across the entire band in complex practical environments still requires further research. Thirdly, there are technical bottlenecks such as gas absorption effects, fiber splicing technology, and standardization, which need to be addressed through collaboration across the industrial chain.
Looking to the future, with the continuous advancement of material science and microfabrication technology, and the active promotion of enterprises and research institutions worldwide, the challenges facing hollow-core fiber are expected to be gradually overcome. The cost is likely to decrease significantly with large-scale production, and the performance will continue to improve. In terms of applications, hollow-core fiber will play a key role in next-generation backbone communication networks, data center interconnections, quantum communication, and high-power laser systems. It may even become the core technology supporting the development of 6G networks, enabling faster, more reliable, and more secure information transmission. In addition, the combination of hollow-core fiber with other technologies such as photonic integrated circuits and quantum computing will further expand its application boundaries, bringing new changes to the field of photonics and beyond.
In conclusion, hollow-core fiber, as a new type of optical fiber with a unique hollow structure and innovative light-guiding mechanism, has broken through the limitations of traditional solid-core fibers and become a frontier technology reshaping the optical communication and photonics industry. From its conceptual proposal in the 1960s to the current commercial trials, hollow-core fiber has gone through decades of technological iteration and breakthroughs. With its ultra-low latency, low nonlinearity, wide bandwidth, and high-power transmission capabilities, it has shown broad application prospects in multiple fields. Although there are still challenges in cost, process, and performance, the continuous progress of research and development will surely promote the large-scale application of hollow-core fiber, bringing a new era of high-speed, high-efficiency, and high-reliability optical communication.
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