Hollow core fiber (HCF) is a special fiber structure whose core area is hollow (usually air or vacuum) instead of the solid glass/plastic medium of traditional optical fiber. The light in hollow core fiber mainly propagates through the air core, rather than relying on the principle of total reflection, so it can reduce the attenuation of optical signals during transmission and supporting a wider range of spectral transmission.
Cable structure
The hollow-core fiber cable has a central channel filled with air, surrounded by a circle of glass chains. Its cross-section looks like a honeycomb with an empty hole in the middle. The structure is like an undamped data transmission channel in space. The interior is in a vacuum state, which allows the optical signal carrying the data to truly approach the speed of light.
Working Principle
Traditional optical fiber: Relying on total internal reflection (TIR), light is repeatedly reflected and transmitted at the interface between the solid core (high refractive index) and the cladding (low refractive index).
Hollow core fiber: it use the photonic bandgap effect (PBG) or antiresonant effect to guide light. Through periodic structural design (such as photonic crystal arrangement), light of a specific wavelength is confined to propagate in the hollow core to avoid contact with solid materials.
Hollow Core Fiber Types
Photonic bandgap hollow-core fiber (PBGF):
The cladding is composed of periodically arranged tiny air holes, forming a photonic bandgap, which prevents light from entering the cladding and forces the light to be transmitted in the hollow core.
Anti-resonant hollow-core fiber (ARHCF):
The anti-resonance effect of the thin-walled glass tube structure is used to confine the light in the hollow core and reduce the interaction with the glass material.
Bragg fiber:
The light is guided through a multi-layer dielectric reflector (similar to Bragg reflection).
Characteristics of hollow-core fiber
1. Low loss:
Currently, hollow-core fiber can achieve a loss of 0.174dB/km, which is on par with the performance of the latest generation of glass-core fiber. At the same time, the theoretical minimum limit of hollow-core fiber in the communication window can be as low as 0.1dB/km, which is smaller than the theoretical limit of 0.14dB/km of ordinary glass-core fiber, and can be deployed over longer distances without repeaters.
2. Low latency:
Light is mainly transmitted in the core area close to the air hole, with a lower refractive index than solid-core glass, faster transmission speed, and latency reduced from 5us/km to 3.46us/km, which is 30% lower than the existing optical fiber system. It is very important for the transmission of current and future latency-sensitive services.
3. Ultra-low nonlinearity:
The nonlinear effect of hollow-core fiber is 3 to 4 orders of magnitude lower than that of conventional glass-core fiber, which greatly increases the optical power of the fiber, thereby increasing the transmission distance. This is of great significance for certain applications, such as photonics experiments and ultrafast optical research.
4. High laser damage threshold:
Hollow-core fiber can achieve more than 99% of the optical power transmission in the air, and the overlap between the light field and the material is minimal. There is lower material absorption at the same transmission power, so it has a higher laser damage threshold.
5. Ultra-wide frequency band:
With the continuous optimization of hollow-core fiber structure design, it can provide ultra-wide frequency bands exceeding 1000nm, easily supporting bands such as O, S, E, C, L, U, and supporting wavelengths that are difficult to transmit with traditional optical fibers such as ultraviolet and terahertz.
6. Large aperture and flexibility:
Since the center is air or vacuum, its aperture is much larger than that of solid optical fiber, but the bending radius can be very small, so it can be easier to connect with other devices, and is suitable for applications that require bending and complex shapes.
HCF Fiber Application
High-power laser transmission: such as industrial laser cutting and medical surgical lasers, to avoid thermal damage to traditional optical fibers.
- Low-latency communication: high-frequency financial transactions and short-distance interconnection of data centers, using the fast propagation characteristics of the air core.
- Gas sensing: The hollow part can be filled with the gas to be tested, and high-sensitivity detection can be achieved through the interaction between light and gas.
- Quantum communication: Reduce the interaction between photons and materials and maintain the stability of quantum states.
Challenges and limitations
- Manufacturing difficulty: complex microstructures require precision wire drawing technology, which is costly.
- Bending sensitivity: some hollow optical fibers are prone to light leakage when bent.
- Coupling loss: special adaptation is required when connected to traditional optical fibers, which may introduce additional losses.
Comparison: conventional fiber vs hollow core fiber
| Characteristics | Hollow core fiber | Conventional fiber |
| Core materials | Air/vacuum | Glass/Plastic |
| Transmission mechanism | Photonic bandgap/antiresonance | Total internal reflection |
| Loss | Ultra-low (theoretical) | Low (about 0.2 dB/km) |
| Nonlinear effects | Extremely weak | Strong (significant at high power) |
| Delay | Even lower (close to the speed of light) | High (affected by the refractive index of the material) |
Conclusion
Hollow core fiber has broken through the physical limitations of traditional optical fibers through innovative structural design and has shown significant advantages in specific fields. Although the current cost and technical threshold are high, with the advancement of manufacturing technology, it may play a greater role in high-speed communications, laser technology and other fields in the future.
FAQ
What is hollow core fiber?
Hollow-core fiber (HCF), also known as hollow photonic crystal fiber or air-core fiber, is an optical fiber that uses air as the transmission medium.
Can use hollow core fiber to make fiber patch cords or fiber pigtails?
Yes, hollow-core fiber can be used to make fiber patch cords and pigtails, and is used in specific fields (such as ultrafast laser transmission, medical treatment, sensing, etc.).
Where is hollow-core fiber mainly used?
Hollow-core fiber is currently mainly used in medical lasers, data center interconnection and quantum communication.
Is the central mode field diameter of hollow-core fiber 9um?
The mode field diameter of hollow-core fiber is 20~40um, which is much larger than the 9.2um of traditional single-mode fiber.
Is hollow-core fiber G.654E fiber?
Currently, the ITU-T G.654.E standard does not cover hollow-core fiber, and the industry chain needs to promote it in a coordinated manner.
Are hollow-core fiber products expensive?
Currently, the price of hollow-core fiber is relatively high (5-8 times that of traditional fiber), and it is expected to drop to less than 2 times in 2026. Only then can it be used in large-scale commercial applications.
What connectors are currently available for hollow-core fiber patch cables?
Hollow-core fiber patch leads are mainly used for ultrafast laser transmission (such as femtosecond and picosecond lasers) to reduce nonlinear effects and pulse distortion. Standard interfaces include FC, SMA, ST, LC, etc.
Can hollow-core optical fibers be spliced?
Yes, hollow fiber can be spliced, but traditional fusion splicers are not compatible with hollow-core fibers and require special equipment (such as low-loss splicing technology, loss <0.1dB). Hollow-core fibers are prone to breakage and require enhanced protection such as armored PU tubes.
Where can I buy hollow core fiber?
The main hollow-core fiber on the market include Corning’s InfiniCor series, OFS’s AllWave series, etc. China YOFC also developed hollow-core fiber.
References:
- 1. Papers from the Optoelectronic Research Center (ORC) of the University of Southampton;
- 2. “Anti-resonant hollow core fiber may become an ideal medium for ultra-high-speed optical transmission systems”, Li Han of China Mobile;
- 3. “30th Anniversary of Photonic Crystal Fiber: A Brief History of Microstructured Fiber”, Thorlabs;
- 4. “Characteristics and Application Development Trends of Photonic Crystal Fiber”, Jiangsu Hengtong Fiber, Chen Wei;
- 5. “Unveiling the Secret of Hollow Core Fiber: The “Light Speed Road” of Future Communications”, ZTE Documents;
- 6. “Hollow Core NANF Fiber, What is Anti-resonant Nodeless”, Optical Communication Women;
- 7. “The Latest Progress of Hollow Core Fiber HCF”, Fiber, Zhihu.
