Hollow-core fiber (HCF), also known as hollow core fiber or air-core fiber, is a new type of fiber that subverts the traditional structure. Traditional G.652, G.654E and other optical fibers are solid glass core light transmission, while the center of hollow core fiber is air cavity, optical signals are mainly transmitted in the air, rather than glass medium, which is the core transmission medium of the next generation of high-speed, low-delay optical communication.
1. Basic principles and structure
1.2. Structural differences: the center of the fiber has no silica glass core, which is a hollow air channel, surrounded by a special microstructure glass cladding to form a dedicated optical waveguide structure.
1.22. Light transmission principle: abandon the traditional optical fiber "total internal reflection" principle, rely on photonic band gap effect, anti-resonance constraint effect will be firmly limited to the transmission of light in the central air cavity, to prevent optical signal leakage.
1.3. Medium innovation: The transmission medium is changed from solid glass to air, breaking through the transmission performance bottleneck of solid optical fiber from the physical level.
2. Mainstream classification
At present, the mainstream of commercial and scientific research is hollow core fiber, which is mainly divided into two categories, among which anti-resonant hollow core fiber is the mainstream of current technology:
2.1. Photonic band-gap hollow-core fiber (HC-PBF): rely on photonic band-gap effect to restrict the beam, excellent noise reduction performance, but limited bandwidth, early application more.
2.2. Anti-resonant hollow core fiber (HC-ARF): the latest generation of mainstream technology, with ultra-wide transmission bandwidth, ultra-low loss, ultra-low nonlinear advantages, suitable for high-speed long-distance transmission, is the core selection of 6G, computing network.
3. Core Advantages
3.1. Ultra-low transmission delay: light propagates much faster in the air than glass medium, and the delay is greatly reduced. It is the core demand of financial high-frequency trading and AI computing cluster interconnection.
3.2. Ultra-low transmission loss: completely avoid the inherent Rayleigh scattering loss of solid fiber, long-distance transmission signal attenuation is smaller, can further extend the repeat-free transmission distance.
3.3. Nearly zero nonlinear effect: air medium has no glass nonlinear defects, completely solves the signal distortion and crosstalk problems of high-speed WDM systems, greatly improves the transmission capacity of single fiber, and adapts to 400G/800G/1.6T ultra-high-speed systems.
3.4. Anti-interference, strong stability: insensitive to temperature changes, radiation, strong light incidence, environmental adaptability is better than traditional solid optical fiber.
3.5. Ultra-wide working band: can cover visible light, infrared and other bands, applicable to a wider range of scenes.

4. Existing short board
4.1. High cost: The microstructure preparation process is complex, and the cost of the fiber body and supporting devices is much higher than that of conventional solid fiber.
4.2. Construction difficulty: the hollow core structure is fragile, the bending resistance and extrusion resistance are weak, the welding and connection process requirements are extremely high, and the operation and maintenance are difficult.
4.3. Insufficient large-scale application: not yet fully popularized, industrial chain supporting is not as mature as traditional optical fiber, mostly used in high-end special scenarios.

5. Typical application scenarios
5.1. AI Computing Center Interconnection (DCI): Distributed supercomputing, long-distance interconnection of large-scale computing clusters, low latency characteristics can break through the bottleneck of computing communication and support large-scale GPU clusters to work together.
5.2. Financial high-frequency trading line: extremely low delay transmission, to meet the strict requirements of securities and futures high-frequency trading on communication delay.
5.3. Backbone ultra-high-speed transmission network: 800G, 1.6T and above ultra-high-speed trunk wavelength division system, improve single fiber capacity and transmission distance.
5.4. Quantum communication, laser transmission: low nonlinearity, low loss characteristics, suitable for quantum signals, precise transmission of high-energy lasers.
5.5. 6G communication bearer network: adapt to the future 6G ultra-low latency, ultra-large bandwidth network transmission requirements.
5.6. Special industrial scenarios: aerospace, military, high radiation, high temperature and other harsh environment communication transmission.

6. SUMMARY
Hollow core fiber is a disruptive technology in the field of optical communications. It breaks the physical performance ceiling of traditional solid fiber through "air transmission" and comprehensively surpasses the four core indicators of delay, loss, capacity and nonlinearity. Although it has not yet been popularized by the whole people due to cost and construction technology constraints, it is already the core development direction of high-end computing power networks, ultra-high-speed backbone networks, 6G, and quantum communications, and is a key carrier for the iterative upgrade of optical networks in the future.