Amidst the rapid advancements in optical communications technology, a new type of optical fiber is quietly transforming the data transmission landscape: hollow-core fiber. Unlike traditional optical fibers, it confines light to an air core for transmission, delivering unprecedented performance improvements.

In today's era of explosive data growth, optical communications, as the cornerstone of the information society, are constantly attracting attention for its technological innovations. Hollow-core fiber (HCF), a new breakthrough in the optical fiber field, offers multiple significant advantages over traditional solid-core fiber.

Hollow-core Fiber vs. Traditional Fiber: Fundamental Differences in Operating Principles
Traditional optical fibers, based on the principle of total internal reflection, confine light to a solid glass core for transmission. Hollow-core fiber, on the other hand, utilizes a unique cladding structure, exploiting photonic band gaps or antiresonance effects, to confine light to an air core for transmission.

This fundamental difference has enabled hollow-core fiber to achieve a qualitative leap in multiple performance metrics.

Significant Advantages of Hollow-Core Fiber
Low Latency: Light propagates in hollow-core fibers at speeds approaching the speed of light in a vacuum, approximately 50% faster than in glass. This characteristic can reduce latency by approximately 33%, crucial for time-critical applications such as 5G networks, data centers, and supercomputers.

Low Nonlinearity: Hollow-core fibers virtually eliminate the Kerr optical nonlinearity present in traditional solid-core fibers, significantly improving signal fidelity and power handling capabilities, making them ideal for transmitting continuous-wave or high-energy pulsed lasers.

Low Signal Loss: The latest generation of hollow-core fibers exhibits approximately 10,000 times lower backscatter than traditional fibers, resulting in less signal attenuation and greater transmission distances.

High Laser Damage Threshold: Over 99% of the optical power in hollow-core fibers is transmitted in air, resulting in minimal overlap between the optical field and the material. This allows them to withstand higher powers and reduces the risk of dielectric breakdown damage caused by high-intensity radiation.

Low Chromatic Dispersion: Because the transmission medium is air, the material dispersion of hollow-core fibers is three orders of magnitude lower than that of solid-core fibers, reducing transmission losses due to material dispersion.

Low Temperature Sensitivity: Hollow-core fibers are remarkably resistant to environmental changes, particularly temperature fluctuations. This unique property makes them ideal for applications in high-precision interferometry, synchronization, and time-frequency metrology.

Challenges and Difficulties in Hollow-Core Fiber Splicing
Despite the many advantages of hollow-core fibers, the splicing process is significantly more complex than that of traditional optical fibers. Splicing, the permanent connection of two optical fibers, is crucial to the performance and reliability of optical fiber networks. The unique structure of hollow-core fibers
presents multiple challenges for splicing:

Structural Collapse
During the splicing process, air holes in hollow-core fibers are susceptible to collapse. When the heat generated by the arc discharge softens the glass, surface tension causes the hollow structure to shrink or even collapse completely.

A study has shown that when the degree of air hole collapse increases (d/∧ = 0.94), light with a wavelength of 780nm or 800nm can no longer be stably transmitted in the core mode of the fiber. This collapse not only alters the fiber's geometry but also severely affects its optical properties.

Mode Matching Difficulties
When splicing hollow-core fiber with conventional single-mode fiber, the mismatch in mode field diameters can lead to additional connection loss. Traditional single-mode fiber (such as G.652) and hollow-core fiber utilize different light-guiding mechanisms, resulting in significantly different mode field
characteristics, leading to significant mode mismatch at the interface.

Complex Optimization of Splicing Parameters
Hollow-core fiber splicing requires precise control of discharge intensity, discharge time, and fiber feed parameters. Improper parameter settings can lead to a variety of problems:

Overly thick or thin splices: This is usually caused by excessive fiber feed, excessively fast feed, or an excessively strong discharge arc.

Bubbles or cracks in the splice: This can be caused by poor fiber cutting or electrode aging.

Increased loss after heat shrinking: This is due to contamination of the fiber after stripping the protective jacket. When the heat shrink tubing is tightened after splicing, the remaining contaminants compress the fiber, causing deformation.

End-face Preparation Challenges
Hollow-core fiber end-face preparation requires extremely high standards. Poor cuts, such as end-face tilt, burrs, or unclean end-faces, can reduce splice quality. Compared to traditional optical fibers, the microstructure of hollow-core fibers makes achieving an ideal end face more challenging.

Hollow-core fibers, a disruptive technology in optical communications, are gradually overcoming challenges in their manufacturing and splicing processes, demonstrating tremendous potential for application. As the technology matures, hollow-core fibers are expected to contribute to new optical communication networks and provide a stronger impetus for the development of a digital society.

Although hollow-core fiber splicing currently faces many challenges, continued R&D investment and technological advancements are gradually addressing these challenges, paving the way for widespread hollow-core fiber adoption. In the future, hollow-core fibers are expected to gradually replace traditional optical fibers in specific application scenarios and become the core infrastructure of high-speed communication networks.