Hollow-core Fiber

In order to break through the limitations of traditional solid-core quartz optical fibres, researchers have been persistent in their research and exploration. During the research process, Hollow Core Fibre (HCF), which has an air core, came into being. The structure of the hollow core fiber is relatively special compared to the traditional optical fibers, and through its specific cladding structure, it can restrict the light to be transmitted in the air core, which changes the transmission medium of light in the optical fibers, and fundamentally avoids the problems due to the intrinsic limitations in the material. The emergence of air core optical fibre provides an ideal solution to the current limitations of traditional optical fibres.

Hollow-core fibres are available in a variety of structures, and researchers have been optimizing the structure of hollow-core fibres in order to obtain fibres with better performance. From the very beginning, when the Bragg clad hollow core fibre was proposed, researchers have never stopped studying hollow core fibres, but the progress has been very slow. It was not until 1996 that the concept of photonic crystal fibres was introduced, which greatly accelerated the development of air-core fibres. Three years later, the first air-conducting photonic bandgap-type hollow-core fibre was successfully manufactured. Subsequently, researchers proposed the Kagome-type hollow-core fibre. The structure of this fibre is similar to that of a photonic bandgap fibre, but it does not support photonic bandgap transmission, although the Kagome fibre is capable of transmitting in multiple transmission bands simultaneously and covers a wider spectral range overall. Scientists have studied Kagome fibres in depth and have proposed the mechanism of Anti-Resonant Reflective Optical Waveguide (ARROW). The anti-resonance hollow-core fibre was discovered during the study of Kagome. Compared with other hollow-core fibres, anti-resonance hollow-core fibres have a simpler structure and show better performance when the core boundary is negative curvature (the core boundary curvature is in the opposite direction of the curvature of the core circle), and its outer ring of tubular structure does not have much effect on the performance of the fibre. Therefore, anti-resonance hollow-core fibres are becoming the focus of researchers.

Characteristic advantages

The advantages of hollow-core optical fibres are shown in the following aspects:

1) Low delay. According to the formula for the transmission speed of light in a medium with refractive index n, v = c/n, it can be known that when the refractive index of the medium is larger, the transmission speed of light is smaller. Compared to the glass material air has a refractive index of 1, which indicates that the transmission speed of light in the air core light is the speed of light, far more than in the glass medium transmission.

2) Low dispersion. Compared with the solid core optical fibre, because the transmission medium of the hollow core optical fibre is air, which greatly reduces the material dispersion brought about by the transmission loss. Generally speaking, the material dispersion of air-core optical fibre is three orders of magnitude lower than that of solid-core optical fibre.

3) Low non-linearity. Similar to the low material dispersion, due to the low nonlinear refractive index coefficient of air relative to glass materials such as silica makes it have a lower nonlinear effect. At 28dBm into the fibre, 190m long air-core fibre 800G PCS-64QAM real-time signal transmission, after sweeping the wave are not observed significant nonlinear cost (<0.2dB), while the equivalent length of single-mode fibre has exceeded the BER threshold, which further verifies the ultra-low nonlinear effect of air-core fibre.

4) High laser damage threshold. During high-power laser transmission in optical fibres, the fibre material absorbs the laser energy, leading to the formation of thermal accumulation at material defects or uneven temperature distribution between the core and cladding, which results in fibre damage. The hollow core optical fibre can achieve more than 99% of the optical power transmission in the air, the optical field and the material overlap is very small, so in the same transmission power has a lower material absorption, also has a higher laser damage threshold. In addition to the advantages listed above, air-core optical fibres also have the advantages of low thermal sensitivity, resistance to irradiation, and ultra-wide transmission bandwidth. These advantages have greatly facilitated the development and application of hollow-core fibres.

Applications

Based on these advantages of hollow core optical fibres, their applications are mainly in the following categories. The first category is the use of hollow core optical fibres for optical communications and high power laser transmission due to their aberration-free optical transmission.

The ultra-wide transmission bandwidth and low dispersion of hollow-core fibres make it possible to break the current limitations of communication capacity, and the low delay of hollow-core fibres can significantly increase the transmission speed of optical communication. This makes hollow core optical fibres have great potential and development prospects in optical communications, and has generated more and more applications in recent years.

The high laser damage threshold, high beam quality, and low nonlinearity of hollow-core fibres make them a great potential for high-power laser transmission in micromachining, minimally invasive surgery, and multiphoton microimaging.

Another type of application for hollow-core fibres is as a platform for light-matter interaction. In a focused laser beam, the light-matter interaction mainly occurs near the focal point, while in a hollow-core fibre, the light can maintain a higher energy always transmitted in the core, and the effective length of the light-matter interaction is significantly increased, which can effectively reduce the threshold of the light-matter interaction and improve the efficiency. In addition, the hollow core structure of the optical fibre makes it more operable. In practice, the core can be filled with functional materials. This crosses the two disciplines of material science and optics, greatly expanding the application areas of hollow-core optical fibres. The filling material can be solid, liquid, gas, according to the different needs of the filling material selection, which greatly enriches the use of hollow core optical fibre range and methods.

With more and more applications of optical fibres in harsh environments, optical fibre sensing technology will be widely expanded to space applications, so it is necessary to improve the radiation resistance of optical fibres and optical fibre devices, and one of the main solutions to overcome the problem of spatial radiation is the use of hollow-core optical fibres.

Summary

Hollow-core fibre breaks through the inherent delay limit and nonlinear Shannon limit of existing solid-core quartz single-mode fibre, providing a new high-performance base for intelligent computing networks and distributed large models, and is expected to change the optical communication industry based on solid-core quartz fibre for half a century. We have reason to believe that hollow-core optical fibre will occupy an increasingly important position in the future of science and technology as well as in the field of life. Just as traditional optical fibres have informed our technological development over the last few decades, the research and development of hollow-core optical fibres is also progressing, and the huge potential and prospects of hollow-core optical fibres will make them pivotal in the present and future technological revolution.