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Discussion on Hollow Fiber

Since Dr. Gao Kun, a British Chinese scientist, proposed optical fiber in 1966, optical fiber has almost run through the development of the entire human society and has greatly changed people’s way of life. Up to now, with the continuous advancement of science and technology, people have higher and higher requirements for optical fiber performance. Due to its own limitations, such as material absorption, dispersion, nonlinearity, and low damage threshold, traditional glass optical fibers make it in Fields such as optical fiber communication, high-power laser output, ultrafast optics, and nonlinear optics have shown limitations, restricting the development and progress of related industries.

In order to break through the limitations of traditional optical fibers, researchers have carried out more in-depth research and exploration. During the research process, the hollow core fiber (Hollow Core Fiber, HCF) with the core of air came into being. Compared with traditional optical fibers, the structure of hollow-core optical fibers is special. Through its specific cladding structure, light can be confined in the air core for transmission, which changes the transmission medium of light in the optical fiber and fundamentally avoids Problems due to intrinsic limitations in materials. The emergence of hollow-core optical fiber provides an ideal solution to solve the limitations of the current traditional optical fiber.

The structure of the hollow-core fiber is various. In order to obtain a fiber with better performance, researchers have conducted in-depth research and design on the structure of the hollow-core fiber. Since the initial proposal of Bragg-clad hollow-core fiber, researchers have never stopped research on hollow-core fiber, but the progress is very slow. Until 1996, the concept of photonic crystal fiber was proposed, which greatly accelerated the development of hollow-core fiber.

In 1999, when the first air-guided photonic bandgap hollow-core fiber was manufactured, people carried out a lot of research. During the research process, the structure was improved, and the Kagome-type hollow-core fiber was proposed. The structure of this kind of fiber is similar to that of photonic bandgap fiber, but it does not support photonic bandgap transmission, and compared with photonic bandgap fiber, Kagome fiber can transmit in multiple transmission frequency bands at the same time, and in the overall covered spectrum Wider range.

For the Kagome fiber, the researchers conducted in-depth research on it and proposed the anti-resonant reflective optical waveguide (ARROW) light guiding mechanism. The antiresonant hollow-core fiber was discovered during the researchers’ study of Kagome. Compared with other hollow-core fibers, the structure of anti-resonant hollow-core fibers is simpler, and researchers have found through research that its core boundary is negative curvature (the curvature of the core boundary is opposite to the curvature direction of the core circle) and can perform It has better performance, and the outer ring of tubular structure has little effect on the performance of the optical fiber. Therefore, anti-resonant hollow-core fibers have gradually become the focus of researchers’ research.

Bandgap Photonic Crystal Fiber

Kagome Hollow Fiber

Negative Curvature Hollow Core Fiber

For different kinds of optical fibers, they all have their own unique advantages and disadvantages.

Taking traditional solid-core optical fiber as an example, it is the earliest developed type of optical fiber with the most mature technology and the most widely used at this stage. Nowadays, solid-core optical fiber can be seen in almost every field of society. However, inherent defects such as nonlinearity, dispersion, and light damage of solid-core optical fiber materials also limit the development of optical fibers.

As technology iterations become faster and faster, people’s requirements for optical fiber performance will become higher and higher. The performance of the existing solid-core optical fiber can no longer meet the demand, and it is necessary to seek progress and development, but the intrinsic defects of materials limit the development of solid-core optical fiber, which makes researchers have to turn their perspective to a more advantageous space. Core fiber up. Compared with traditional solid-core optical fibers, hollow-core optical fibers can ensure that light is transmitted in the air, which greatly reduces the influence of material properties on optical fiber performance.

The advantages of hollow-core optical fiber are embodied in the following aspects:

1) Low latency. According to the formula v=c/n for the transmission speed of light in a medium with a refractive index n, it can be known that the higher the refractive index of the medium, the lower the transmission speed of light. Air has a refractive index of 1 compared to the glass material, which means that light travels at the speed of light in a hollow core, much faster than in a glass medium.

2) Low dispersion. Compared with solid-core optical fiber, since the transmission medium of hollow-core optical fiber is air, this greatly reduces the transmission loss caused by material dispersion. In general, the material dispersion of hollow-core fibers is three orders of magnitude lower than that of solid-core fibers.

3) Low nonlinearity. Similar to low material dispersion, air has lower nonlinear effects due to its low nonlinear index of refraction relative to glass materials such as silica.

4) High laser damage threshold. When the fiber is used for high-power laser transmission, the fiber material will absorb the laser energy, resulting in heat accumulation at the material defect or uneven temperature distribution between the core and the cladding, resulting in fiber damage. Since the hollow core fiber can transmit more than 99% of the optical power in the air, the overlap between the optical field and the material is extremely small, so it has lower material absorption under the same transmission power, and has a higher laser damage threshold.

In addition to the advantages listed above, the hollow core fiber also has the advantages of low thermal sensitivity, radiation resistance, and ultra-wide transmission bandwidth. These advantages greatly facilitate the development and application of hollow-core optical fibers.

According to these advantages of the hollow-core fiber, its application mainly includes the following categories. The first category is used for optical communication and high-power laser transmission due to the distortion-free optical transmission of the hollow-core fiber.

The ultra-wide transmission bandwidth and low dispersion of the hollow-core fiber make it possible to break the limitation of the current communication capacity, and the low delay of the hollow-core fiber can significantly increase the transmission speed of optical communication. This makes hollow-core optical fibers have great application potential and development prospects in optical communication, and more and more applications have been produced in recent years.

The characteristics of high laser damage threshold, high beam quality, and low nonlinearity of hollow core fiber make it have great application potential in the field of micromachining, minimally invasive surgery, multiphoton microscopic imaging, etc. required for high-power laser transmission.

Another application of hollow-core optical fibers is as a platform for the interaction of light and matter. In a focused laser beam, the interaction between light and matter mainly occurs near the focal point, while in a hollow-core fiber, light can maintain high energy and is always transmitted in the core, and the effective length of light-matter interaction is significantly increased. This can effectively lower the threshold and improve the efficiency of light-matter interaction.

In addition, due to the hollow core structure of the optical fiber, it has more operability. In practical applications, the core can be filled with functional materials. This intersects the two disciplines of materials science and optics, greatly expanding the application field of hollow-core optical fibers. The filling material can be solid, liquid, or gas, and the filling material can be selected according to different requirements, which greatly enriches the application range and methods of the hollow-core optical fiber.

With the increasing application of optical fibers in harsh environments, optical fiber sensing technology will be widely extended to space applications, so it is necessary to improve the radiation resistance of optical fibers and optical fiber devices. One of the main solutions to overcome space radiation problems is to use hollow core fiber.

With the continuous deepening of research, we have reason to believe that hollow-core optical fiber will occupy an increasingly important position in the future technology and life fields. Just as traditional optical fibers have run through the development of our science and technology in the past few decades, the research and development of hollow-core optical fibers are also making continuous progress. The huge development potential and prospects of hollow-core optical fibers will also enable them to be used in the current and future scientific and technological revolutions. occupies an important position.