An Introduction to Ultra-low Attenuation Hollow Core Fiber

Recently, there has been increasing attention on hollow-core fiber optics. During a conference two days ago, Microsoft announced its plan to lay approximately 15,000 kilometers of hollow-core fiber over the next two years. This suggests that many professionals and businesses are increasingly favoring this innovative type of fiber. So, what are the advantages of hollow-core fiber over traditional quartz fiber? Follow this guide to learn more about this new type of fiber.

Shortcomings of traditional quartz fiber

As we all know, compared to copper cables, optical fiber emerges with the goal of low latency and low energy consumption. Traditional quartz fiber has encountered the following bottlenecks in meeting these evolutionary directions and demands.

First, there is a nonlinear Shannon limit in terms of transmission capacity. That is, the capacity of quartz fiber can not be further expanded because of the limitation of nonlinear effects.

Secondly, in terms of transmission delay, quartz fiber is slower than the speed of light due to the existence of a refractive index. The traditional quartz fiber delay is about 5us/km.

Finally, quartz fiber is also limited by quartz material scattering. Its loss limit is only 0.14dB/km.

What is hollow core fiber?

Hollow core fiber (HCF) is an optical fiber that uses air as its transmission medium. Inside a hollow core fiber optic cable, a central channel filled with air is surrounded by a ring of glass chains with a hollow hole in the middle. This vacuum-like structure allows optical signals to travel at speeds infinitely close to the speed of light. This results in ultra-low latency and loss, which allows for greater transmission capacity and longer transmission distances.

The history of HCF

The development of hollow core optical fiber seems to repeat the development of quartz fiber loss in the 1970s.

Early hollow-core optical fiber was proposed by Professor Philip Russo of the University of Bath in the United Kingdom, the concept of hollow core optical fiber relationship.

The first generation of hollow-core optical fiber is based on the photonic band effect photonic crystal hollow-core optical fiber; it has about 1.2DB of surface scattering loss limit of limit; its loss can only be made at least about 1.4DB per kilometer.

The second generation of hollow-core optical fiber’s light-sensing mechanism is based on the light reflection effect of a new type of optical fiber. This kind of fiber was first discovered and prepared in 2002. At that time, the loss was about 1DB per meter, which was not considered suitable for optical communications. By 2011, structural improvements improved its loss to 40 DB per kilometer.

By 2018, our team also achieved 2DB per kilometer through structural optimization. Fu Jen University in the UK recently reported a loss of 0.11dB per kilometer at this year’s OFC conference, exceeding the lowest loss limit of traditional quartz fiber development for many years.

However, the current HCF in engineering properties also faces some difficulties. One is that the yield is not comparable to traditional quartz optical fiber, so the price is relatively high. Then, fusion splicing hollow core fiber is also slightly more complex than fusion splicing conventional optical fiber.

Recent advances in hollow-core fiber technology

Our team has accumulated more than ten years of research experience in this field and has accomplished ten world-first research results. In particular, this year, we completed the laying of the first hollow-core fiber optic network and also set a world record for the laying of an ultra-low-loss fiber optic cable.

We can provide an integrated solution for characterizing the fusion splices of hollow-core fiber structures and monitoring their usage. The critical splices do not grow in the same way as the prefabricated rods of traditional quartz fibers. It is structured by stacking to achieve its optical properties, so it is very different from the conventional fiber preparation method. The stacking process, which we call stacking and pulling, is first to stack the structure we want. Then, we realize the desired structure through high temperature, high precision, and precise air pressure control.

We Can now realize 0.2~0.3dB/km loss between hollow and solid cores under high echo reflection. The average loss in the fusion of hollow core fiber and HCF is about 0.1dB, but it needs to rely on a relatively more expensive fusion splicing machine.

The value of hollow core fiber

These four ‘lows’ can summarize the value of hollow core fiber in this optical communication.

Low latency

First, we know that the hollow core fiber is faster than the traditional quartz fiber at the optical level by about one-third of the order of magnitude. In addition, at the electrical level, compared with the conventional quartz fiber, the nonlinear coefficient of the HCF is about 3 to 4 orders of magnitude lower. Therefore, in DSP processing, almost no need for a nonlinear compensation process can significantly simplify the input signal processing and reduce the time delay.

Low power consumption

Secondly, in terms of reducing power consumption, DSP dispersion compensation reduces this important power consumption, and DSP nonlinear compensation makes power consumption almost zero. In addition, the lower the loss, the lower the power consumption. Therefore, in the future, in the transmission process, the number of relay stations and optical modules can also be reduced to reduce power consumption.

Low cost

Lower transmission delay means air-core fiber data centers can cover a wider area. A city’s original coverage, which required four data centers, can now be simplified to three. At the same time, it can be relocated to the far outskirts of the city to cover a larger area. In addition, the simplification of the DSP can also significantly reduce the equipment cost of air-core fiber.

Low system complexity

Empty core fiber’s nonlinear benefit compared to traditional quartz fiber is reduced by three to four orders of magnitude, so there is no Raman. At the same time, hollow-core fiber dispersion compensation is reduced by nearly an order of magnitude. In addition, it helps reduce dependence on advanced processes and builds a secure, autonomous, and controllable communication environment.

Conclusion

Hollow core optical fiber has a comprehensive application scenario, but it will still take some time before it is widely used. However, the excellent communication performance of hollow core fiber includes low latency, low power, low cost, etc. HCF will overcome the original quartz fiber’s shortcomings and improve communication quality and speed.

Disclaimer: This guide is based on Professor Gao Shoufei's speech at the OSIC 2nd OptoWise Conference Forum. For learning and communication purposes, I have organized, modified, and added to Professor Gao Shoufei's speech to create this guide. The speaker retains the copyright for this article. Please let me know if you notice any errors so I can make the necessary corrections.