Why Is Hollow Core-Optical Fiber So Popular?

What is hollow core fiber? Hollow-core optical fiber, also referred to as “hollow optical fiber ” in many articles on the Internet, is called Hollow-core fiber (HCF) in English and is a new type of optical fiber.

The traditional optical fibers we commonly use now are all glass-core optical fibers. Inside the optical fiber is a core made of quartz glass (the main component is silicon dioxide ).

As the name suggests, hollow-core optical fiber no longer has a physical core inside the optical fiber but is “empty” – there is only air, inert gas, or vacuum. So, what are the advantages of hollow-core optical fiber compared to traditional glass-core optical fiber? Why Is Hollow-Core Optical Fiber So Popular? Why is it receiving special attention within the Optical Communication Industry?

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Why Is Hollow-Core Optical Fiber So Popular

The reason for studying hollow-core optical fibers is not that reducing the size of the core inside can reduce costs. This is because optical signals have more advantages when propagating in the air than glass fibers.

In high school physics, we learned an important formula:

v is the speed of light in a given medium. c is the speed of light in a vacuum, which is well known to be about 300,000 km/s. n is the refractive index of the medium.

The speed of light propagation in different media is different.

The refractive index of air is approximately 1. The refractive index of other media is greater than 1. For example, the refractive index of water is 1.33, crystal is 1.55, and diamond is 2.42. Depending on the composition of the glass, the refractive index is approximately 1.5 to 1.9.

This means that the propagation speed of light in traditional glass core optical fiber is significantly lower than c.

According to experimental data, if hollow-core optical fiber is used, the propagation speed of optical signals will be increased by about 47% compared to traditional glass-core optical fiber.

This will significantly reduce the latency of optical fiber communications (by about one-third). According to calculations by relevant research institutions, the latency of glass-core optical fiber is about 5 microseconds per kilometer, and that of hollow-core optical fiber is 3.46 microseconds per kilometer. For a distance of 1,000 kilometers, the latency can be reduced by 1.54 milliseconds.

This latency improvement is of great significance for high-frequency financial securities transactions, as well as industry scenarios such as telemedicine and industrial manufacturing.

Hollow core optical fiber has many advantages, which I will introduce later.

The Development and Evolution of hollow-core Optical Fiber

Next, let’s look at the technical implementation of hollow-core optical fiber. Simply put, optical fiber’s principle is to “trap” light in wired cables. Traditional solid optical fiber, from the inside to the outside, includes three parts: the core, the cladding, and the coating (sometimes a plastic sheath on the outside).

Basic-Components-of-a-traditional-Fiber-Optic-Cable

When light enters the optical fiber, the refractive index n1 of the optical fiber core is higher than the refractive index n2 of the cladding, and total reflection occurs. Then, the light will continue to reflect and eventually propagate forward.

For hollow-core optical fiber, since the refractive index of air is lower than that of the cladding, total reflection will not occur. Therefore, if hollow-core optical fiber wants to achieve the “encirclement” of light, new technical ideas must be adopted.

As early as the 1960s, when Charles Kao published his paper on the creation of optical fiber, some people had proposed the idea of hollow-core optical fiber. However, the material technology then was not mature enough for its realization.

In 1987, American applied physicists Eli Yablonovitch and Sajeev John first proposed the concept of photonic crystal, breaking the deadlock.

Photonic crystal, or photonic bandgap material, is an artificial microstructure formed by a periodic media arrangement with different refractive indices.

Simply put, photonic crystals have the function of “wavelength selection, “which can selectively allow the light of a certain wavelength band to pass through while preventing the light of other wavelengths from passing through.

The colorful gems you see, as well as the glittering, colorful metallic luster of butterfly wings, peacock feathers, and beetle shells in nature, all originate from the special periodic microstructure of photonic crystals, which can selectively reflect specific wavelengths of light.

periodic-microstructure-of-photonic-crystals

Based on the theory of photonic crystals, in 1991, Philip Russell (P.St. J. Russel) of the University of Southampton in the United Kingdom first proposed the concept of photonic crystal fiber ( PCF).

In 1996, Philip Russell’s colleagues, Jonathan Knight and Tim Birks of the Optoelectronics Research Center at the University of Southampton, successfully developed a solid-core photonic crystal fiber sample and confirmed the transmission characteristics of light in photonic crystal fibers.

The above picture is a cross-section of the optical fiber at that time. As you can see, there are many small holes and no obvious core.

The birth of photonic crystal fiber has attracted the attention of the optical research field. Many teams have begun to join in the research of photonic crystal fiber, which has also accelerated the progress of related research.

In 1998, Jonathan Knight and others announced the discovery of the “photonic band gap wave guiding effect in optical fiber” and produced the world’s first photonic bandgap photonic crystal fiber (PBG-PCF ).

In 1999, Philip Russell et al. published a paper in Science proposing the hollow core single-mode photonic band gap photonic crystal fiber (HC-SM-PBG-PCF). Soon after, RFCregan et al . formally developed a sample. (Note that this should be the earliest hollow core fiber in the world. )

Cross-section of hollow-core photonic crystal fibers with different structural designs

The figure above shows that the entire photonic bandgap photonic crystal fiber (PBG-PCF) looks like honeycomb briquettes. Therefore, it was also called holey fiber (HF) and micro-structured fiber (MSF).

The core of the optical fiber is hollow and filled with air. The optical fiber cladding is a large number of air holes arranged in a periodic pattern, all with precisely set aperture size, hole spacing, and period.

When a light signal enters an optical fiber, the photons will enter the cladding from the air core. The periodically arranged air holes in the cladding will form a photonic crystal structure, which prevents photons of a certain frequency from passing through the cladding and “bounces ” them back to the core. This way, the photons can only continue propagating along the air core.

After the emergence of photonic bandgap photonic crystal fiber, although scientists have been trying to improve it, they still cannot solve the loss problem. The loss of this type of fiber has always been at the dB/Km level, and it is difficult to prepare.

This has hindered the application of hollow-core optical fibers. Therefore, scientists continue to explore and try to find new hollow-core optical fiber structures.

The researchers proposed Kagome hollow-core fibers. Later, based on the research on Kagome hollow-core fibers, they proposed antiresonant hollow-core fibers, which became the mainstream research direction in the industry.

Antiresonant-hollow-core-fiber-early-type

In 2019, Francesco Poletti‘s team at the Optoelectronics Research Centre at the University of Southampton invented the famous Nested Antiresonant Nodeless Fiber (NANF), reducing the loss of hollow-core optical fiber to 1.3dB/km.

Just one year later, in 2020, Lumenisity, the industrialization subsidiary of the University of Southampton, reduced the loss of NANF optical fiber to 0.28dB/km, which shocked the entire industry.

We can take a closer look at the structure of NANF fiber:

The center of NANF optical fiber is an air-filled core. Around the core are parallel glass tubes. Each glass tube is nested with another glass tube.

This is called single nesting. If you embed another one, it is double nesting.

The purpose of nesting is related to “resonance.”

Resonance is also called interference. When two waves move in unison, and their amplitudes are maximized, it is called resonance. When the energy of a certain frequency point is minimized, it is called anti-resonance or anti-resonance.

The glass tubes are nested to form a “resonant cavity.”

The transmission spectrum shows multiple peaks. The peaks are separated into multiple high-reflection areas, also called anti-resonance windows. In these windows, incident light from the hollow core causes high reflection, greatly reducing the optical fiber’s leakage loss.

There is no contact between the glass tubes on the sides, which is called nodeless. If there is a node, it will cause greater loss.

NANF optical fiber solves the bottleneck limitation of photonic bandgap photonic crystal fiber, and its theoretical loss and transmission bandwidth are better than the current glass core optical fiber, so it has attracted much attention in the industry.

Photonic bandgap hollow core fiber vs. nested antiresonant nodeless fiber

British Telecom, Comcast, euNetworks, and other companies have adopted Lumenisity’s NANF hollow-core fiber technology in the past few years.

British Telecom uses NANF to construct mobile network bearer networks and conducts quantum key distribution tests on NANF.

Comcast worked with Lumenisity to deploy a 40-kilometer hybrid hollow-core fiber and traditional fiber link in Philadelphia to test and verify compatibility and other aspects.

euNetworks has deployed a 14km stretch of Lumenisity hollow-core fiber between London and Basildon in the UK to connect two data centers critical to financial transactions. Because of the huge commercial value of hollow-core optical fiber, Microsoft directly acquired Lumenisity on December 9, 2022. The transaction price is unknown, but it is not low.

At present, leading domestic fiber manufacturers, such as Changfei and Hengtong, are actively developing hollow-core fiber technology. Many universities are also conducting research in this area, not to mention the three major operators, who are closely watching the relevant progress of hollow-core fiber technology.

In the next few years, the research and implementation of hollow-core optical fibers will be further accelerated.

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Advantages of Hollow-core Fiber

Let’s talk about the advantages of hollow core optical fibers:

Lower latency. This has been introduced in detail before.
Lower loss in transmission. The transmission loss of hollow-core optical fiber is also an important technical indicator of optical fiber. The lower the optical fiber loss, the longer the distance the optical signal can be transmitted in the optical fiber, and the easier it is for the signal to be identified and demodulated at the other end. When optical signals are transmitted in the air, the loss is less than when they are transmitted in quartz glass. As mentioned earlier, the current hollow-core optical fiber can achieve a loss of 0.174dB/km, which is on par with the performance of the latest generation of glass-core optical fiber. According to research institutions, the theoretical minimum loss limit of hollow-core optical fiber can be as low as 0.1dB/km, which is smaller than that of ordinary glass-core optical fiber ( 0.14dB/km ).
Support more optical bands. Hollow-core optical fiber is not picky about light and can easily support multiple bands of light, such as O, S, E, C, L, and U.
Reduced nonlinear effects. The nonlinear effect of hollow-core optical fiber is 3 to 4 orders of magnitude lower than that of conventional glass-core optical fiber, which allows the input optical power to be greatly increased, thereby increasing the transmission distance.
Capable of transmitting high-power laser. When traditional glass-core optical fibers transmit high-power lasers, they absorb laser energy, causing heat accumulation at material defects or uneven temperature distribution between the core and cladding, causing optical fiber damage. In the case of hollow-core optical fiber, more than 99% of the optical power is transmitted in the air, and the light field and material weight are extremely small. Therefore, there is lower material absorption at the same transmission power, and it has a higher laser damage threshold. Simply put, high-power lasers do not easily burn it (kilowatt level). In addition to the advantages listed above, hollow-core optical fiber has low dispersion, thermal sensitivity, radiation resistance, etc. This is why the industry pays close attention to developing hollow-core optical fiber technology.

Application Scenarios of Hollow-Core Optical Fiber

The first scenario is, of course, communication. The low loss and low latency of hollow core optical fibers are very suitable for optical fiber communications, especially the latency-sensitive communication scenarios mentioned above.
The second scenario is sensing, which uses optical fiber for environmental perception. Hollow-core optical fiber has greater flexibility and large aperture characteristics. It can be used in optical sensing to measure temperature, pressure, flow, and chemical composition.
The third scenario is laser application. As I just said, hollow-core optical fiber can withstand high-power lasers. Therefore, it can transmit laser beams, such as laser cutting and etching, in industrial manufacturing and deep in the human body to improve the imaging and treatment of diseased tissues. Transmitting lasers is a form of transmitting energy that has many potential applications.

Final words

In short, hollow-core optical fiber is a good thing. It has many advantages and a very broad application prospect. It is necessary to increase attention and investment in this technology.

At present, hollow-core optical fiber is still working hard to reduce its loss and improve performance indicators.

To accelerate the implementation of this technology, we also need to pay attention to the following points:

Standardization of the internal structure of optical fiber, what kind of architecture should be adopted for finalization and large-scale production.
How can we improve the process, reduce manufacturing difficulty, and achieve mass production with a high pass rate?
Verify and prepare solutions for engineering problems encountered in the current network deployment. The simplest one is how to weld if the hollow core fiber is broken.
How to accelerate the industrial chain layout and provide supporting support regarding materials, devices, etc.

As time goes by, I hope that these questions can be answered. I also hope that hollow-core optical fiber can enter the mature commercial stage as soon as possible and bring further capacity improvement to our network.