What Exactly Is Hollow Core Fiber?

1.What is hollow core fiber?

Hollow-core fiber (HCF) uses air as the transmission medium, replacing the traditional optical fiber that uses "glass core" as the transmission medium. With ultra-low latency, ultra-low nonlinearity, potential ultra-low loss and wider passband bandwidth, hollow-core fiber can help OTN systems achieve greater transmission capacity, longer transmission distance, and smaller transmission time delay.

2.Hollow core optical fiber light guide principle

Different from the total reflection principle of conventional optical fiber waveguide light guides, the core of hollow-core optical fibers is air, and light guidance depends entirely on the restriction of light by the cladding. For example, highly reflective silver can be used to constrain the reflection of light so that it can only be transmitted in the air core. This technology was first proposed in the 1960s. It coats the inner wall of a glass capillary with a reflective film and then transmits mid-infrared light in the middle. However, because the hole is relatively large, it is easier to coat, but the larger the hole, the more modes of transmission will occur. In this structure, it is more difficult to achieve long-distance single-mode transmission.

With the advancement of technology, in the 1980s and 1990s, people proposed specially designed cladding structures, such as hollow photonic crystal fibers. Its light guiding principle is the photonic crystal band gap effect. Similar to the concept of band gaps in semiconductors, the cladding air hole structure of this fiber has strict periodicity. When this periodic structure is destroyed by the introduction of the fiber core, a defect state or local state with a certain bandwidth is formed, and only light waves of a specific frequency can propagate in this defect area, while light waves of other frequencies cannot. propagates, thereby forming a constraint on light. With this structure, the refractive index of the core layer does not need to be greater than that of the cladding, so more practical hollow-core optical fibers emerge with various structures.

Hollow core fiber structure diagram

3.Why is hollow core fiber needed?

As we all know, for nearly half a century, optical networks represented by single-mode optical fiber systems have been a solid connection base in the communications world with their advantages such as "large capacity, low power consumption, and low latency." However, quartz (glass), as an optical fiber core material, has inherent limits, including capacity bottlenecks and performance limits.

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Capacity bottleneck: Due to the channel bandwidth restriction of quartz material, the upper limit of single-fiber single-mode C+L band capacity is about 100Tbps. Even if the O/S/U band is expanded, it still cannot break through the P level.

Performance limits: There are theoretical limits including nonlinearity, attenuation, delay, etc., which limits the further improvement of transmission performance (such as distance, delay).

In recent years, with the continuous breakthroughs in hollow-core fiber-related technologies, it is predicted that the transmission capacity, distance and delay of hollow-core fiber systems will be comprehensively improved in the future, becoming the most suitable solution for ultra-low-latency scenarios such as data centers and computing power networks. The best choice will also give priority to commercial trials in these scenarios.

4.Hollow core fiber VS glass core fiber?

Compared with the currently widely used glass-core optical fiber, hollow-core optical fiber has significant advantages in the following aspects.

Low latency: Light is mainly transmitted in the core area close to air holes. The refractive index is lower than that of solid glass, and the transmission speed is faster. The latency drops from 5us/km to 3.46us/km. The transmission latency is compared with existing optical fibers. The system is reduced by 30%. It is very important for current and future delay-sensitive service transmission.

Ultra-low nonlinearity: The nonlinear effect of hollow core fiber is 3 to 4 orders of magnitude lower than that of conventional glass core fiber, which allows the input optical power to be greatly increased, thus increasing the transmission distance. Various equipment manufacturers in the industry, including ZTE, have started research on related optical systems based on this feature, such as 128QAM high-order modulation and high-power amplifier technology, which are expected to increase system capacity and transmission distance by at least 2 times.

Potential ultra-low loss: The current loss achieved by hollow-core optical fiber is 0.174dB/km, which is the same as the performance of the latest generation of existing glass-core optical fiber.At the same time, the theoretical minimum limit of the communication window of hollow-core optical fiber can be as low as 0.1dB/km, which is smaller than the theoretical limit of 0.14dB/km of ordinary glass-core optical fiber.

Ultra-wide operating frequency band: With the continuous optimization of hollow-core fiber structure design, it can provide an ultra-wide frequency band exceeding 1000nm, easily supporting O, S, E, C, L, U and other bands.

5.What is the application progress of hollow core fiber industry?

In view of the technical advantages of hollow-core optical fiber, domestic and foreign universities and companies in the optical fiber field have currently conducted relevant research. The most well-known is lumenisity, a subsidiary of the University of Southampton, which will reduce the attenuation coefficient of hollow-core optical fiber to 0.174dB/km in 2022, representing the highest technical level in the industry. Of course, limited by the maturity of hollow-core fiber technology, including fiber performance improvement (decrease in attenuation coefficient), manufacturing process maturity and standard improvement, it is currently mainly focused on testing and scientific research. For example, in 2021, British Telecom announced that it would cooperate with Lumenisity and global network company Mavenir at the British Telecom Laboratory to test a 10km long hollow-core optical fiber cable. Therefore, it is predicted that it will still take some time for it to be commercialized.

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