Everything You Ever Wanted To Know About Hollow Core Fiber

Schematic diagram of hollow core fiber structure

1. What Is Hollow Core Fiber

Hollow core fibre, or HCF for short, replaces the traditional optical fiber with a glass core as the transmission medium, using air as the transmission medium.

Hollow core fiber, like traditional glass core fibre, consists of three parts: the core, the cladding and the coating, but the main difference lies in the core and the cladding.

The core of a hollow core fiber is air, and the cladding is based on a microstructural design, usually consisting of a series of tiny air holes. These air holes are aligned along the length of the fiber to form a specific periodic structure, and the cross-section of the cladding resembles a honeycomb made up of a network of fine silicon filaments.

2. Why You Need Hollow Core Fiber

In recent years, with the development of emerging technologies such as big data, cloud computing and the Internet of Things (IoT), there is an increasing demand for high-speed, high-capacity, low-loss optical communication systems. Traditional glass-core optical fibers, with glass as the fiber core material, have intrinsic limits, including capacity bottlenecks and performance limits.

For example, constrained by the channel bandwidth of the glass material, there is an upper limit to the amount of information it can transmit, just as there is a limited flow of water in a water pipe. Moreover, due to the theoretical limits of nonlinearity, attenuation, and time delay, optical signals may be deformed, weakened, or slowed down when transmitted in glass fibers, which limits the further improvement of the transmission performance (e.g., distance and time delay).

In order to overcome the problems of traditional optical fibers in the transmission process and to improve the efficiency and performance of optical communications, hollow core optical fibers are applied.

3. How Does Hollow Core Fiber Work?

Traditional optical fiber is based on the principle of total reflection of light to achieve light transmission in the glass core.

When light enters the centre of the fiber to propagate, the refractive index n1 of the fiber core is higher than that of the cladding n2, and the loss of the core is lower than that of the cladding, so that the light will undergo the phenomenon of total reflection, and its light energy will mainly be transmitted in the core, and with the help of successive total reflection, light can be transmitted from one end to the other.
total reflection of light
The core of a hollow core optical fiber is air, and since the refractive index of air is smaller than that of the cladding medium and does not satisfy the condition of total reflection, a specially designed cladding structure is required for transmitting light in a hollow core.

For example, based on the photonic bandgap effect of hollow photonic crystal fiber, its cladding consists of a series of tiny air holes, with a precisely set aperture size, hole spacing and period, the core is air.

When light is incident on the interface between the core and the cladding, it is strongly scattered by the periodically arranged air holes in the cladding. This multiple scattering produces coherence, allowing light waves that meet a specific wavelength and angle of incidence to return to the core for further propagation.

With this structure, the guidance and propagation of light is achieved with the refractive index of the fiber core layer being less than that of the cladding.
Schematic diagram of hollow core fiber structure
The photonic bandgap waveguide mechanism above is a complex optical phenomenon, and we can try to explain it in layman’s terms:

Imagine a long corridor with many columns, the columns are arranged in a certain pattern, and you try to throw a ball and see how it rolls through the corridor.

Because of the pillars, the ball will not go straight through the gallery, but will bounce and roll between the pillars. In some cases, the ball may not be able to move forward because of the arrangement of the pillars, as if it were ‘trapped’.

In photonic bandgap waveguiding, the photon acts as the ball, and the photonic crystal (a structure with periodic refractive index changes) acts as a corridor with many pillars. The air holes in the photonic crystal are arranged in a certain pattern just like the pillars.

When a light wave enters this structure, it is reflected and scattered between the air holes, like a ball bouncing between the pillars. Some frequencies of light waves will not propagate through the cladding because of this special arrangement, just as a ball cannot move forward under certain circumstances. These light waves are then confined to the core, just as the ball is ‘trapped’ in a certain area of the gallery.

4. What Are The Advantages Of Hollow Core fiber

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

  • Low time delay

The low refractive index of the internal air core and the optimized structural design, so that the light in the hollow core optical fiber propagation speed faster, delay from 5us/km down to 3.46us/km, the transmission delay compared to the existing optical fiber system to reduce the 30%, for the current and future delay-sensitive business transmission is very important.

  • Ultra-low non-linearity

The interaction between light and medium in the air core is weakened, thus reducing the generation of nonlinear effects.

The nonlinear effect of an air-core fiber is three to four orders of magnitude lower than that of a conventional glass-core fiber, which allows the optical power of the incoming fibers to be significantly increased, thus improving transmission distance.

Various equipment manufacturers in the industry have launched research on optical systems based on this feature, such as 128QAM high-order modulation and high-power amplifier technology, etc., which is expected to improve system capacity and transmission distance by at least two times.

  • Potential ultra-low loss

Currently, the loss of hollow core optical fiber is 0.174dB/km, and the performance of the latest generation of glass core optical fiber is the same, the theoretical minimum limit of hollow core optical fiber in the communication window can be as low as 0.1dB/km, which is lower than the theoretical limit of 0.14dB/km of ordinary glass core optical fiber.

5. Where Hollow Core Fiber Can Be Used?

So where will hollow-core fiber be used?

  • Long-distance transnational communications and satellite communications

Hollow core optical fiber has the characteristics of high efficiency, high speed and high capacity long distance communication, which is the preferred solution for future long distance communication.

  • Data transmission in data centers

Ideal for large-capacity cloud computing.

  • Medical equipment

Hollow core optical fiber can be used in the manufacture of medical equipment to provide clearer and more accurate images.

  • Sensor and monitoring system construction

Empty-core optical fibers are used to build sensors and detection systems for real-time monitoring and remote control of industrial equipment.

  • Secure communications

Hollow core optical fiber can be used to build secure communications, improving the security and reliability of communications.

Echo Huang

Echo Huang is an expert wordsmith and marketing professional at Bonelinks with more than 8 years of experience in high technology businesses – fiber optics, IoT, and telecommunication. She is very glad to share industry knowledge and communicate with others.

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