What is Hollow Core Fiber (HCF)?
Hollow core optical fibers are different from traditional solid glass or plastic core optical fibers, as their interior is empty and can be filled with air, inert gas, or vacuum. This unique structural design method significantly changes the optical propagation characteristics of optical fibers, giving them multiple performance advantages over traditional solid glass core optical fibers. Due to the faster propagation speed of light in air compared to glass, hollow core optical fibers have lower latency and losses compared to traditional optical fibers. Microsoft Lumensity claims that its hollow fiber optic speed is 47% faster than standard quartz glass. In addition, hollow core fibers do not pick up light and can easily support light in multiple bands such as O, S, E, C, L, U, etc.
Hollow core fiber, like traditional glass core fiber, consists of three parts: core, cladding, and coating. The main difference lies in the core and cladding. The core of hollow fiber is air, and the cladding is designed based on microstructure, usually consisting of a series of tiny air holes arranged in a honeycomb like structure. When light is incident on the interface between the fiber 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 specific wavelengths and incident angles to return to the core layer and continue propagating. The function of microstructure is to confine optical signals within the fiber core for propagation, and the performance of hollow core fibers is mainly determined by the microstructure.
Hollow fiber reduces the refraction of light by the medium due to the propagation of light in the air, thereby greatly reducing the transmission delay. The signal loss of hollow fiber is significantly lower than that of traditional fiber, making it suitable for ultra long distance transmission and reducing the need for signal amplifiers. Hollow core fiber significantly reduces nonlinear effects (such as self phase modulation within the fiber) during high-power optical transmission, making it widely applicable in high-power laser transmission and quantum communication.
Hollow core fibers can be simply classified into two categories based on their microstructure design and working principle: photonic bandgap hollow core fibers (PBG-HCF) and anti resonant hollow core fibers (AR-HCF). The development of hollow core optical fibers has mainly gone through the evolution process from photonic bandgap fibers to anti resonant fibers.
Photon bandgap hollow core fibers rely on the photonic crystal structure in the fiber cladding to form a photonic bandgap to restrict the propagation of light beams in the hollow core. The difference in refractive index of photonic crystals means that the light beam can only propagate in the core and cannot leak into the cladding. However, this structure is prone to losses, with a predicted loss of approximately 4dB per kilometer, which limits its use in long-distance networks.
The anti resonant hollow fiber coherently reflects light back and forth between the tubular glass films inside the fiber, confining the light near the air core and transmitting it along the axis. The principle of anti resonance is quite complex, and some humans argue that it is similar to thin film interference. This type of optical fiber utilizes the principle of anti resonant reflection and forms a complex microstructure through special structural design, such as designing multiple layers of specially arranged capillaries. This structure prevents total reflection of light during transmission, and the nested structure of capillaries can significantly reduce the attenuation of hollow core optical fibers.
