# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 2 Main Types of Hollow-Core Fibers
### Main Differences  
All hollow-core fibers, except for capillary fibers with metallic mirror, not discussed here, exhibit low-loss guidance of radiation in specific bands of wavelengths only, depending on the refractive index of materials used and the geometry of the light-guiding structure. Fibers of type (c) with nested capillaries (Nested Anti-resonant Nodeless Fiber-NANF) can have low-loss bandwidth of several hundred nanometers [12].  
HC-PBGF fibers (Figure 1b) have a photonic barrier exhibiting periodic variation in the refractive index, made of multiple concentric layers of identical, usually hexagonal, gas-filled cells, sometimes called a \"photonic crystal\", because of regularly repeating units and their directional arrangement. The typical number of layers is 6-9. All solid parts of this fiber are made of the same material.  
The first HC-PBGF was reported in 1999 [11]. The fabrication of such a fiber using the \"stack-and-draw\" method is difficult (Section 9), mostly due to the following factors:  
* Assembling large number of tubes (263 for fiber in Figure 1b) is labor-intensive and requires clean room;
* The dimensions of parts change during the consolidation of preform and the drawing of fiber due to the surface tension of softened glass and variations in the gas pressure in tubes.  
Low-loss bandwidth of HC-PBGF can be considerably reduced via interference from lossy surface modes localized at the core-cladding interface; these modes can anti-cross  
Figure 1: Structures of HCFs: **(a)** Bragg Fiber; **(b)** Photonic Bandgap Fiber (Blaze Photonics HC-1550-02); **(c)** Anti-Resonant fiber of NANF type. Filling gas is shown as white, fused silica as dark grey. In the left drawing, lighter and darker shades of grey indicate low-index and high-index materials of the Bragg reflector. Outer part of cladding and protective coating not shown.  
Figure 2: HCFs with Kagome lattice: **(a)** “pure” Kagome; **(b)** with “negative curvature” surfaces towards core; **(c)** with hybrid lattice including “levitating” anti-resonant capillaries (right). Drawings show ideal structures without distortions introduced during consolidation of preform and drawing of fiber.  
with air-guided modes, giving rise to narrow spectral regions of high loss [11]. Examples of loss spectra are shown in Section 4.1.  
Kagome fibers have a very characteristic photonic structure, including a combination of hexagonal and triangular cells, with two triangular cells adjacent to each larger hexagonal one (Figure 2). Kagome is a name used in Japan for a hexagonal weaving pattern. Despite the superficial similarity to HC-PBGF, the escape of light from the core is blocked by anti-resonance (Section 5.1), not a photonic bandgap. Kagome fibers have wider bandwidth than HC-PBGFs, but the blocking of light escape is usually less effective and fiber attenuation higher. Number of preform parts is comparable.  
Loss can be reduced in hybrid Kagome designs (Figure 2b,c) via the addition of \"negative curvature\" parts (bulges) and/or single or nested tubes constituting additional, nodeless structures around the cavity, at the expense of more complex manufacturing [11]. Thin supports for \"levitating\" tubes (Figure 2c), not shown due to fine dimensions, reduce undesirable light scattering, but can break when the fiber is sharply bent or twisted.  
The lowest attenuation was achieved in NANFs: similar to, at 1550-1600 nm, and lower, at \(\leq\)1350 nm, than in the best solid-core fibers [12; 13].  
Specific types of HCFs are presented in Sections 3-5, with a focus on properties important for communication networks and data transmission, in particular improvements over conventional (solid) fused silica fibers.