# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
###### Abstract  
Hollow-core optical fibers (HCFs) have unique properties like low latency, negligible optical nonlinearity, wide low-loss spectrum, up to 2100 nm, the ability to carry high power, and potentially lower loss then solid-core single-mode fibers (SMFs). These features make them very promising for communication networks and similar applications. However, this class of fibers is still in development. Current applications are almost exclusively limited to low-latency data links for High-Speed Trading (HST); other uses are in the trial stage now. In this paper, we comprehensively review the progress in the development of HCFs including fiber design, fabrication and parameters (with comparisons to conventional single-mode fibers) and support technologies like splicing and testing. A variety of HCF applications in future telecom networks and systems is analyzed, pointing out their strengths and limitations. Additionally, we review the influence of filler gas and entry of contaminants on HCF attenuation, and propose a new fusion splicing technique, avoiding the destruction of the fiber's photonic cladding at high temperature.  
hollow-core fiber; photonic bandgap fiber; anti-resonant fiber; nested anti-resonant nodeless fiber; latency; telecommunications; data transmission; DWDM; optical fiber splicing +
Footnote †: journal: Applied Physics Letters  
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Footnote 1: email: {[EMAIL_ADDRESS]}  
Footnote †: journal: Applied Physics Letters  
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Footnote 1: email: {tomasz.osuch@pw.  
* The length and cost of fiber needed is low in comparison to the economic or other benefits obtained (acceptable cost/benefit ratio);
* Technical requirements cannot be met using traditional fibers. Examples include mid-infrared transmission (\(\lambda\geq 1.75\)\(\upmu\)m), phase-sensitive systems operating in variable temperatures, high-power applications, and systems affected by backscattering.  
Issues currently preventing wide adoption of HCFs in telecom networks are also reviewed, with possible solutions.  
Prospective applications of hollow-core fibers, apart from HST, include the linking of antennae at microwave radio sites, high-capacity DWDM networks, radiation-resistant data links, supercomputers, radars with phased antenna arrays and short synchronization links. Adoption in more cost-sensitive facilities like data centers, LANs, or 5G networks will likely wait for large-scale manufacturing and lower cost of fibers.  
This review is mostly limited to dielectric HCFs made of fused silica, as fibers made of other materials lack an adequate optical and mechanical performance, in particular, attenuation low enough for transmission over kilometer distances. Applications other than transmission in telecom and data systems, like sensors and power delivery, are not covered.  
The rest of this paper is arranged as follows. The main types of hollow-core fibers are introduced in Section 2, and reviewed in more detail later: Bragg fibers in Section 3, photonic bandgap fibers in Section 4, and anti-resonant fibers in Section 5. In Section 6, the consequences of changing the optical transmission medium from glass or polymer to gas are briefly presented, including issues like gas impurities and water penetration into HCF.  
Selected properties and parameters of HCFs important for communication networks are reviewed in Section 7, with comparison to solid-core single-mode fibers. The most important models of the guiding mechanism in HCFs and simulation tools are presented in Section 8, while manufacturing is shortly looked at in Section 9. The splicing and termination of HCFs are described in Section 10. Transmission properties of HC-PBGF and NANF fibers are evaluated in Section 11. Possible applications of HCFs are reviewed in Section 12, and some technologies required for deployment of HCFs in networks in Section 13. The conclusions are presented in Section 14.