# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 5 Fibers with Anti-Resonant Guidance
### NANF and DNANF Fibers
In addition to \"basic\" ARF/NANF/DNANF fibers with all round tubes shown in Figure 12, several other designs exhibiting a low (predicted) confinement loss were recently proposed, including NANF with elliptical inner tubes (Figure 15a, CL \(\approx 10^{-3}\) dB/km) [51], DNANF with two parallel smallest tubes located in the same radial distance from the fiber axis (Figure 15b, CL \(\approx 10^{-5}\) dB/km) [52], and ARF with non-nested stadium-shaped tubes elongated in the radial direction (Figure 15c) to increase the distance between the cavity and the outer jacket, and improve the azimuthal confinement of light [53]. Unfortunately, the substantial reduction in confinement loss alone cannot proportionately decrease the total fiber attenuation, as other loss components like SSL (Section 6.2), absorption by gas contaminants (Section 6.3), or bending loss remain. The chromatic dispersion of NANF fibers designed for the 1550 nm band is low: 1.5-4 ps/nm\(\cdot\)km (Figure 16), with flat characteristics over a wide range of the spectrum: 200 nm and more [12]. This property is valuable in long-distance DWDM systems, eliminating or substantially reducing the need for dispersion compensation. Figure 16: Chromatic dispersion of typical NANF [12] and Corning SMF-28e+ [30] fibers. In NANF, CD slope rises close to resonant wavelengths; negative CD at short wavelengths is possible [12; 40]. Figure 15: Alternative ARF structures with low confinement loss: (a) NANF with elliptical inner tubes [52], (b) DNANF with two parallel inner tubes [53], (c) ARF with “stadium” tubes [54]. Despite a lack of rotational symmetry of cladding, the PMD measured in 10 km of concatenated NANF fibers in a loose-tube duct cable was moderate: 0.29 ps/\(\surd\)km [54], close to the requirement for SMF, \(\leq\)0.20 ps/\(\surd\)km [29]. The reasons for this are explained in Section 7.2. NANF cables are manufactured commercially under the trade name CoreSmart by a British company Lumenisity--a spinoff from the University of Southampton, acquired by Microsoft in December 2022, with a first deployment for HST connection to the London Stock Exchange in March 2021, and with installed lengths of up to 45 km. Table 3 is a comparison of Lumenisity CoreSmart NANF fiber to solid single-mode fiber. In the absence of published product specifications, data from research papers, primarily [12; 13; 49; 54; 55], were substituted and some parameters estimated. Fused silica NANF with low loss, 0.85-1.25 dB/km in several bands close to 2 \(\upmu\)m (\(\approx\)1980 nm, \(\approx\)2040 nm, \(\approx\)2100 nm), was also reported by researchers from the Beijing University of Technology in China in 2021 [56]. This fiber was designed for optical power delivery, with a large core diameter (46 \(\upmu\)m) and cladding made of five nested tubes. NANF/DNANF fibers are promising for communication networks, especially high-capacity DWDM networks. Advantages over solid single-mode fibers include the following:  
* Low latency (\(-\)31,7% vs. SMF-28e+);
* Low and weakly wavelength-dependent chromatic dispersion;
* Low optical nonlinearity and ability to handle high optical power;
* Resistance to ionizing radiation. * Low-loss bandwidth and minimum attenuation are currently comparable, but:
* NANFs suffer from \"water peak\" absorption in the 1350-1490 nm band (Figure 14);
* The loss of NANFs can be further reduced [13], while that of solid-core fibers cannot. Other disadvantages, applicable to all HCFs, include:
* Difficult splicing (Section 10);
* Sensitivity to contamination in adverse environments (Sections 6.3 and 6.4);
* Large, non-standardized diameter of most fibers (Section 7.1). An increase in core diameter reduces \(\eta\) and fiber attenuation; therefore, HCF designers prefer large core diameters and mode field. The \(\eta\) coefficient is wavelength-dependent, rising at short wavelengths. Consequences of effective removal of glass from the transmission path are substantial:  
* Loss can be, in theory, reduced to \(\approx\)0.001 dB/km [48; 52]--see Section 6.2.