# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 5 Fibers with Anti-Resonant Guidance
### NANF and DNANF Fibers  
The evolution of anti-resonant fibers was rather long. It began with Kagome fibers, presented in Sections 2.1 and 5.2, and was later followed by the introduction of round \"negative curvature parts\" surrounding the core for a lower overlap of optical power with glass. Once this approach was proven effective, the whole multi-layer Kagome cladding was replaced in 2011 with a single layer of tightly packed (touching) glass tubes or \"capillaries\" separating the empty core from the jacket tube [46]. This design was soon improved in 2013 [47] via the introduction of small gaps between the tubes, as shown in Figure 12a. The loss of this fiber at medium-infrared wavelengths was reduced substantially below bulk absorption in the glass. A Nested Anti-resonant Nodeless Fiber (NANF) fiber was proposed by [PERSON] of the University of Southampton in 2014 [48]. In an NANF, the number of anti-resonant walls is doubled (Figure 12b), reducing confinement loss and allowing the designers to extend the low-loss bandwidth by differentiating the wall thickness and resonant wavelength of each tube. The lowest attenuation of NANF, reported in 2021, was 0.22 dB/km at 1625 nm [49]--comparable to the attenuation of commercial SMF [30] at the same wavelength. Confinement loss can be further reduced via the inclusion of an additional small tube, resulting in Double Nested Anti-Resonant Nodeless Fiber (DNANF)(Figure 12c). Figure 11: SEM images of Kagome fibers: (**a**) with round core; (**b**) with “hypocycloid” core delimited by round surfaces [42]. The white bar corresponds to 50 μm. Please compare with perfect structures shown in Figure 2a,b. Reproduced with permission. Figure 12: Main types of Anti-Resonant fibers: (**a**) with non-touching single tubes, (**b**) with nested tubes (NANF), (**c**) with double-nested tubes (DNANF). Fibers of type (**b,c**) had a minimum loss of 0.23 dB/km [49] and 0.174 dB/km [13], respectively. For an actual image of the last fiber, see Figure 13a. This design was proposed and studied in 2014 by [PERSON] and [PERSON] [50] and [PERSON] [48]. The reduction in confinement loss down to \(10^{-5}\) dB/km was predicted using computer simulations, as each membrane added between the cavity and outer cladding (\"jacket\", see Section 7.1), preferably close to the core cavity, reduces the proportion of light propagating in the radial direction. In NANF and DNANF, nodes exist only between outer surfaces of tubes and the jacket tube, where the power density is very low. Light scattering is very low despite a considerable degree of fusion between the nested tubes and jacket observed in several fibers--see the example shown in Figure 13a. The lowest total loss so far, 0.174 dB/km at 1530 nm, was achieved in a DNANF in 2022 [13]. The photonic structure of this fiber is shown in Figure 13a, and the loss spectrum in Figure 14. Figure 13b is a comparison of the mode field distributions in this DNANF and equivalent NANF with an identical core diameter (28 \(\upmu\)m). In a DNANF, the outer part of mode field is kept farther away from the fiber jacket by the relatively large middle tube and radial confinement loss reduced tenfold. Figure 14: Loss spectra of lowest-loss DNANF fiber [13]: (blue) measured, (red) estimated for fiber without water peak. Figure 13: (**a**) SEM image of 0.174 dB/km DNANF, (**b**) simulated mode field distributions and radial confinement loss in this fiber and equivalent NANF [13]. Reproduced with permission. With marginal loss contribution from bulk absorption in fused silica or other material, and proper scaling, NANFs can be optimized for different spectral ranges, including 850 nm and visible light, where a lower loss in comparison to solid fused silica multimode fibers was demonstrated [49]. The number of tubes is usually five, six or seven, sometimes eight or ten.