# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 5 Fibers with Anti-Resonant Guidance
### Origins of Fiber Loss  
The attenuation in HCF is produced via several mechanisms:  
1. Escape of light through cladding, causing confinement loss (CL);
2. Surface scattering loss (SSL) [11];
3. Absorption by contaminants of filler gas, e.g., water vapor (Section 6.3);
4. Absorption of bulk silica glass, important at long wavelengths [26];
5. Micro- and macrobending loss;
6. Loss resulting from structural imperfections of photonic cladding [60]. Each loss component is wavelength-dependent in a different way. Loss analysis for NANFs is presented in [19; 61], and for HC-PBGF in [26]. CL is used to measure how effectively the cladding blocks escape of guided light from the fiber's core at a particular wavelength. It depends both on cladding design and the accuracy of controlling its geometry during fiber manufacturing. CL is also known as \"Leakage Loss\". CL can be, in theory, reduced as desired via the modification of cladding, e.g., adding layers of holes or nested tubes, with predicted values in DNANF fibers down to \(\approx\)10\({}^{-5}\) dB/km [48; 52]. However, manufacturing imperfections and variations in key dimensions have a considerable influence [60]. For example, the thickness of walls around the core of HC-PBGF shall be a half of the thickness of the struts to minimize the attenuation. SSL is produced by thermally excited Surface Capillary Waves (SCW) in molten glass, which is frozen when the fiber is cooled after drawing. Light guided in the fiber is scattered on uneven surfaces encircling the hollow core; the resulting attenuation in HC-PBGF is proportional to \(\lambda^{-3}\)[11]. Recent analyses and experiments [62] indicate that the application of surface shear during fiber drawing can reduce the RMS height of SCW from \(\approx\)0.4 nm in fibers drawn without shear to \(\approx\)0.15 nm at most important SCW wavelengthsshorter than 1 \(\upmu\)m, with an expected threefold reduction in SSL. Surface scattering is partly backscattering, beneficial for fiber measurement with OTDR. SSL is decreased by increasing the core diameter to minimize the overlap of guided light with glass surfaces around the fiber's core, and is negligible in NANF/DNANF fibers. With adequately low \(\eta\), HCF can have low attenuation despite a high loss of fused silica or other structural material. In particular, an effective onset of phonon absorption in SiO\({}_{2}\) fiber is shifted from 1500 nm in SMF to 1800-2200 nm [26]. In an HC-PBGF with \(\eta\) = 0.5%, the minimum loss of leakage-free hollow-core fiber, limited by phonon absorption in fused silica, can be as low as 0.01 dB/km at 1800 nm, rising to 1 dB/km only at 2150 nm [11; 20]. For an NANF with \(\eta\) = 0.01% (100 ppm), the minimum loss produced by phonon absorption in glass is below 0.001 dB/km at wavelengths close to 2000 nm. Loss resulting from Rayleigh scattering in glass is not important in HCFs because its value in bulk glass (\(\approx\)0.14 dB/km @ 1550 nm) is multiplied by \(\eta\)--more at long than at short wavelengths. This enabled the development of NANFs operating at visible wavelengths, with a loss lower than that in solid-core fused silica fibers, e.g., 2.8 dB/km at 660 nm [63]. The same applies to hollow-core polymer optical fiber (HC-POF). Solid POFs have a core of PMMA and thin cladding of fluorinated polymer. These fibers have too high attenuation for telecom networks, 150-200 dB/km at 650 nm, and 80-100 dB/km at 480 nm and 520 nm [64], and are restricted to short data links in vehicles, industrial automation, etc. The attenuation of PMMA HCF with \(\eta\leq\) 0.005 can be, in theory, reduced to \(\leq\)1 dB/km in the 600-850 nm band [65]. This is less than the typical attenuation of OM2/3/4/5 multimode solid-core fibers used in LANs and data centers: 2 dB/km at 850 nm [66]. The attenuation of state-of-the art fused silica HC-PBGF is primarily a sum of:  
* CL, which usually reaches high values in several bands, usually leaving narrow low-loss transmission band(s) (Figure 4).