# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 5 Fibers with Anti-Resonant Guidance
### Absorption and Scattering by Filler Gas and Contaminants  
Because 99-99.999% of radiation propagating in HCF interacts with gas filling the core rather than material of which the photonic structure is made, ensuring low attenuation depends on the properties and the purity of this gas, including absorption by gas, its impurities and deposits on surfaces around the core, as well as Rayleigh scattering. The advantages of gas over glass or polymer include a low refractive index, \(\approx\)1.000273 at 1550 nm for air at room temperature and standard atmospheric pressure at sea level [67], and very weak Kerr, Raman and Brillouin effects at atmospheric or lower pressure. Unfortunately, filling gas can be contaminated with species:  
* Present in the working area of the fiber manufacturing plant: water vapor, carbon dioxide, or fuel for gas burners;
* Out-gassing from hot glass, mostly chlorine and fluorine present in commercial fused silica tubes in concentrations up to 2000 ppm, and 3200 ppm for Heraeus F300 and F320-08, respectively [68];
* Diffusing into the fiber through open ends and imperfectly sealed splices, like hydrogen, water vapor, and methane [69; 70]. Argon is used as protective gas during fiber drawing to prevent the burning and oxidation of oven parts, and is also suitable as filler. In most experiments, purified air is used. Attenuation produced by Rayleigh scattering in a dry, dust-free air or pure argon at 101.3 kPa pressure (standard atmospheric pressure at sea level) is approx. 0.035 dB/km at 600 nm and 0.14 dB/km at 400 nm, respectively, if a complete loss of scattered light is assumed [71; 72]. This attenuation is proportional to the effective cross-section of Rayleigh scattering of a particular gas and its pressure. While helium has the lowest cross-section of Rayleigh scattering--Ca. 60\(\times\) lower than air [72]--and few absorption lines in the spectral range of interest (Table 4), this gas can easily diffuse out from fiber or splice, and appears unsuitable. The attenuation of HCF is increased by the absorption of gas and its contaminants. In particular, water is deposited on the inner surfaces and reacts with fused silica, producing silanol (SiOH) absorbing light at 1364 nm, while the initial concentration of water vapor decreases with time. Non-polar gases like nitrogen, oxygen or argon do not attach to fused silica surfaces [69]. The absorption lines (for elements) and bands (for compounds), and Raman scattering cross-section (\(\sigma\)) characterizing the intensity of Raman scattering are listed in Table 4. Only the strongest absorption lines or bands for each gas are listed. Data were compiled from the NIST Basic Atomic Spectroscopic Data for elements website [73], HITRAN on the Web website [74] and papers [75; 76] for compounds. \begin{table}
\begin{tabular}{l c c} \hline \hline \multicolumn{1}{c}{**Gas**} & \multicolumn{1}{c}{**\(\sigma\) [10\({}^{-32}\) m\({}^{2}\)]**} & \multicolumn{1}{c}{**Absorption Lines or Bands 1 [nm]**} \\ \hline Helium (\({}^{4}\)He) & 0.39 & 668, 707, 1083, 2058 \\ Argon (Ar) & 23.8 & 697, 707, 750, 764, 795, 801, 810, 812, 912, 966 \\ Nitrogen (N\({}_{2}\)) & 26.5 & 744, 747, 868, 1247, 1358, 2099–2206 \\ Oxygen (O\({}_{2}\)) & 22.0 & 777, 845, 927, 1253–1283 \\ Chlorine (Cl\({}_{2}\)) & no data & 822, 838, 859 \\ Fluorine (F\({}_{2}\)) & no data & 686, 704, 713 \\ Hydrogen (H\({}_{2}\)) & 5.30 & 656, 1282, 1875 \\ Methane (CH\({}_{4}\)) & 59.2 & 1631–1679 \\ Propane (C\({}_{3}\)H\({}_{8}\)) & 346 & 1685–1765 \\ Ammonia (NH\({}_{3}\)) & 38.9 & 1471–1547, 1926–2025, 2200–2348 \\ Water vapor (H\({}_{2}\)O) & 19.2 & 1117–1153, 1352–141, 1816–1934 \\ Carbon dioxide (CO\({}_{2}\)) & 62.2 & 1432, 1438, 1998–2026 \\ \hline \hline \end{tabular} \({}^{\dagger}\) All absorption lines are relatively weak.  
\end{table} 
Table 4: The strongest absorption lines or bands in non-ionized state between 600 nm and 2200 nm, and Rayleigh scattering cross-sections [\(\sigma\)] at 733 nm, of possible filler gases and their contaminants [72; 73; 74; 75; 76].