# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 7 Selected Properties of HCFs
### Backscattering  
Light propagating in glass or gas, but not in a vacuum, is subject to scattering due to the microscopic non-uniformity of the bulk material--the omni-directional Rayleigh scattering produced by random non-uniformities much smaller than the wavelength (their dimensions in fused silica are in order of 0.2 nm), or scattering resulting from the thermally generated surface capillary waves \"frozen\" after drawing of HCF (see Section 6.2). The scattering cross-sections of gases is listed in Table 4. In a contaminated HCF, additional scattering may by produced by particles of dust, droplets of condensed vapor, or product(s) of its reaction with silica, deposited on surfaces around core. It may be a Mie scattering, as such particles have sizes comparable to the wavelength of light.  
In solid-core optical fibers, only Rayleigh scattering in fused silica (usually doped) is observed, and this is the mechanism imposing a low limit on fiber attenuation at wavelengths up to 1550 nm. A small part of scattered radiation reflected backwards is captured by the fiber (ca. 0.12% in SMF, and 0.5% in 50/125 \(\upmu\)m MMF) and propagated back to the light source. Backscattering is detrimental, producing interference in bidirectional data transmission, noise in analog TV links, feedback and instability in optical amplifiers, or complicating the monitoring and compensation of fiber latency (Section 12.2.1). On the other hand, backscattering enables researchers to measure fiber attenuation, non-uniformities and length with optical time domain reflectometer (OTDR). Without backscattering, OTDR would only be able to detect and locate fiber ends, splices and defects.  
The lowest level of backscattering in SMF is approximately \(-\)76 dB/m at 1550 nm for fibers with pure silica core, and about \(-\)72 dB/m for typical SMF with a germania (GeO\({}_{2}\))-doped core. In an NANF, backscattering from fused silica parts of the fiber at the same wavelength is very low, about \(-\)115 dB/m [48; 87], because only 0.001-0.03% of light interacts with glass parts of the fiber and the fiber acceptance angle is smaller than in SMF (NA = 0.02-0.03 vs. \(\approx\)0.14). With backscattering intensity ca. 40 dB lower, OTDR measurements of NANFs would be almost impossible, as this difference nullifies the OTDR dynamic range available at \(\leq\)20 m resolution: 15-25 dB one-way, or 30-50 dB of intensity difference.  
The situation is changed by the gas in the core. In a typical NANF filled with dry air at a normal atmospheric pressure, the intensity of backscattering is approx. \(-\)100 dB/m, ca. 25 dB below that in a typical SMF [87], just enough to allow for OTDR measurements in NANF fibers of medium lengths, e.g., 10 km. The Doppler effect in air at room temperature causes the spectral widening of backscattered light by some 500 MHz [88]. This broadening does not affect ordinary OTDR with a receiver responding to the intensity of the returning light, but OFDR with a narrowband receiver fails to detect the backscattering from gas.  
As presented in Section 6.3, the type and pressure of filler gas affect the intensity of backscattering in an NANF. Vacuum NANF would produce almost no backscattering, being perfect for bidirectional transmission, but very difficult to test with OTDR.  
In HC-PBGF, scattering at surface capillary waves, negligible in NANF, dominates (Section 6.2). In a fiber with small, \"7-cell\" core, e.g., NKT Photonics HC-1550 (Figure 4, Table 1), backscattering is quite strong, with approximately -77 dB/m estimated for a perfect fiber, and -60 dB/m measured in real fibers [87]. When the core is larger, \(\approx\)30 \(\upmu\)m diameter or \"19-cell\" size, scattering is weaker, but still allows for measurements with typical OTDR [89].