# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 4 Photonic Bandgap (Photonic Crystal) Fibers
### Fibers with Regular Photonic Structure  
Photonic bandgap fiber was invented by [PERSON] at the University of Bath in 1991 [12], with a paper about it published in 1995 [8]. The basic idea--surrounding a central part reserved for light propagation (hollow or otherwise) with a periodic photonic structure composed of alternating low- and high-refractive-index materials, blocking the leakage or entry of light due to anti-resonance in periodic nodes (hollow channels)--proved versatile and became the basis for extensive research. HC-PBGF cladding is a periodic, multi-layered structure composed of glass and gas and known as two-dimensional photonic crystal, in the center of which a defect (core) is included. The core is usually made by removing 7, 19, or 37 adjacent cells of this structure--in fact, just the walls between them--forming a circle [10]. Radiation at frequencies within the bandgap cannot pass this cladding (in the radial direction--the bandgap effect is, in general, direction- and polarization-dependent) and is confined to the core and the nearest layers of cladding. The effectiveness of such guidance is limited, and a reduction in leakage loss to less than 1 dB/km usually requires 4-8 layers of cladding. Unlike conventional solid fibers with high index cores and low index cladding, light guidance in HC-PBGF is narrowband. In practice, the bandgap of a nominally uniform cladding often varies from one layer to another due to the distortion of the fiber structure during the consolidation of the preform. This can be beneficial: multiple layers of cladding with stepped bandgaps extend the low-loss bandwidth of the fiber, although a higher attenuation must be accepted or the number of layers increased. HC-PBGFs have either a single low-loss band, which can be more than 100 nm wide, or several narrow bands (Figure 4). Multiple narrow bands can be related to the presence of additional large holes in the cladding (Figure 5), introduced to capture and leak away light from higher-order modes. One direction of HCF development, from early HC-PBGFs to current NANFs, is decreasing the air filling fraction of the cladding from \(\approx\)90% to 5-10% in order to reduce overlap between optical power and glass, fiber loss, and nonlinearity--see Sections 6.1 and 6.2. Attenuation was considerably lower than in Bragg fibers. The loss of early HC-PBGFs, in excess of 1000 dB/km, was reduced to 1.7 dB/km at 1560 nm in 2004 [20] and 1.2 dB/km in 2005 [18] by researchers from the University of Bath, but no further loss reduction in this class of fibers was achieved. Such fibers are suitable for data links in LANs, data centers, or radio sites, but not metro and long-distance networks. Overall, HC-PBGFs have four serious disadvantages when applications in data transmission and telecom networks are considered:  
1. Propagation of higher-order modes in the photonic cladding, and modal dispersion, even when the core diameter is typical for single-mode fibers (10-20 \(\upmu\)m) [21];
2. High polarization mode dispersion (PMD), frequently in the order of 100 \(\upsur\)ps/km, resulting from the non-circularity of the core and distortions during consolidation and drawing. A PMD over 55 ps/\(\upsur\)km was reported for a fiber placed in a loose-tube cable [22; 23; 24; 25];
3. High surface scattering loss (SSL) [26]--see Section 6.2;
4. Many HC-PBGFs have low loss only in a few narrow (10-30 nm) sections of the spectrum (Figure 4) [20; 27; 28] because of interference between the lossy surface modes and the fundamental mode [11]. However, fibers of this type with a broad (\(\approx\)150 nm) transmission bandwidth centered around 1550 nm have been developed. An example of a \"wideband\" HC-PBGF fiber is NKT Photonics HC-1550, whose data are presented in Table 1. Figures 5a and 6 show gradually variable sizes of holes. Figure 4: Loss spectra of HC-PBGFs: low-loss experimental fiber (red) [20], and OFS AccuCore fibers with six shunts: typical production fiber (blue) [28], and experimental fiber (green) [22].