# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 4 Photonic Bandgap (Photonic Crystal) Fibers
### Commercial Fibers and Their Parameters  
The main suppliers of commercial HC-PBGFs are OFS Fitel (a Furukawa group company) in the U.S. and NKT Photonics (a subsidiary of Hamamatsu Photonics K.K.) in Denmark. Fibers from NKT Photonics share a common design (Figures 5a and 6), tailored for wavelengths between 800 nm and 1550 nm by sizing of core and photonic cladding. HC-1310 and HC-1550 fibers for telecom wavelengths [35] have dimensions typical for single-mode fibers (Table 1) but are unsuitable for telecom networks due to high attenuation: 60 dB/km @ 1310 nm, and 30 dB/km @ 1550 nm and propagation of higher-order modes. Figure 8: Chromatic dispersion of OFS AccuCore with six shunts—four lengths concatenated [22], and Corning SMF-28e+ [30]. The dispersion of individual lengths of AccuCore differed substantially. Figure 7: MFD vs. wavelength: OFS AccuCore with six shunts [34] and Corning SMF-28+ [30]. OFS Fitel makes HC-PBGFs installed in commercial networks, mostly low-latency links for high speed trading, presented in Section 12.1.3. Selected data of OFS AccuCore fibers [21, 22, 27, 28, 34, 36] are listed in Table 2.  
HC-PBGF has three advantages over solid single-mode fibers:  
* Low latency (\(-31.5\%\) vs. SMF-28e+) [27, 36];
* Low optical nonlinearity and ability to handle high optical power;
* Resistance to ionizing radiation. As regards disadvantages, an important one is high PMD, resulting from the following factors:  
* Lack of 90\({}^{\circ}\) rotational symmetry: eight shunts would be better (Section 7.2);
* Considerable distortions of fiber structure, especially the core (Figures 5b,c and 6). PMD in similar un-spun HC-PBGFs was often in excess of 100 ps/\(\sqrt{\text{km}}\), rising when the long-wavelength edge of transmission band was approached [23, 24, 25]. High PMD makes AccuCore fiber suited only for short links with NRZ coding (no added latency due to complex coding/decoding and FEC processing) operating at 10 Gb/s rate, which typically tolerate instantaneous differential group delay (DGD) of 30 ps, and a PMD of 10 ps--see Section 11. Because of radiation resistance and latency stability (see Section 7.3), similar fibers are also useful in nuclear facilities, space equipment, and multi-antenna microwave systems. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **HC-1300** & **HC-1550** & **SMF-28e+** \\ \hline Core diameter (nom.) [\(\mu\)m] & 10.0 & 11.5 & 8.2 \\ Mode field diameter (nom.) [\(\mu\)m] & 7.5 @ 1300 nm & 9.0 @ 1550 nm & 9.2 @ 1310 nm \\ Cladding diameter (nom.) [\(\mu\)m] & 125 & 120 & 10.4 @ 1550 nm \\ Range of operating wavelengths [nm] & 1290–1330 & 1490–1680 & 1260–1675 \\ Attenuation (max.) [dB/km] & 60 @ 1300 nm & 30 @ 1550 nm & 0.35 @ 1310 nm \\ Attenuation (typ.) [dB/km] & 30 @ 1300 nm & 30 @ 1550 nm & 0.20 @ 1550 nm \\ Chromatic dispersion (typ.) [ps/nm\(\cdot\)km] & 30 @ 1300 nm & 18.5 @ 1550 nm & 0.18 @ 1550 nm \\ Chromatic dispersion (typ.) [ps/nm\(\cdot\)km] & 66 @ 1300 nm & 42 @ 1550 nm & 17 @ 1550 nm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Technical data of NKT Photonics HC-1310 and HC-1550 PBGFs for telecom wavelengths and conventional single-mode fiber Corning SMF-28e+. Adapted with permission from Ref. [35], © NKT Photonics 2022, and Ref. [30], © Corning Inc., 2021.