# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 7 Selected Properties of HCFs
### Polarization Mode Dispersion  
This parameter is important in telecom fibers for transmission at very high rates and over long distances. While fiber dispersion can be digitally compensated, this introduces latency due to signal buffering and processing, and is avoided in low-latency links.  
Figure 17: Main dimensions of hollow-core fibers: (**a**) HC-PBGF, (**b**) NANF.  
PMD in solid single-mode fibers can be reduced by the spinning of hot fiber during drawing, introducing permanent rotation and the periodic reversal of orientation of locally added components of a differential group delay (DGD), thus preventing the accumulation of DGD along the fiber. Fiber spinning was developed in 1981 for sensor fibers [81], and introduced to telecom fibers in 1994 [82; 83].  
In HC-PBGF (Figure 4), one source of PMD is the non-circularity of the central cavity, probably due to the distortion of the preform during consolidation. High PMD observed in OFS AccuCore fibers (see Sections 4.2 and 4.3) is partly created by the deliberate introduction of a small non-symmetry of the core--ellipticity and local increase in wall thickness, to eliminate high-order modes at the expense of increasing PMD and PDL [32]. The PMD value for this fiber listed in Table 2 applies to fiber spun during drawing at a high rate of approx. 100 rev/m, corresponding to a spin period of 10 mm, and placed in an underground cable of a loose-tube design. PMD in un-spun fiber in the same conditions was \(\approx\)55 ps/\(\sqrt{\text{km}}\), but micro- and macro-bending experienced by the fiber on a 150 mm diameter spool reduced it to \(\leq\)3 ps/\(\sqrt{\text{km}}\)--more effectively than spinning [21; 22]. This behavior of HC-PBGF may spur the development of cables ensuring similar conditions for fibers inside.  
PMD in NANF/DNANF results mostly from the ellipticity of the core, as in conventional single-mode fibers, but another source of PMD exists. Single-mode NANFs with a number of regularly spaced anti-resonant tubes exhibit some degree of \"spill\" of mode field into gaps between the tubes (Figure 18) as \"spikes\". If these spikes are not arranged with a 90\({}^{\circ}\) rotational symmetry, one state of polarization will experience transmission latency different from that of the orthogonal state, resulting in DGD. In typical NANF designs with 5-8 tubes (nested or not), one with 8 tubes exhibits 90\({}^{\circ}\) rotational symmetry, while one with 6 tubes exhibits the largest asymmetry (Figure 19). In fibers with five and seven tubes, the difference in propagation conditions between orthogonal polarizations is considerably lower.  
An NANF fiber has a structure with 90\({}^{\circ}\) rotational symmetry, incorporating eight tubes shall exhibit lower PMD than fibers with five, seven and, in particular, six tubes (Figure 19).  
However, this mechanism of DGD creation is weak due to the following factors:  
* Minimal interaction of optical power with tubes--only 0.001-0.03% of it is propagating in the glass, or other solid material the fiber is made of;
* Small angle between the directions of adjacent extreme field distributions [84]: 36\({}^{\circ}\), 30\({}^{\circ}\), 25.7\({}^{\circ}\) and 22.5\({}^{\circ}\) in an NANF with five, six, seven and eight tubes, respectively.  
Figure 18: Distribution of power density in non-symmetrical NANF with seven tubes for 2 \(\upmu\)m band: (**a**) false color image, (**b**) image adjusted to enhance peripheral parts of guided mode at 2 \(\upmu\)m.  
PMD measured in loose-tube duct cables with Lumenisity NANF fibers was quite low, 0.29 ps/\(\sqrt{\text{km}}\), and transmission experiments at 400 Gb/s (95 Gbaud) rate and over distances up to 1128 km performed at BT laboratories were successful [54].  
The distortion of mode field increases the loss of splice to conventional single-mode fiber with a circular mode field, e.g., by 0.08 dB for an NANF with six tubes (Figure 19a) [84].