# Hollow-Core Optical Fibers for Telecommunications and Data Transmission
## 10 Splicing and Termination of Hollow-Core Fibers
### Common Issues  
HCF-HCF and HCF-SMF splices shall meet the following requirements:  
* Low insertion loss, preferably \(\leq\)0.25 dB;
* Low reflectivity, preferably \(\leq\)\(-\)40 dB;
* Hermeticity to prevent the contamination of HCF (Sections 6.3 and 6.4);
* Mechanical strength and long-term reliability (\(\geq\)20 years). The cladding of HCFs includes photonic structures with thin walls, down to \(\approx\)0.4 \(\upmu\)m, and numerous holes or tubes with a diameter in the 5-75 \(\upmu\)m range. A delicate photonic bandgap or tubular structure is prone to collapse during arc fusion, when the glass is softened and flows, and gas pressure rapidly increases. A short fusion time like 0.1-0.3 s, and carefully controlled power are mandatory [100]. There is a compromise between splice loss, rising with the length of collapse, and mechanical strength, best when the interface between fibers is fully fused [101]. Damage to photonic cladding can reduced via the following steps:  
* Shortening the fusion time to \(\approx\)0.2 s so that heat does not fully penetrate the fiber's cladding;  
Figure 20: HCF cable with SMF pigtails: (**a**) version with single splices; (**b**) version with double splices, and short intermediate MFD fiber (IMF) inserted in the middle. * Reducing the arc power and current to 50-70% of value for solid-core SMF;
* Sweeping the arc along certain length of fibers to reduce inner temperature;
* Applying overpressure in all voids in the cladding. For example, splicing NKT Photonics HC-1550-02 HC-PBGF fiber [35] (see Table 1, Figure 6) with a 125 um jacket diameter to conventional SMF-28 fiber on an Ericsson FSU 975 fusion splicer with \(\approx\)1 mm electrode gap required pre-fusion and fusion currents of 5 mA (for 0.2 s) and 9 mA (for 0.3 s) [102], corresponding to a discharge power of \(\approx\)2.5 W and \(\approx\)4.5 W, respectively (only a part of this power is transferred to fused fibers), while currents recommended by the manufacturer for splicing two solid SMF fibers on the same machine were 10.5 mA (for 0.3 s) and 16.3 mA (for 2 s), respectively [103]. The fact that currently manufactured HC-PBGF and NANF fibers intended for data transmission and telecom networks have larger diameters, \(\geq\)200 um (Tables 2 and 3), and require a higher arc power (approximately proportional to cross-section of glass jacket) is beneficial for fusion splicing, because a high-frequency AC electric arc at normal air pressure becomes unstable at currents below 5 mA. Otherwise, the only solution would be to use costly low-pressure arc fusion splicers or filament splicers. Current settings adequate on a particular arc fusion splicer are generally not transferable to machines of a different type because the power generated by the arc is proportional to the voltage drop, which changes with the gap between electrodes at a rate of approx. 500 V/mm. This gap varies roughly between 1 mm and 5 mm, depending on whether the machine is designed for splicing 3 mm wide 12-fiber fiber ribbons, or only single fibers. Advanced fusion splicing machines designed for a wide array of specialty fibers, like FITEL S184 PM-SLDF ver.2 or more affordable S183 PMII [104], have factory programs for splicing HCFs, rotational alignment, and even automatic adjustment of fusion parameters. Nevertheless, the user must be ready to experiment to find the best settings, dependent, in particular, on fiber diameter and jacket thickness. As most HCFs have a diameter larger than 200 um, a large diameter cleaver is needed. The cleaning of cut, but not sealed, HCF with a liquid solvent (isopropyl alcohol, acetone, etc.) is forbidden, as the solvent would impregnate all voids and the fiber would lose its properties. Cleaving and splicing shall be conducted in dust-free, low humidity environment, because a low pressure inside fiber [69; 70] would lead to the suction of dust and wet air inside and increase the fiber loss--see Sections 6.3 and 6.4. Other solutions are different for HCF-HCF and HCF-SMF splicing.