This paper describes a newly developed butt joint type hollow-core fiber connector with protected fiber ends. It can typically realize nearly 0.5-dB insertion and 45-dB return loss without physical contact. It has been confirmed that the HCF con. This paper describes a newly developed butt joint type hollow-core fiber connector with protected fiber ends. It can typically realize nearly 0.5-dB insertion and 45-dB return loss without physical contact. It has been confirmed that the HCF connector is suitable for actual use because it maintains stable connection characteristics in mating cycles, and it can be cleaned with an ordinary connector cleaner.••Studies of hollow-core fiber (HCF) have progressed,,,,,,, and recently a double nested antiresonant nodeless fiber (DNANF) with a propagation loss of 0.174 dB/km has been reported. This value indicates that DNANF has the potential to be used in optical communication networks. The single-mode fiber (SMF) that is currently used has an upper transmission capacity limit of 100 Tb/s due to the limitation of the power density of optical signals, but an HCF can expand the mode field diameter (MFD) propagated in a single mode without non-linearity effect nor fiber fuse phenomena. HCF is also attracting attention for its low-latency characteristic compared with silica fiber and its low dispersion, making it suitable for quantum communication. On the other hand, an optical connector is in. 2.1. Connection tolerances of butt-coupled HCFFor this study, we used IXF-ARF-45–240, Exail, which is a commercially available HCF with an anti-resonant structure. The fiber cross-section is shown in Fig. 1, and its characteristics are shown in Table 1.Fig. 1. Cross-sectional shape.Table 1. HCF characteristics.We investigated the relationship between misalignment and coupling loss when connecting this fiber with a butt joint. The measurement set up is shown in Fig. 2. An ASE (Amplified Spontaneous Emission) light source with a wavelength of 1460–1610 nm was used as the light source. The optical continuous wave reflectometer (OCWR) method was used to measure the return loss. An SMF and an HCF were coupled after numerica. We measured the connection characteristics of the HCF connectors. The measurement setup is shown in Fig. 10. As in Fig. 2, the HCF is excited from an SMF via the NA conversion module with a lens array. To eliminate higher-order modes, a 12-m long HCF with an HCF connector was used as a master plug, and the insertion and return loss of a 1-m long HCF patch cord connected to this master plug were measured. The insertion loss was measured from the ratio of the optical powers at points A and B shown in Fig. 10. The return loss was measured from the ratio of the optical power at point C when an Ag mirror was opposed to point A and the optical power at point C when a patch cord was connected to point A. The maximum return loss that can be measured with this measurement setup was 57 dB.Since it is not possible to polish the HCF endface and remove any dust adhering to it, we designed a prototype optical connector with a structure that protects the HCF endface with a thin glass plate. Although there is a large gap between the HCFs connected in this structure, it was expected that the excess loss could be kept low due to the small NA. The insertion loss of the prototype HCF connector is distributed in a range exceeding 0.3 dB as expected from the gap loss, and it is considered that the expected insertion loss was obtained. Since the dimensional accuracy of the stainless-steel ferrule is insufficient, we obtained a large excess loss. However, since there is room to improve the dimensional accuracy of the ferrule, we believe it is possible to achieve a lower insertion loss.