Low-loss splicing between anti-reflection coated SMF and hollow-core fiber without back-reflection degradation

Anti-reflection (AR) coating is an effective technique for reducing back-reflection and insertion loss (IL) in the interconnection between standard single-mode fiber (SSMF) and hollow-core fiber (HCF). However, the difference in thermal expansion coefficient between the fiber and coating leads to performance degradation during fusion splicing. To address this limitation, we report an embedded fiber splicing technique that uses a tapered borosilicate glass tube (BGT) as a splicing bridge, thereby avoiding direct heating of the AR coating. Theoretical analysis shows that a minimal IL of 0.13 dB can be achieved by optimizing the structural parameters, including the GRIN length and the air-cavity length, when the HCF mode field diameter ranges from 17 to 21 μm. Then, the SSMF with an AR-coated GRIN end is inserted into a tapered BGT, with the untapered side spliced to the HCF. By managing the GRIN length, the air gap, and the splicing offset, a low-IL interconnection can be achieved without the back-reflection penalty. Finally, an SSMF-HCF-SSMF interconnection link with an IL of 0.6 dB and a back-reflection of -33.8 dB is experimentally achieved. Meanwhile, higher-order mode coupling is well suppressed to below -35 dB. The proposed embedded splicing technique can facilitate HCF applications in a harsh environment.