Low loss and high performance interconnection between standard single-mode fiber and antiresonant hollow-core fiber

We demonstrate halving the record-low loss of interconnection between a nested antiresonant nodeless type hollow-core fiber ( NANF) and standard single-mode fiber (SMF). The achieved interconnection loss of 0.15 dB is only 0.07 dB above the theoretically-expected minimum loss. We also optimized the interconnection in terms of unwanted cross-coupling into the higher-order modes of the NANF. We achieved cross-coupling as low as -35 dB into the LP11 mode (the lowestloss higher-order mode and thus the most important to eliminate). With the help of simulations, we show that the measured LP11 mode coupling is most likely limited by the slightly imperfect symmetry of the manufactured NANF. The coupling cross-talk into the highly-lossy LP02 mode (> 2000 dB/ km in our fiber) was measured to be below -22 dB. Furthermore, we show experimentally that the anti-reflective coating applied to the interconnect interface reduces the insertion loss by 0.15 dB while simultaneously reducing the back-reflection below -40 dB over a 60 nm bandwidth. Finally, we also demonstrated an alternative mode-field adapter to adapt the mode-field size between SMF and NANF, based on thermally-expanded core fibers. This approach enabled us to achieve an interconnection loss of 0.21 dB and cross-coupling of -35 dB into the LP11 mode.

Automated summary

We have reduced the interconnection loss between a nested antiresonant nodeless type hollow-core fiber (NANF) and standard single-mode fiber (SMF) and optimized the unwanted cross-coupling into the higher-order modes of the NANF. Simulation results suggest that the LP11 mode coupling is likely limited by the slightly imperfect symmetry of the manufactured NANF.