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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 LP$$_{11}$$ 11 mode (the lowest-loss higher-order mode and thus the most important to eliminate). With the help of simulations, we show that the measured LP$$_{11}$$ 11 mode coupling is most likely limited by the slightly imperfect symmetry of the manufactured NANF. The coupling cross-talk into the highly-lossy LP$$_{02}$$ 02 mode ($$>2000$$ > 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 LP$$_{11}$$ 11 mode.

We present a new approach to permanently interconnect hollow-core fiber (HCF) to solid-core fiber, which does not involve fusion splicing. Our approach is based on a modification of the glue-based fiber-array technology routinely used for fiber pigtailing of planar lightwave circuits. The resulting interconnection provides for a low insertion loss due to the fact that the HCF microstructure is not deformed during the gluing (low temperature) process that is almost impossible to achieve with the standard (high temperature) fusion splicing method. Furthermore, this low temperature technique enables the deposition and preservation of thin films deposited at the solid-to-hollow core fiber interface, allowing for additional functionality without the introduction of extra losses or any increase in complexity. To demonstrate this, we have applied an anti-reflection (AR) coating. A further feature of our approach is the ability to control very precisely the length of the graded-index (GRIN) fiber mode field (MF) adapter inserted in between the standard singlemode fiber (SMF-28) and the HCF. We show experimentally how the length of the GRIN fiber MF adapter influences the coupling between the SMF-28 and the fundamental as well as higher-order modes of the HCF. We coupled between SMF-28 (10 µm Mode Field Diameter, MFD) and the fundamental mode of a 19-cell hollow-core photonic bandgap fiber (HC-PBGF, 21.1 µm MFD) with the lowest-ever reported insertion loss of 0.30 dB per interface.

We made and characterized two Fabry-Perot interferometer samples made of the latest-generation hollow core fiber with sub-1-dB/km loss. Thanks to this low transmission loss, we achieved a finesse of over 140 and 120, for interferometer lengths of 5 and 23 m, respectively. This resulted in transmission peaks as narrow as 47 kHz. Our all-fiber Fabry-Perot interferometers have standard single-mode fiber pigtails (for easy integration in conventional fiber optic systems) and employ fiber mode field adapters to enable low-loss coupling between the pigtails and the low-loss hollow core fiber. The high-reflectivity mirrors (>98 %) were deposited directly on the fiber mode field adapters, which were glued to the hollow core fiber, resulting in permanently-aligned Fabry-Perot interferometers. We also measured how the position of the transmission peaks change with temperature (an important performance metrics for most applications, e.g., when used as a narrow-band band-pass filter) and found that it changed 14.5 times less in our Fabry-Perot interferometer relative to a similar device made of standard single mode fiber.

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