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.

By modifying the design of the interconnection between standard and hollow-core fibers, we achieved low insertion loss while creating a gap in between. This will allow for insertion of thin optical elements such as filters.

Hollow-core fibers (HCFs) have been under intense research interest thanks to their many advantages including low latency, low nonlinearity, and temperature insensitivity. The most recent progress on the double nested antiresonant nodeless fiber (DNANF) demonstrated fiber losses of only 0.174 dB/km. Transmission of ultra-short, high-peak-power pulses can greatly benefit from low nonlinearity of HCFs. However, the waveguide dispersion in HCFs such as DNANF is typically 2-3 ps/(nm·km) in the low-loss transmission region, still causing unwanted pulses broadening. Here, we demonstrate a low-loss interconnection between HCF and a dispersion-compensating fiber (DCF), enabling to obtain HCF+DCF link with zero-net dispersion. To adapt the relatively small mode-filed diameter (MFD) of DCF (4.9 μm) to the MFD of the HCF, we first splice a short segment of graded-index (GRIN) multi-mode fiber on the DCF. The GRIN fiber is then polished to a specific length to obtain an optimal MFD adaptation to our HCF, which was a nested antiresonant nodeless fiber (NANF) with 26.3 μm MFD at 1550 nm. We obtained a loss of only 0.55 dB for the whole DCF-GRIN-NANF component. By depositing an anti-reflective coating on the mode-field adapter end-face, the interconnection loss can be further reduced to 0.39 dB.

We demonstrate design of fiber Fabry-Perot c avities based on large-core graded-index multimode fibers. Smallest full width at half maximum along with maximum transmission is reached for core diameters over 200µm core and reflectivity above 99%.

We report simultaneous low coupling loss (below 0.2 dB at 1550 nm) and low back-reflection (below −60 dB in the 1200-1600 nm range) between a hollow core fiber and standard single mode optical fiber obtained through the combination of an angled interface and an anti-reflective coating. We perform experimental optimization of the interface angle to achieve the best combination of performance in terms of the coupling loss and back-reflection suppression. Furthermore, we examine parasitic cross-coupling to the higher-order modes and show that it does not degrade compared to the case of a flat interface, keeping it below −30 dB and below −20 dB for LP11 and LP02 modes, respectively.