Beschreibung:
<jats:title>Abstract</jats:title><jats:p>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<jats:inline-formula><jats:alternatives><jats:tex-math>$$_{11}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
<mml:msub>
<mml:mrow />
<mml:mn>11</mml:mn>
</mml:msub>
</mml:math></jats:alternatives></jats:inline-formula> 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<jats:inline-formula><jats:alternatives><jats:tex-math>$$_{11}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
<mml:msub>
<mml:mrow />
<mml:mn>11</mml:mn>
</mml:msub>
</mml:math></jats:alternatives></jats:inline-formula> mode coupling is most likely limited by the slightly imperfect symmetry of the manufactured NANF. The coupling cross-talk into the highly-lossy LP<jats:inline-formula><jats:alternatives><jats:tex-math>$$_{02}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
<mml:msub>
<mml:mrow />
<mml:mn>02</mml:mn>
</mml:msub>
</mml:math></jats:alternatives></jats:inline-formula> mode (<jats:inline-formula><jats:alternatives><jats:tex-math>$$>2000$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
<mml:mrow>
<mml:mo>></mml:mo>
<mml:mn>2000</mml:mn>
</mml:mrow>
</mml:math></jats:alternatives></jats:inline-formula> 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<jats:inline-formula><jats:alternatives><jats:tex-math>$$_{11}$$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML">
<mml:msub>
<mml:mrow />
<mml:mn>11</mml:mn>
</mml:msub>
</mml:math></jats:alternatives></jats:inline-formula> mode.</jats:p>