LAUSR

We design, fabricate, and demonstrate the first hollow-core air-gap anti-resonant fiber coupler in a dual hollow-core anti-resonant fiber (DHAF) structure. The coupling takes place through an air gap between air cores, promising a limitless operation window beyond material transmission. The DHAF follows the same waveguide mechanism as the hollow-core anti-resonant fiber (HAF). The coupling is attainable over the entire transmission bands determined by a resonant frequency of a HAF. A coupling length, thus coupling strength, is controllable by adjusting fiber design parameters. In addition, at a fixed design and length, the coupling strength linearly responds to longitudinal mechanical tension, enabling continuous variable coupling ratio in a single coupler. Furthermore, we confirm that the coupler is polarization insensitive, and does not require precise polarization alignment of an input beam. We demonstrate its robust coupling performance and applicability in forming a fiber laser ring cavity, and delivering and power splitting ultrafast laser pulse. The air-core air-gap coupling promises applications in mid-infrared and ultraviolet regions where the current coupling technology is limited.