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Core Modal Spatial-Structuring in Inhibited-Coupling Hollow-Core Fibers

A profound knowledge of the cladding mode properties and their interaction with the core modes is decisive for successfully designing inhibited-coupling (IC) guiding hollow-core photonic-crystal fibers (HC-PCFs). In IC fibers, the core and cladding modes coupling is robustly minimized due to a strong mismatch between their transverse phases and a small overlap between their fields. In the latest years, great attention has been given to IC fibers, especially since the introduction of the hypocycloid core contour concept [3], which entailed a dramatic loss reduction and preceded the development of single-ring tubular-lattice (SR-TL) HC-PCFs [4]. The interest on SR-TL HC-PCFs is due to their noteworthy properties such as geometric simplicity and the absence of connecting nodes in the fiber cladding, which favors IC guidance. Efforts on SR-TL HC-PCFs development entailed the accomplishment of ultra-low loss and broad transmission bandwidth [5]. Here, we show that the alteration of the lattice-tubes azimuthal position in SR-TL HC-PCFs causes modifications in the mode loss hierarchy and can be used to select the propagation of specific higher order modes (HOMs). It can be done by conveniently enlarging the spacing between the cladding tubes and by providing additional leakage to the fundamental mode while keeping low impact in the confinement loss (CL) of specific HOMs. It allows obtaining uncommon intensity profiles and polarization properties at the fiber output allowing it to act as a mode shaper.