Theoretical and Numerical Investigation of Tube-Shape Effects in Hollow-Core Inhibited-Coupling Fibers

Hollow-Core Inhibited-Coupling Fibers (HC-ICFs) have emerged as a leading platform for broadband and low-loss light guidance, with transmission performances capable of overcoming some of the fundamental limitations of conventional solid-core fibers. By confining light predominantly in a hollow region and minimizing its interaction with the surrounding dielectric material, these fibers offer wide transmission bandwidths, reduced nonlinear effects, and a versatile platform for applications ranging from high-power beam delivery to sensing and gas–light interaction. Despite these advances, understanding the physical mechanisms governing confinement loss remains central to the optimization of HC-ICFs. Within each transmission band, two distinct contributions have been identified: mode-coupling loss (MCL), originating from the residual coupling between the fundamental core mode and dielectric modes of the cladding, and tube-tunneling loss (TTL), associated with power leakage through the finite dielectric structure. In this thesis, I focus on the MCL contribution investigating how the geometry of the cladding tubes affects the inhibited-coupling mechanism. An artificial-index fiber configuration is employed to suppress the tunneling contribution and isolate MCL, allowing the influence of tube geometry on core–cladding coupling to be investigated. The conventional circular tube is progressively deformed into elliptical geometries with different aspect ratios, enabling the resulting changes in confinement loss to be related to the spatial and spectral properties governing modal coupling. Within this framework, the azimuthal fourier decomposition of the fundamental core mode and the cut-off frequencies distribution of the dielectric modes provide complementary information on the strength and spectral location of core–cladding coupling. The investigation is further extended beyond pure ellipticity through flattened and sharpened profiles, designed to modify the local curvature in the core-facing region of the tube boundary and investigate its influence on the confinement mechanism. The results show that tube shape has a significant influence on the confinement-loss spectrum and on the spectral characteristics associated with core–cladding coupling. In particular, elliptical deformation leads to a marked modification of the transmission region, with an intermediate aspect ratio around 1.3 providing the broadest transmission band among the investigated elliptical configurations. The flattened and sharpened profiles further show that local modifications of the core-facing tube boundary produce distinct changes in the spectral response, highlighting the relevance of tube curvature and shape in the design of HC-ICFs. Overall, the results provide further insight into the relationship between cladding geometry and inhibited-coupling guidance and indicate geometrical shaping as a useful degree of freedom for tailoring the transmission properties of hollow-core fibers.