Multi-moded, anti-resonant hollow-core fibre shows great promise for a range of applications from high power laser delivery to novel, non-linear experiments. Anti-resonant fibers typically guide multiple transverse modes due to their large core size, albeit with often large differential modal loss. Understanding the behaviour of higher order modes in these fibres is of crucial importance if we are to exploit the benefits of hollow-core optical guidance in few- and multi-moded applications, or to design more robustly single mode fibres. In this work we conduct thorough numerical investigations into the origins of confinement loss in tubular anti-resonant fibres and its dependence on the fibre's key structural parameters. We show that away from the resonances in the glass, leakage loss can be interpreted as originating from resonant out-coupling between the core modes of interest and the lossy modes of the cladding tubes, and is highest when these are phase-matched. Using this insight, we show that the loss can be estimated a posteriori from knowledge of the fields of the core-guided and cladding tube modes and their propagation constants. Such a quantitative estimate is satisfactory by considering only the three lowest mode groups of the cladding tube. This deeper understanding paves the way to a more informed approach to designing few and multi-moded hollowcore fibers for various applications.
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