Exploring the effect of nested capillaries on core-cladding mode resonances in hollow-core antiresonant fibers

Optimal suppression of higher-order modes (HOMs) in hollow-core antiresonant fibers comprising a single ring of thin-walled capillaries was previously studied, and can be achieved when the condition on the capillary-to-core diameter ratio is satisfied (d/D approximate to 0.68). Here we report on the conditions for maximizing the leakage losses of HOMs in hollow-core nested antiresonant node-less fibers, while preserving low confinement loss for the fundamental mode. Using an analytical model based on coupled capillary waveguides, as well as full-vector finite element modeling, we show that optimal d/D value leading to high leakage losses of HOMs, is strongly correlated to the size of nested capillaries. We also show that extremely high value of degree of HOM suppression (similar to 1200) at the resonant coupling is almost unchanged on a Wide range of nested capillary diameter d(Nested) values. These results thus suggest the possibility of designing antiresonant fibers with nested elements, which show optimal guiding performances in terms of the HOM loss compared to that of the fundamental mode, for clearly defined paired values of the ratios (d(Nested)/d and d/D. These can also tend towards a single-mode behavior only when the dimensionless parameter d(Nested)/d is less than 0.30, with identical wall thicknesses for all of the capillaries.