Influence of Double-Nested Tubes on Avoided Crossings in 5-Tube Nested Hollow-Core Fibers

Avoided crossings in 5-tube hollow-core fibers are investigated for the impact of double-nested rings on confinement loss and single-mode guidance. Higher-order mode extinction ratio of 104 is achieved with a wide range of double-nested tube dimensions, while maintaining a confinement loss lower than 1 × 10–3 dB/km for the fundamental mode of the fiber over a broad 170 nm band in the visible wavelength regime. At the telecommunication wavelength of 1550 nm, confinement loss of the fundamental mode is less than 0.012 dB/km, and higher-order mode extinction ratio is larger than 6500 for a broad 190 nm band. For a fixed ratio of nested to cladding tube diameter, the diameter of double-nested tubes has negligible influence on the modal purity and small influence on the confinement loss. The fiber also exhibits a very low dispersion of less than ± 3 ps/nm/km over a broad 200 nm bandwidth.