Implementing elliptical cladding elements in terahertz negative curvature hollow-core waveguides

We report a negative-curvature hollow-core waveguide for the terahertz (THz) band that employs ellipse-nested half-ellipse (ENHEF) cladding elements 3D-printed in cyclic olefin copolymer (COC). The five-ring elliptical structure supports anti-resonant guidance, resulting in low interaction between the guided field and the polymer, which reduces material absorption. Numerical optimization predicts a confinement loss of 0.041 dB/m and a total loss, including material absorption, of 0.35 dB/m at 0.412 THz, coincident with the frequency of the lowest measured propagation loss of 3.89 dB/m for a 6 mm-core fiber fabricated via fused deposition modeling (FDM). The design also achieves strong higher-order-mode (HOM) suppression, with an extinction ratio of 704 at 0.476 THz. Direct measurements of the COC filament provide the refractive index and bulk loss used in the simulations, ensuring model-experiment consistency. The bending loss was also evaluated numerically by introducing curvature-induced refractive-index modification into the simulation model. The ENHEF concept combines simple additive fabrication with low-loss and single-mode guidance, offering a scalable route to practical THz delivery fibers.