Positive and negative curvatures nested in an antiresonant hollow-core fiber.

We propose a negative curvature hollow-core fiber that has a nested elliptical element in the antiresonant tubes. The additional elliptical element effectively adds two curvatures, namely, a positive and a negative curvature. Our numerical study shows that it enhances the confinement of the light in the core. Moreover, the nested elements provided an extra degree of freedom that can be exploited to suppress higher-order modes through the change of the ellipticity. The resulting low confinement loss and single-mode guidance properties of the proposed fiber make it a suitable candidate for applications in ultrashort pulse delivery and gas-based nonlinear systems.