Low-Loss and Weakly Coupled Eight-Mode Nodeless Hollow-Core Anti-Resonant Fiber With Three-Layer Nested Tubes in Each Cladding Unit

A low-loss and weakly coupled eight-mode nodeless hollow-core anti-resonant fiber (HC-ARF) is designed and analyzed. The cladding structure comprising six cladding units with three-layer nested tubes suppresses mode coupling between the eight core modes (LP₀₁-LP_31a, LP_31b-LP₄₁) and cladding modes. By optimizing the core radius and nested tubes, the confinement losses of the LP₀₁-LP₄₁ modes can be controlled to be below 1.72 × 10⁻⁴ dB/m. The effective refractive index difference between adjacent core modes is greater than 1.0 × 10⁻⁴, and the corresponding differential mode group delay is larger than 0.38 ps/m in the wavelength range between 1480 nm and 1670 nm. As a result, weak coupling can be accomplished between LP₀₁-LP₄₁, and these eight core modes can propagate through the HC-ARF with similar low losses. Numerical analysis demonstrates that the eight weakly coupled modes can be stably supported at a bending radius of 20 cm. At smaller bending radii of 5 and 3 cm, five and two weakly coupled core modes, respectively, can still be supported.