Disentangling Loss Mechanisms to Bridge Inhibited-Coupling and Anti-Resonant Models in Hollow-Core Fibers

Hollow-core photonic crystal fibers based on inhibited coupling mechanism are now the leading platform for broadband, low-loss light guidance. Yet, despite major advances, the physical origin of confinement loss in these fibers remains only partially understood. Here, we present a unified framework that resolves this long-standing issue by identifying and quantifying the two dominant loss mechanisms. Using coupled-mode theory and azimuthal Fourier decomposition, complemented by an artificial-index fiber model, and an empirical model derived from power leakage at the outer dielectric boundary, we separate the total confinement loss into the mode-coupling loss, arising from residual coupling between the core mode and cladding dielectric modes, and the tube-tunneling loss, associated with power leakage through the finite dielectric structure. We derive semi-analytical expressions for both contributions and show that their sum accurately reproduces the full numerical loss spectrum across several inhibited coupling fiber designs. This framework reconciles numerical and analytical descriptions of confinement loss for the first time, offering a predictive and physically transparent tool for optimizing next-generation hollow-core fibers.