  Simple structures are always a pursuit but sometimes not easily attainable.
It took researchers nearly two decades for conceiving the structure of
single-ring hollow-core negative curvature fiber (NCF). Recently NCF eventually
approaches to the centre of intense study in fiber optics. The reason behind
this slow-pace development is, undoubtfully, its inexplicit guidance mechanism.
This paper aims at gaining a clear physical insight into the optical guidance
mechanism in NCF. To clarify the origins of light confinement, we boldly
disassemble the NCF structure into several layers and develop a multi-layered
model. Four physical origins, namely single-path Fresnel transmission through
cascaded interfaces, near-grazing incidence, multi-path interference caused by
Fresnel reflection, and glass wall shape are revealed and their individual
contributions are quantified for the first time. Such an elegant model could
not only elucidate the optical guidance in existing types of hollow-core fibers
but also assist in design of novel structure for new functions.
