In an optical fiber, a Ray that is confined primarily to the core.

A guided ray satisfies the relation given by the formula above, where R is the angle the ray makes with the Fiber axis, r is the radial position, i.e. , radial distance, of the ray from the fiber axis, nr is the Refractive Index at the radial distance r from the fiber axis, and NA is the Refractive Index at the Core radius, a, i.e. , at the core-cladding interface. Guided rays correspond to bound modes, i.e. , guided modes, in terms of Modes rather than rays. Synonyms: bound ray, trapped ray.