Guided ray

Guided ray: In an optical fiber, a ray that is confined primarily to the core.

Note: A guided ray satisfies the relation given by where θ_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, n_r is the refractive index at the radial distance r from the fiber axis, and n_a 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.

Source: from Federal Standard 1037C and from MIL-STD-188