A meridional ray is a path of light that passes directly through the central longitudinal axis of a fiber core each time it reflects off the cladding. In multimode fiber, these rays represent the shortest and most direct paths, arriving at the receiver faster than skew rays, which spiral around the outside of the core.
The dispersion in this step-index fiber is terrible because the meridional rays arrive way before the higher-order modes.
Understanding meridional versus skew rays helps explain modal dispersion, which is why multimode fiber is limited to short distances compared to singlemode.
Related terms
Numerical aperture
A measure of how wide a cone of light a fiber can accept and guide, written as a number such as 0.20.
Multimode fiber
Fiber with a 50 or 62.5 µm core that carries many light paths at once, used for short links inside buildings and data centers.
Core
The central glass region of an optical fiber where the light actually travels.
Cladding
The layer of slightly lower-index glass around the core that keeps light trapped inside it.