meridional ray
A ray that passes through the axis of the optical fiber.
meridional ray: a fiber-bound light path through the core axis
A meridional ray is a light ray that travels through an optical fiber and passes through the fiber's central axis. Unlike skew rays, which spiral around the core without crossing the axis, meridional rays cross it repeatedly as they bounce between the core-cladding boundary. This axial passage makes them predictable and easier to analyze mathematically, which is why they serve as the standard reference case in fiber optics theory.
In a step-index fiber with a core diameter of 50 micrometers and a numerical aperture (NA) of 0.2, meridional rays enter at angles within the fiber's acceptance cone and follow planar paths containing the fiber axis. They undergo total internal reflection at incident angles greater than the critical angle, determined by the refractive index difference between core (typically 1.48) and cladding (typically 1.46). Each bounce reflects the ray back toward the axis, then away again, creating a zigzag pattern in a cross-sectional view.
Meridional versus skew rays
Skew rays never intersect the axis and follow helical paths with higher mode numbers, typically carrying power over longer distances in multimode fibers. Meridional rays are easier to trace but represent only a subset of total propagating modes. In single-mode fibers operating at 1310 nm or 1550 nm, the distinction matters less because only one fundamental mode (LP01) exists, but in multimode fibers carrying hundreds of modes, both ray families coexist and must be accounted for when calculating loss, dispersion, and bandwidth.
The term reflects its geometric origin: meridional refers to a great circle or plane passing through both poles of a sphere, analogous to the plane containing the fiber axis and the ray's path. Engineers use meridional ray models to establish baseline performance and to identify mode-dependent loss, where different ray families experience different attenuation due to core irregularities or bends. Understanding meridional behavior is essential when designing launch conditions, modeling coupling efficiency, and predicting modal noise in coherent systems.