Skew rays and photon turning points in graded-index multimode fibers

Abstract

When rays are transmitted without passing through the fiber axis they are called skew rays. These rays follow a helical path through the fiber. The light ray propagating in a graded-index fiber does not necessarily reach every point in the fiber core. The ray is contained within two cylindrical caustic surfaces, and for most rays a caustic does not coincide with the core-cladding interface. Hence, the caustics define the classical turning points of the light ray within the graded fiber core. These turning points defined by the two caustics may be designated as occuring at r = r0 and r = r1, i.e., the radii where the wavevector’s radial component is zero.