An analysis was made of the modal properties of the step-index concentric-core fiberguide of circular cross section, which has found application in duplex (two-channel) transmission and in secure communications. The total number of modes capable of propagating in the guide and their longitudinal propagation constants were evaluated for a variety of geometrical parameters and operating conditions. The intensity variation across the guide’s cross section was also determined, and a criterion, the core-to-ring intensity ratio, was established to designate whether any mode is confined exclusively to the core region, to the ring region, or spread out significantly over both regions. Information about the distribution of each mode’s energy across a cross section is essential in determining the amount of cross talk present in a guide. It was also demonstrated that when the propagation constants for a core and a ring mode coincide a resonant transfer of energy takes place between the core and ring regions. This resonance condition is extremely sensitive to changes in normalized frequency and the radii.
Peter D. Gianino, "Modal characteristics of step-index concentric-core fiberguides of circular cross section," Appl. Opt. 22, 1734-1740 (1983)