Optical vortices and orbital angular momentum in strongly coupled optical fibers

We have studied the effect of strong coupling on the propagation of optical vortices (OVs) and evolution of their orbital angular momentum (OAM) in parallel multimode optical fibers. Based on the perturbation theory that goes beyond the limits of weak orthogonality approximation we have established that strong coupling does not lead to alteration of the structure of supermodes as compared to the case of weak coupling. The strong coupling affects only the propagation constants of such supermodes, which we have found analytical expressions for. We have also studied the evolution of OVs and emphasized the difference between the powers stored in partial OVs and powers located at the fiber cores. We have studied OAM in such fibers, as well as corrections to the total OAM due to interference effects and shown that the influence of such effects on forming the total OAM under strong coupling is negligible. We have also demonstrated that in such systems it is sufficient to take account only of the coupling of OVs with equal by modulus topological charges, whereas other types of coupling are negligible.