Angular momentum of the fields of a few-mode fiber: I. A perturbed optical vortex
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This paper presents the results of studies of the physical nature of the electrodynamic angular momentum of a stable CV+1+ vortex in a few-mode fiber. It shows that the angular momentum of a CV+1+ vortex can be conventionally divided into orbital and spin angular momenta. The longitudinal component of the fundamental HE11+ mode on the axis of the fiber has a pure screw dislocation with a topological charge of e=+1. The longitudinal component of a CV+1+ vortex also has a pure screw dislocation on the axis of the fiber with a topological charge of e=+2. Therefore, perturbation of a CV+1+ vortex by the field of the fundamental HE11+ mode removes the degeneracy of the pure screw dislocations of the longitudinal and transverse components of the field and breaks down the structural stability of the CV+1+ vortex. As a result, an additional azimuthal flux of energy with an angular momentum opposite to that of the fundamental flux is induced. An analogy is drawn between the stream lines of a perturbed CV vortex and the stream lines of an inviscid liquid flowing around a rotating cylinder. Studies of the evolution of a CV vortex in a parabolic fiber show that they are structurally stable when acted on by the perturbing field of the HE11+ mode. However, perturbing a CV+1+ 1 vortex of a stepped fiber with the field of the HE11+ mode destroys the structural stability of the vortex. It is found that the propagation of a circularly polarized CV vortex can be represented as a helical wavefront screwing into the medium of the fiber. The propagation of a linearly polarized vortex in free space is characterized by the translational displacement (without rotation) of a helical wavefront.
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