ANGULAR-MOMENTUM OF THE FIELDS OF A FEW-MODE FIBER - I - A PERTURBED OPTICAL VORTEX

This paper presents the results of studies of the physical nature of t he electrodynamic angular momentum of a stable CV+1+ vortex in a few-m ode fiber. It shows that the angular momentum of a CV+1+ vortex can be conventionally divided into orbital and spin angular momenta. The lon gitudinal component of the fundamental HE11+ mode on the axis of the f iber 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 di slocation 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 fundam ental transverse components of the field and breaks down the structura l stability of the CV+1+ vortex. As a result, an additional azimuthal flux of energy with an angular momentum opposite to that of the fundam ental flux is induced. An analogy is drawn between the stream lines of a perturbed CV vortex and the stream lines of an inviscid liquid flow ing around a rotating cylinder. Studies of the evolution of a CV vorte x in a parabolic fiber show that they are structurally stable when act ed on by the perturbing field of the HE11+ mode. However, perturbing a CV+1+ vortex of a stepped fiber with the field of the H-11(+) mode de stroys the structural stability of the vortex. It is found that the pr opagation of a circularly polarized CV vortex can be represented as a helical wavefront screwing into the medium of the fiber. The propagati on of a linearly polarized vortex in free space is characterized by th e translational displacement (without rotation) of a helical wavefront . (C) 1997 American Institute of Physics.