TOPOLOGICAL PHASE OF OPTICAL VORTICES IN FEW-MODE FIBERS

It has been found that as they propagate, the natural optical vortices of a few-mode parabolic fiber acquire a topological phase in addition to the dynamic phase. The magnitude of this phase is numerically equa l to the polarization correction to the propagation constant of the CV and IV vortices. An analysis revealed that this phase is a new type o f optical manifestation of the topological Berry phase. The already kn own Pancharatnam and Rytov-Vladimirskii phases are associated with cha nges in the magnitude and direction of the angular momentum how of the wave. In the fields of natural vortices of a few-mode fiber all the e xplicit parameters of the wave remain unchanged during propagation. Ho wever, the direction of the momentum density vector of the vortex unde rgoes cyclic variations along the trajectory of the energy flow line. These cyclic variations of the implicit vortex parameter are responsib le for the new type of topological phase. Unlike the study made by van Enk (Ref. 6), where the topological phase was only related to the ang ular momentum for the lowest-order Gaussian beams (l = +/- 1), this to pological phase describes guided vortices with any values I of the top ological charge. The results can be used to estimate the stability of CV and IV vortices relative to external perturbing influences on the o ptical fiber. (C) 1998 American Institute of Physics. [S1063-7850(98)0 2804-3]