OPTICS OF SINGULARITIES OF A LOW-MODE FIBER - II - OPTICAL VORTICES

Processes of the formation of directed optical vortices of a low-mode fiber are studied experimentally and theoretically The state of the fi eld of a directed optical vortex is shown to be characterized by two q uantum numbers: the spirality (direction of circulation of field polar ization) sigma(z) and the topological charge l. There are two types of optical vortices in low-mode fibers: stable and unstable. The states (+1, +1) and (-1, -1) correspond to stable vortices, the states (+1, - 1) and (-1, +1) correspond to unstable vortices. Periodic variations o f the sign of the topological charge l and the direction sigma(z) are observed in the process of propagation of an unstable vortex. Vortex f ields are represented in terms of physically observable circularly pol arized wave combinations. It is found that small perturbations of the state of polarization of a stable vortex do not result in its destruct ion or conversion into an unstable vortex. It is found that a stable v ortex has two components of the energy flux of the Poynting vector: P- z directed along the axis and the azimuth P-phi circulating about the fiber axis. In the process of propagation of a stable vortex, the dire ction of circulation of P-phi does not vary and depends only on the si gn of the topological charge. In an unstable vortex, periodic variatio n of the direction of circulation of P-phi is observed.