Numerical simulation of a speckle pattern formed by radiation of optical vortices in a multimode optical fibre

The formation of optical vortices in optical fibres is studied. Speckle patterns of optical fibres produced due to the interference of guided modes and optical vortices are simulated. The spatial statistical characteristics of speckle structures are obtained. It is shown that speckle structures, formed by the radiation of optical vortices rotating in the same direction, have the characteristic azimuthal angular dimensions of spots that are considerably larger than their axial size, whereas speckle patterns, formed by vortices rotating in the opposite directions, do not virtually differ from speckles produced due to the interference of usual guided modes. A method is proposed to distinguish speckles produced by optical vortices rotating in the same direction from speckles formed by usual guided modes with the predominant excitation of modes with comparatively small azimuthal indices. A comparison with experimental data is performed.