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This paper discusses the speckle structures of the radiation of multimode fiber lightguides in the presence of optical vortices. It is shown that the interference pattern of the radiation of optical vortices with an identical direction of rotation creates a speckle structure whose speckles rotate as the plane of observation is displaced close to the surface of the output end. The effect of various factors on the spatial characteristics of the speckle structures is considered.Multimode fiber lightguides (FLGs) are widely used to create fiber-optic sensors of physical quantities, in local communication lines, and also in devices for delivering optical radiation. When coherent-radiation sources are used, the technical characteristics of such systems largely depend on the parameters of the speckle structure (SpS) of the output radiation. The parameters of such devices are usually calculated on the assumption that interference involves the radiation of waveguide modes, which are actually electromagnetic waves with a planar wave front. However, optical vortices (OVs), which are waves with a rotating wave front, can arise and propagate in multimode FLGs. The parameters of SpSs formed by a group of OVs differ from the parameters of SpSs that arise when waveguide modes interfere (below in the text called waveguide modes with a planar wave front). 1,2 The presence of OVs with an identical direction of rotation causes the modal noise to increase when the output radiation is spatially filtered, 3 as well as changing the character of the displacement of the speckles when there is action on the lightguide.In most cases, the characteristics of the SpSs of lightguides were studied either in near-field diffraction or in farfield diffraction. Therefore, one of the less-studied phenomena associated with the presence of OVs is the speckle-rotation effect close to the surface of the output end of the FLG as the plane of observation changes. Some theoretical and calculational materials that relate to the influence of OVs on the parameters of the SpS appear in Refs. 4 and 5.Experimental apparatus 2 based on a MIKMED-V-1-20 microscope with an MFT-11T trinocular head was used to investigate the SpSs of FLGs. A helium-neon laser, as well as semiconductor lasers with wavelengths 633, 650, and 532 nm, were used as the radiation sources, whose radiation was focused by the microscope's optical system onto the input end of the lightguide. The spatial position of the focused beam was monitored by means of a video camera. The technique for adjusting the optical system and determining the tilt angle of the beam is described in Ref. 2. To excite ordinary waveguide modes in the FLG under investigation, the center of the focused beam was brought into coincidence with the axis of the lightguide. To create OVs, the center of the beam was displaced from the center of the core toward the cladding of the FLG, while the tilt angle of the beam was specified so that the conventional trajectory over which the beam moves inside the FLG had a helical form.In t...

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