The Multifocus Structure of Radiation upon Femtosecond Filamentation of an Optical Vortex in a Medium with an Anomalous Group Velocity Dispersion

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We analyze numerically the femtosecond filamentation of a vortex beam with the topological charge m = 1 at wavelength λ₀ = 1800 nm in fused silica under anomalous group velocity dispersion conditions. We show that, as a pulse with a fivefold excess of its power over the critical power propagates through the medium, a multifocus ring structure of radiation with a peak intensity of higher than 10¹³ W/cm² and surface energy density of about 0.2 J/cm² is formed. The self-action of the optical vortex is accompanied by the transformation of the spectral energy mainly into the Stokes region. A comparison is made with the case of a normal dispersion of the group velocity upon the self-action of a vortex beam at the wavelength of  λ₀ = 400 nm.