| Summary: | We investigate a simple paraxial vector beam, which is a coaxial superposition of two single-ringed Laguerre–Gaussian (LG) beams, linearly polarized along the horizontal axis, with topological charges (TC) <i>n</i> and −<i>n</i>, and of two LG beams, linearly polarized along the vertical axis, with the TCs <i>m</i> and −<i>m</i>. In the initial plane, such a vector beam has zero spin angular momentum (SAM). Upon propagation in free space, such a propagation-invariant beam has still zero SAM at several distances from the waist plane (initial plane). However, we show that at all other distances, the SAM becomes nonzero. The intensity distribution in the cross-section of such a beam has 2<i>m</i> (if <i>m</i> > <i>n</i>) lobes, the maxima of which reside on a circle of a certain radius. The SAM distribution has also several lobes, from 2<i>m</i> till 2(<i>m</i> + <i>n</i>), the centers of which reside on a circle with a radius smaller than that of the maximal-intensity circle. The SAM sign alternates differently: one lobe has a positive SAM, while two neighbor lobes on the circle have a negative SAM, or two neighbor pairs of lobes can have a positive and negative SAM. When passing through a plane with zero SAM, positive and negative SAM lobes are swapped. The maximal SAM value is achieved at a distance smaller than or equal to the Rayleigh distance.
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