– A Bessel beam with OAM of charge = 1 has a circular caustic. Fourier phase pattern, with azimuthally linearly increasing phase Φ = , confined to an infinitesimal thin ring. Fourier transform by a lens. Fourier wavefront shaping determines the direction of the propagating rays, and the caustic ( ) forms in real space as the envelope of the rays in a transverse plane ( ). A subset of the rays, each coming from a specific point on the ring, is shown in ( ). The complete family of rays is composed of a continuum of bundles like the one shown in but displaced in . The corresponding field has a propagation-invariant transverse intensity ( ) with a pronounced central ring according to the circular caustic. – A more complex phase function yields an astroid caustic, which includes four cusp caustics.
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