We demonstrate that the orbital angular momentum of beams, either Laguerre- Gaussian or Bessel beams, is intrinsically related to the symmetries of the diffraction aperture providing information of their amount of topological charge they carry. It is well known that the orbital angular momentum is a constant of motion in physics and, according to the Noether's Theorem this is due to one of the associated symmetry groups of the equations that describe the particular system. In Optics, since the three dimensional Helmholtz and paraxial wave equations are rotationally invariant then there exist solutions describing optical beams with rotating wavefronts that carry OAM. They are called vortex beams. In particular, it has been demonstrated that Laguerre-Gaussian and Bessel beams, which may have optical vortices with a topological charge m, carry a well-defined OAM equals to mh per photon [2,3]. As a conserved quantity of the field it should not be affected by diffraction.
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