Structured Light and Darkness in Spiral Vortex Beams

The problem of the structural stability of structured vortex beams in both theoretical and experimental optical aspects is considered. We have refined the standard structural stability condition of structured vortex beams by the requirement the fine structure of the intensity pattern to be constant up to scale and rotation when propagating. We have revealed that the critical points pattern in the structurally stable non-rotating beams forms three separatrices, which do not allow the external rotating energy flow to involve the beam fine structure into rotation that together with the scale transformations can significantly distort the beam. Analyzing the chaotic speckle structure of a perturbed structured beam, we have shown that measuring their amplitude and phase spectra by the intensity moments technique in LG and HG bases makes it possible to restore the initial mode structure of the beam.