Structural stability of spiral beams and fine structure of an energy flow
The problem of structural stability of wave systems with great numbers of degrees of freedom directly concerns the issue of redistribution of energy fluxes in structured vortex beams that ensure their stability under propagating and focusing. A special place in this variety is occupied by spiral vortex beams capable of mapping complex figures, letters and even words. Spiral beams contain an infinite set of Laguerre-Gauss beams with a strong sequence of topological charges and radial numbers, their amplitudes and phases are tightly matched. Therefore, the problem of structural stability plays a special role for their applications. Using a combination of theory and computer simulation, supported by experiment, we analyzed the structure of critical points in energy flows for two main types of spiral beams: triangular beams with zero radial number and triangular beams with complex framing of their faces with both quantum numbers. Structural stability is provided by triads of critical points, both inside and outside the triangle, which direct the light flux along the triangular generatrix and hold the framing when rotating the beam. The experiment showed that a simple triangular spiral beam turns out to be stable even with small alignment inaccuracies, whereas a complex triangular beam with a framing requires careful alignment.