Observation of multiramp fractional vortex beams and their properties in free space

We have experimentally investigated the evolution properties of multiramp fractional vortex beams (MFVBs) in free space, by using a fundamental Gaussian beam reflecting from a phase-modulated spatial light modulator. The issue about the total vortex strength of such MFVBs is systematically addressed, and our result reveals the dependence of the total vortex strength depends on both the non-integer topological charge $\alpha $ and the multiramp number $m$ contained in initial multiramp phase structures. In the near-field region, vortices contained in MFVBs are unstable and it is hard to effectively confirm the vortex strength for such fields. However, in the far-field region, the evolution of vortices in fields becomes stable and the behavior of vortex strength is confirmed experimentally via measuring vortex structures by interference method. These findings give us an understanding of such complex MFVBs and may lead to potential applications in light signal process and propagation.