  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.
