Measuring the topological charge of optical vortices with a twisting phase.

We analyzed the propagation characteristics of the intensity of a vortex beam after it passes through a twisting phase. It was found that the doughnut-like intensity pattern of a vortex beam would separate into several bright and dark fringes. The number of dark fringes between two bright spots is equal to the topological charge (TC) of the vortex beam. Meanwhile, the intensity pattern varies with the sign of the TC. Based on this property, we proposed a convenient method to measure the TC of a vortex beam by observing its intensity pattern after passing through a twisting phase. This detection technique is mainly based on the use of a twisting phase, and the effect of parameters in the twisting phase is demonstrated and clearly studied. By choosing proper parameters in the twisting phase, the separation speed of a vortex beam's intensity could be controlled in the experiment. The experimental results are in good agreement with the theoretical analyses.