LAUSR

Determination of the topological charge (TC) of a vortex beam is of significant importance in various applications such as high-capacity optical communications and micromanipulations. Though many kinds of methods, until now, have been proposed to measure the TC, most of them will fail if the measured vortex beam is partially blocked by an opaque obstacle. In this Letter, we have introduced an efficient method to determine the TC of an incomplete vortex beam (partially blocked by an obstacle) using its reconstructed phase distribution under propagation. The numerical and experimental results showed that the sign and magnitude of the TC can be simultaneously determined by counting the number of phase singularities and the directions of the phase variations around the singularities occurring in the reconstructed phase pattern, respectively. Our method works even when half of the vortex beam is blocked.