First-order statistics of the phase in optical vortex speckles

We present the theoretical analysis of first-order statistics of the phase in a far-field speckle field, which originates from an optical vortex passing through a random phase screen. By using the concept of the equiprobability density ellipse, we show that the standard deviation of the phase in a speckle field varies non-monotonically in the radial direction and, more interestingly, it exhibits a minimum at a certain radial position determined by the topological charge. In the limit of zero topological charge, the phase statistics naturally converges to the expectation corresponding to the incident Gaussian beam.