Generating a twisted Gaussian Schell-model beam with a coherent-mode superposition

Twist phase is a nontrivial second-order phase that only exists in a partially coherent field, providing a new degree of freedom for manipulating statistical properties of random light. However, in the existing experimental methods either it is difficult to control the twist phase or requires a large number of pseudo/random modes to approximately synthesize the twisted beams. In this work, we demonstrate a simple and efficient approach, based on the superposition of mutually orthogonal Laguerre-Gaussian modes with appropriate mode weights, to generate the twisted beams with a controllable twist phase. We show that a smaller number of modes are required to synthesize the twisted beams, compared to the pseudo-mode superposition, because the orthogonal coherent modes are used in the present method. We experimentally generate the twisted Gaussian Schell-model beams with controllable strength of twist phase and measure their degree of coherence and average intensity behavior during propagation. The experimental results agree well with the theoretical predictions. Our approach will promote the application of the twisted partially coherent beams in optical imaging and free-space optical communications.