Computation of topological charges of optical vortices via nondegenerate four-wave mixing

In this paper, we report an experiment, which demonstrates computation of topological charges of two optical vortices via a nondegenerate four-wave-mixing process. We show that the output signal photon carries orbital angular momentum which equals to the subtraction of the orbital angular momenta of the probe light photon and the backward pump light photon. The {sup 85}Rb atoms are used as the nonlinear medium, which transfers the orbital angular momenta of lights.

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