Computation of topological charges of optical vortices via   non-degenerate four-wave-mixing
  In this paper, we report an experiment, which demonstrates computation of topological charges of two optical vortices via non-degenerate four-wave-mixing process. We show that the output signal beam carries orbital angular momentum which equals to the subtraction of the orbital angular momenta of the probe light and the backward pump light. The &#8312;&#8309;Rb atoms are used as the nonlinear medium, which transfer the orbital angular momenta of lights. 
Wei Jiang, Qun-feng Chen, Yong-sheng Zhang and G. -C. Guo