I want to generate vortex beams by simulating a vortex phase plate in the optical path like this.

The effect I want to achieve is as shown in the first image

 Different topological loads will produce different imaging effects

by using grid sag , I wrote a dat file to create a special surface sag which is shown in the figure below. But when I looked at the simulation results using a physical propagation POP, it was very different from what I expected. I want to know how to simulate the Vortex phase plate and produce vortex beams correctly