Step by Step Propagation With a Short Focal Length Lens about lightpipes HOT 3 OPEN

In general, and especially with lenses, you have to be careful not to violate the Nyquist criterium. The maximum spatial frequency of your problem (= max(derivative of the phase with respect to x and y coordinate) ) should be smaller than the maximum spatial frequency given by the grid-spacing (= N/2/size)

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When you propagate from a source plane (here optical field after the thin lens) to a target plane at various distances, it is best practice to preserve the source and repeatedly propagate from source plane to new target plane. This avoids buildup of errors during the propagation. For example, when you want to propagate the optical field Field at the source plane to target planes that are in the range z_start to z_end away from the source plane, you could use:

Nz=50                                    # number of steps in range z_start to z_end
z0 = z_start
dz = (z_end -z_start)/Nz

I1=np.zeros((Nz,GridDimension))
I2=np.zeros((Nz,GridDimension))

for i in range(Nz):
   F = Forvard(Field,z0)
   I = np.array(Intensity(F))
   I1[i,:] = I[int(GridDimension/2),:]
   I2[i,:] = I[:,int(GridDimension/2)]
   z0 += dz

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FELman2011:
Unfortunately I'm simulating an effect that has to be propagated step by step (if you are more insterested - self foucing via kerr effect), so I can't run from this problem. In a matter of fact, the errors of propagation with steps and without them are quite small so my problem is mainly the inefficient running time.

FredvanGoor:
Thanks, thats a good tip.

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