How to achieve 0.3mm wavelength propagation and return

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How to achieve 0.3mm wavelength propagation and return

When I choose a wavelength of 0.3mm, I cannot calculate it back

F.propagate(0cm)
I = F.get_intensity()
plot square root of field intensity (for improved contrast)
F.plot_intensity(I, xlim=[-9 mm, 9* mm], ylim=[-9* mm, 9* mm], square_root = True)

F.propagate(80*mm)
I = F.get_intensity()

plot square root of field intensity (for improved contrast)
F.plot_intensity(I, xlim=[-9* mm, 9* mm], ylim=[-9* mm, 9* mm], square_root = True)

F.propagate(-80*mm)
I = F.get_intensity()

plot square root of field intensity (for improved contrast)
F.plot_intensity(I, xlim=[-9* mm, 9* mm], ylim=[-9* mm, 9* mm], square_root = True)

But it can be achieved at 3000nm