How to simulate Fourier Transform at the Focal Plane (when input_distance = f)

[udithhaputhanthri]

Hi,
Thank you for your fantastic work.

I have implemented the lens system as given in this script. I also followed your article as well.

When I set up input_distance = output_distance = focal_length= 25cm, Ideally I should get the Fourier Transform at the output. But the implementation does not give that. Could you please guide me on how I can get that?

Input field (intensity)

What I got from the simulation (I checked for multiple scale factors in scale_propagation function (in this script). Below are images for scale_factor= 30)

What should I get (Fourier Transform)

[rafael-fuente]

Hi,

I edited the script to accommodate the parameters you mentioned (input_distance = output_distance = focal_length= 25cm)

import diffractsim
diffractsim.set_backend("CPU") #Change the string to "CUDA" to use GPU acceleration

from diffractsim import MonochromaticField, ApertureFromImage, Lens, nm, mm, cm, um


zi = 25*cm # distance from the image plane to the lens
z0 = 25*cm # distance from the lens to the current position
radius = 6*mm


# set up simulation
F = MonochromaticField(
    wavelength=488 * nm, extent_x=1.5 * mm, extent_y=1.5 * mm, Nx=2048, Ny=2048,intensity = 0.4
)

F.add(ApertureFromImage("./apertures/QWT.png",  image_size = (1.0 * mm, 1.0 * mm), simulation = F))

F.scale_propagate(25*cm, scale_factor = 20)
F.add(Lens(f = 25*cm , radius = radius))
F.scale_propagate(25*cm, scale_factor = 0.7)

I = F.get_intensity()
F.plot_intensity(I, square_root = True, units = mm, grid = True)

This is the output:

That is the absolute value of the Fourier transform of the aperture, at least within the paraxial approximation.