Using MATLAB to Compute Diffraction Patterns of Complex Apertures

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# Using MATLAB to Compute Diffraction Patterns of Complex Apertures - PowerPoint PPT Presentation

Using MATLAB to Compute Diffraction Patterns of Complex Apertures. Stephen Schultz Department of Electrical and Computer Engineering Brigham Young University. Lens Focus. Fresnel becomes Fraunhofer at focal plane 2D Fourier transform Take 2D transform of complex structures Slits

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### Using MATLAB to Compute Diffraction Patterns of Complex Apertures

Stephen SchultzDepartment of Electrical and Computer EngineeringBrigham Young University

Lens Focus
• Fresnel becomes Fraunhofer at focal plane
• 2D Fourier transform
• Take 2D transform of complex structures
• Slits
• Multiple slits
• Letters
• Pictures
• Fingerprints
Fourier Transform with MATLAB
• Built-in Fast Fourier Transform (FFT)
• Y = fft2(X)
• Performs a Discrete Fourier Transform (DFT)
• Uses the Cooley-Tukey algorithm
• Fastest with sizes 2n
• Process
• Discretize the aperture
• Compute DFT
• Relate integer parameter to spatial coordinates
MATLAB Fourier Transform1. Aperture discretization
• Create gray scale image in graphics program
• Black is blocking
• White is transmitting
• Gray scale would be variable amplitude
• Save as a gif file (In illustrator)
• Save for Web and Devices
• Set the image size (faster with 2n pixels)
• No transparency, not interlaced
MATLAB Fourier Transform2. Computing DFT
• Computing DFT
• Use the two-dimensional FFT command

E=fft2(A)

• Puts center of the beam in the corners
• Use the fftshift command to put it into the center

E=fftshift(fft2(A))

fft2( A )

• fftshift( fft2( A ) )
• FFT is a Discrete Fourier Transform (DFT)

We need to relate the DFT to the FT

• We will do this with a 1D analysis and then extend it to 2D by just replacing the FFT command with FFT2
• Use an FFT, which is a discrete Fourier Transform (DFT)
• MATLAB assumes that it starts at zero rather than –x so you need to shift it using the fftshift command
• Convert the DFT to a discrete time Fourier transform (DTFT)
• The index is scaled by the number in the array
• f=p/N
• Convert the DTFT into a continuous Fourier transform (FT)
• Use the sampling period scaling factor x=Dx n
• fx=f/Dx
• Compare this to the analytic triangle function
• Summary of spatial coordinates
• Relate to discrete frequency
• Relate to spatial frequency
• Fraunhofer diffraction relationship
• Spatial coordinates
MATLAB Fourier Transform3. Spatial Coordinates
• For the FFT the number of frequency points equals the initial spatial points
• The spatial extent of the diffraction pattern
• More pixels causes the analysis plane to be bigger

M=64

M=256

Spatial Resolution3. Spatial Coordinates
• Spatial resolution, Dx, independent of number of pixels
• Need larger image, W, without increasing slit size
Summary of MATLAB Code(2. Fraunhofer Diffraction)
• Fairly simple MATLAB coding (9 lines of code)

[M,N] = size(A);

lambda = 600e-9;

f = 16.5e-3;

W = 4e-3;

I = (fftshift(fft2(A))).^2;

x = linspace(-lambda*f*N/(2*W), lambda*f*N/(2*W), N);

y = linspace(-lambda*f*M/(2*W), lambda*f*M/(2*W), M);

pcolor(x, y, I)

• Most work done using graphics program
Aperture Diffraction
• Rectangular aperture
• Slit size
• Cassegrain telescope
• Obscuration size
• Struts
Optical Image Processing (2. Fraunhofer Diffraction)
• Perform Fourier transform on an image
• Filter the image (low pass or high pass filter)
• Perform another Fourier transform
Experimental Implementation(3. Experimental)
• Helps solidify what is happening
• Complimentary to simulations
• No approximations but few aperture variations
• Easy to look at transition between regimes
• Students learn sensitivities of optical image processing
• Alignment
• Opaqueness
Experimental Implementation(3. Experimental)
• Requirement to make the implementation practical
• Long focal length lenses
• Easier focusing
• Larger features (fx=x/lf)
• Transparencies transferred to chrome mask
• Put various patterns on same mask
• Spatial filtered red laser
• Object is a simple transparency
• Chrome on glass
• Multiple patterns on the same mask
• Sizes from 10-370 mm
• Patterns
• Rectangular slits
• Rectangular blocks
• Circular holes
• Circular blocks
Grid Filtering(3. Experimental)
• Grid transparency
• Switch filter pattern by small shifts in mask

2.7 mm Grid with 110mm Filter Slit

2.7 mm Grid with 30mm Filter Slit

Blurring of an Image(3. Experimental)

Original Image

Blurred Image

Transition(3. Experimental)
• Transition from image to spatial frequency

Image off-focus

Image at focus