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Hologram questions

  1. Aug 1, 2006 #1
    after many experiments with digital holography i finally got the best interference image i could...

    but after analyzing it, i got my object in two locations!
    it was supposed to be on the lower right side of the image, but i got it on the upper right side too....

    any idea why it happened?

    if you need more info, i can explain the setup i used to get the image:
    (without getting into detail about how i control the intensity of each beam and make sure there are no distubances):
    first i split the laser beam, and light my object with one beam (the object beam), the second beam is reflected by glass from (almost) behind the object, and a digital camera without it's lenses pictures the interference of the two rays.

    after i got my image, i use matlab to construct the image back - i simulate a beam with the same wavelength of the original beam that light through the image - it's pretty easy actually, i only need to use inverse fourier transform on my image multiplied by something which holds the info about the distance of the camera and the focal length i want to use for my simulated eye.

    the script is very strait forward, so if anyone want it, i can post it here.

    anyway, i just can't figure out why i got the second object...
     
  2. jcsd
  3. Aug 1, 2006 #2

    berkeman

    User Avatar

    Staff: Mentor

    Just a wild guess, but maybe you need to use a single-sided transform. Is the reflection backwards? Could it be a reflection about the zero spatial frequency axis in the transform?
     
  4. Aug 1, 2006 #3
    i don't think it's backwards... here, i'll post it:

    the default ifft function in matlab is nonsymetric, so i don't think that's the reason...

    any other ideas?
     

    Attached Files:

    Last edited: Aug 1, 2006
  5. Aug 1, 2006 #4
    just to make sure it's not the fault of my script, i'll post it here, so you can take a look:

    Code (Text):

    I=imread('img.tif');
    I=I(750:1773, 1:1024);

    C=complex(I);
    C=double(C);
    lam = 632.817e-9;
    %lam is the wavelength
    d=1.128;

    %d is the distance of the camera from my object
    dx=6.8e-6;
    %dx is the pixel seperation of my camera

    j=1:1024;
    K= -i*pi/lam/d*(j.^2*dx.^2)';
    L= -i*pi/lam/d*(j.^2*dx.^2);
    KL=exp(K)*exp(L);
    C=KL.*C;


    D=ifft2(C);
    %inverse FFT
    F=abs(D);


    F1=F(1:512,1:512);
    F2=F(1:512,513:1024);
    F3=F(513:1024,1:512);
    F4=F(513:1024,513:1024);
    F=[F4,F3;F2,F1];
    clear F4 F3 F2 F1 D KL K L j

    F=F.^0.5;
    %making the faded image of my hologram brighter.

    imshow(F)
     
     
  6. Aug 2, 2006 #5
    I found the cause!

    when i picture the interference i get [tex]I=O^2+R^2+2ORcos(\Delta \phi)[/tex]
    where R is the reference beam's amplitude, and O is the object beam's.
    so i don't really get the phase difference of the two - i get the cosine of it, and the cosine have the same value for positive phase differences as for negative ones.
     
  7. Aug 2, 2006 #6

    Danger

    User Avatar
    Gold Member

    Good to hear that you sorted it out, fargoth. My thought was that you might not be using front-surface mirrors, and thus getting muliple reflections.
     
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