Generating your Own Doughnuts!

To generate your own doughnuts:

  1. Downloading the forked binary grating shown below:

  1. Print it.
  2. Reduce it with a photocopy machine to about 1/4 of an inch (6 mm) on the side. Be sure to turn the original sideways in consecutive reductions, because the photocopy machine does not preserve the horizontal and vertical aspect ratios.
  3. Make your last copy onto a transparency. If you look at it with a short-focal-length lens you will see the fork--it will not be pretty because of the quality of the transparency paper and the printing over it.
  4. Shine a HeNe laser onto the center of your rough "grating." You will encounter one problem: the doughnuts will be too small for you to know if they are even there. The best way to overcome this is to get a front surface mirror and reflect the pattern near you, as shown in the figure below.

  1. Now you need to align the hologram so that the beam hits the middle of the fork. If not, the pattern will not contain doughnuts. Once it is aligned you will see the LG01 and LG0-1 modes in first order. Since this is a binary grating with equal fringe to space ratio, the second order doughnut is suppressed. You will see the third order (LG03) doughnuts much better.
  2. For better performance, take pictures of the pattern with high resolution black and white film--the negative is your grating.

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