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Why is the the first order useful?

  1. Apr 25, 2014 #1
    I've been looking into diffraction for AOLM and stuff like that, and these produce diffraction. Binary Spatial Light Modulators like Digital Micromirror Devices also produce diffraction. This diffraction is then focused and filtered using a pinhole. But, and here is the question:

    Why is always the first order of diffraction used and not the zeroth order?

    The first order has more power than the 2nd, 3rd or other higher orders. That's something I get. But why is the zeroth order of diffraction not used? I've been seeing on several websites that the zeroth order is not 'modulated', but why isn't it? Isn't the zeroth order just as valid to use as the 1st? I have added an image of a 4f system and you can see it discards the zeroth order. Why?

    EDIT: AOLM stands for Acoustic Optical Light Modulator (more commonly known as just AOM) and adds the frequency of soundwaves to light. For an example, if you want to add several MHz to light (of several THz) you can do so using an AOLM.

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    Last edited: Apr 25, 2014
  2. jcsd
  3. Apr 25, 2014 #2
    Do you really expect us to know what AOLM means? You got to be kidding. Even without really knowing what you mean, it's clear that the zeroth order is no good because it is not modulated. That means the signal you're looking for won't show up in the zeroth order fringe.
  4. Apr 25, 2014 #3
    I get the zeroth order is not modulated, but why is the zeroth order not modulated?

    PS: I added an explanation of what an AOLM is to my question
  5. Apr 25, 2014 #4


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    Ask yourself this question: what is the AOM doing to the lens? How does this change with frequency?
  6. Apr 25, 2014 #5
    It uses sound wave to change the refractive index. This index-modulation creates relatively more opaque and less opaque area, which acts like a grating. The zeroth order is made up of light coming through this grating, but I guess that isn't the part that gets refracted by the change in index? And therefore the zeroth order is the unmodulated order?

    If these assumptions of mine are correct, how do they extend to more general applications, such Digital Micromirror Devices that use pinhole filtering? There is no change in refractive index when you use a DMD, only tiny micromirrors (usually 10 micron in size).
  7. Apr 25, 2014 #6


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