Why is the the first order useful?

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Discussion Overview

The discussion centers on the utility of the first order of diffraction in systems utilizing Acoustic Optical Light Modulators (AOLM) and Digital Micromirror Devices (DMD). Participants explore the reasons for preferring the first order over the zeroth order of diffraction, particularly in terms of modulation and signal visibility.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why the zeroth order of diffraction is not used, noting that it is often described as not being modulated.
  • Another participant asserts that the zeroth order is ineffective because it does not carry the modulated signal, implying that the first order is preferred for this reason.
  • A participant seeks clarification on why the zeroth order is not modulated, reiterating their confusion about its validity.
  • One participant explains that the AOLM alters the refractive index, creating a grating effect, which leads to the zeroth order being unmodulated.
  • Another participant inquires about the implications of these assumptions for DMDs, which do not involve refractive index changes, suggesting a need for further exploration of the topic.
  • A later reply mentions that the acoustic modulator creates density waves within the lens, contributing to the discussion on modulation effects.

Areas of Agreement / Disagreement

Participants generally agree that the zeroth order is not modulated, but there is no consensus on the underlying reasons or implications of this observation. The discussion remains unresolved regarding the broader applicability of these concepts to different systems.

Contextual Notes

Participants express uncertainty about the definitions and implications of modulation in the context of diffraction orders, particularly concerning the relationship between refractive index changes and the behavior of light in AOLMs and DMDs.

Choisai
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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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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.
 
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
 
Choisai said:
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

Ask yourself this question: what is the AOM doing to the lens? How does this change with frequency?
 
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).
 

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