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DMD Diffraction pattern

Jinyang Liang10035
Intellectual 295 points

Hey, I am using DMD to project a laser image. In the experiment, I found some diffraction problems:

(1) I used a 1064nm Infrared laser as the input for two DMDs (Discovery 1100 - visible window and Discovery 4000 - NIR window). I saw two sets of diffraction pattern in the output for both DMDs. These two diffraction patterns are the same but very close to each other. I read the material in your website but it doesn't mention anything about this diffraction patterns.

(2) I used a 6328A He-Ne laser for that Discovery 4000 again. At this experiment, I saw two patterns again. However, at this time, these two patterns is kind of far away to each other. The envolop of that weaker diffraction lies at (6,6) order of the stronger diffraction pattern.

I am guessing that these weird result may be caused by the window but then I can't explain the DMD4000 with infrared input. I am also thinking some DMD structure is wavelength sensitive?

Have you ever got this problem before? Any suggestion is appreciated.

Jinyang

  • 0
    TI__Guru 54605 points

    Hello Jinyang,

    (1)  For the Infrared on the visible and NIR window the double pattern is likely caused by reflections between the inner and outer surfaces of the DMD window.  Please refer to http://focus.ti.com/pdfs/dlpdmd/Type_A_window_transmission_Discovery_Website.pdf this shows that at 1064 there is still an appreciable reflection (about 12 % by the coatings).  Above 1300 nm the reflection becomes very small.

    (2)  I am not certain, but the second weaker pattern at (6,6) for the HeNe on the NIR coating is likely caused by a reflection from the underside of the glass back on to the DMD surface that is then diffracted a second time. 

    We have seen this type of behavior when wavelengths are used that are not near the high transmission zone of the window coatings.

    Fizix

  • 0 in reply to Fizix
    Intellectual 295 points

    Hey Fizix,

    Thanks for your reply. This REALLY helps!! I have another two questions about this diffraction pattern.

    (1) I observe the interference pattern at every individual diffraction order for He-Ne with DMD4000-IR window. However, I didn't see the second set of the diffraction order at this case. Since the window mismatches the input wavelength, the high reflection should cause the same problem as I saw for 1064. I am thinking maybe for He-Ne, the second reflected diffraction is superimposed with the first diffraction set. For 1064, somehow, the second diffraction has a relatively large reflected angle, which produces the second diffraction set. Would you please help me figure out this interference pattern?

    (2) I observed a changing pattern when the wavelength mismatches the window. When you slightly move DMD (tilted vertically or horizontally) then output image fluctuated in intensity. It seems that the fluctuation is caused by a Newton-Ring-like interference pattern. Since this only happens for the mismatched situation, I guess the window should be the criminal. Do you think this is caused by the flatness of the window?

    I appreciate you help.

    Jinyang

  • 0 in reply to Jinyang Liang10035
    TI__Guru 54605 points

    Hello Jinyang,

    (1)  You are correct that for the longer wavelength the second diffraction pattern from the bounce is likely too low in angle to be seen.

    (2)  I have observed this behaviour also.  The upper and lower surface of the window can cause interference fringes, especially for mismatched coatings.

    Fizix