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Diffraction Efficiency in Visible Range

Kourosh
Prodigy 20 points

Hi,

    I checked the dimension and tilt angle of mirror for one of the DMD devices which is 0.7 XGA type A DMD, as I understand, it's blaze wavelength for 2nd and 3rd order of diffraction are around 2 and 1.3 micrometer respectively so if I want to use it by a monochromatic laser in visible range, let say 532 nm, highest diffraction efficiency occurs in 7-8th order. I am really confused. if I don't want to use 0 order diffraction mode , should I use 7-8th order of diffraction to get the highest efficiency? I will really appreciate it if anybody let me know how much diffraction efficiency is for 532 nm (roughly) for 2nd and 3rd order of diffraction.

Best,

K

  • 0
    TI__Guru 54510 points

    Hello Kourosh,

    Welcome to the DLP section of the TI-E2E community.

    Diffraction can be a little bit complex.  For a DMD it is an interaction of the wavelength, pixel pitch, mirror tilt angle and incident beam illuminiation angle.

    I do not know the efficiency, but the 2nd and 3rd order will be very low efficiency.  You really  need to work out near the 7th or 8th order as you mentioned.  With illumination At 24° over from the DMD normal coming in from the upper left corner (marked as pin 1) the 7th or 8th order will be coming out of the DMD near normal.

    Is there a specific need to work in the lower orders?  The dispersion at the 7th and 8th orders for 532 will be similar to the dispersion of the 2nd order in the 1.3 - 2 µm wavelength region.

    Fizix

  • 0 in reply to Fizix
    Prodigy 20 points

    Hello Fizix,

            Thanks for your response, I am not willing to use 0th order because I am not sure about the scattering of inter-pixel area. I have this problem with my liquid crystal SLM, I know DMD has completely different configuration and physical structure, but it might an issue for me. the problem with 7-8th order of diffraction is its direction, although it is near normal direction but it's angle with normal is much more than 2nd order relatively. Did you have any experiences with inter-pixel scattering?

    Regards,

    Kourosh

  • 0 in reply to Kourosh
    TI__Guru 54510 points

    Hello Kourosh,

    We have not studied inter-pixel scattering in any depth.

    However, calculations show that for the 13.68 µm pixel at 532 nm wavelength and 24° angle incident illumination that the 8th order should be at -1.1° from the normal.  The 2nd and 3rd would be at +17.3° and +14.0° respectively,

    24° illumination is used because the mirror tilt angle is 12° and this puts the blazed (or near-blazed) order(s) closest to the normal of the DMD (i.e. the window normal).  If 27.2° illumination were used the 8th order would be very close to normal (less than 0.01°).

    Fizix

    P.S. -- NOTE: When refering to the nth order we are actually refering to the (n,n) order.  DMDs can only reach blaze conditions along the 45° diagonal of the diffraction pattern such that m=n for blaze orders [i.e. (1,1), (2,2), (3,3), . . . (n,n)].