• State TI Thinks Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 3 answers
  • Subscribers 29 subscribers
  • Views 218 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

DLP9000: Blazed grating effect due to DMD

Sahil Sharma
Prodigy 40 points
Part Number: DLP9000

Tool/software:

Hi I am trying to used the DMD as  diffraction grating. when I make a grating of 12 pixels on the dmd I see that it diffracts the laser (520) . I see two patterns, one due to blazed grating effect of the dmd ( that is when there is no  pattern and light is shine on it, you still see a 2D diffraction pattern). and the other one due to the pattern. However as per theory, the + and - order of the diffracted light should have same power which is not the case. Can someone help me find my mistake?

The mirrors are tilted at 12 degrees, so in order to make the reflected light from dmd to go perpendicular, the laser light falls on it on angle 24 degree. However I put around 30 degrees in my case as  it worked for me .  Since it is very crucial for me that the power values for both the orders match. Can anyone tell why is it not so?

  • 0
    TI__Expert 4400 points

    Hi Sahil,

    I've assigned this ticket to someone who is familiar with this DMD. Please allow us some time to get back to you on this.

    Regards,
    Michael Ly

  • 0
    TI__Mastermind 46950 points

    Hello Sahil,

    Please read the following two application notes:

    Using Lasers With DLP DMD Technology

    System Design Considerations Using TI DLP® Technology in UVA (363 – 420 nm) (Rev. A) - this one will help understand the different blaze conditions even though you are not working in the UV, the principles are still the same.

    If you still have questions then post back.

    Fizix

  • 0 in reply to Fizix
    Prodigy 40 points

    Hi I read the  file but I am still not sure how to  meet the blazed grating condition.  I am illuminating the dmd with 24 degree angle from the dmd normal but I see that the diffraction orders are still not equal in intensity. I see as shift in them  Can you please help in calculating the blaze condition for my laser of 520 nm wavelength?

  • 0 in reply to Sahil Sharma
    TI__Mastermind 46950 points

    Sahil,

    In the documents you read it provides a discussion about how to maximize one order (a blaze).  To try to make four orders equal in value you need an "anti-blaze" condition (a term I coined for it).  This means to set the order of interest in the calculations to n +/- 0.5 which will result in an anti-blaze condition.  Be aware that it is not always possible to reach a condition you want.  If you try say order 8 + 0.5 and cannot reach an anti-blaze then try 8 - 0.5.

    NOTE:  I pulled 8 out of my head.  You need to calculate the closest order (n = ?) to a blaze as your starting point.

    Fizix

  • 0 in reply to Fizix
    Prodigy 40 points

    To be honest I did not grasp the concept clearly, I will tell you want I am doing and I hope you can tell me what I should do. So when I keep the incident angle around 24 degree,  the reflected light from the dmd is almost normal to the dmd surface and shows 2D diffraction with several spots. then I project a binary grating of 12 pixel pitch on the DMD. after this, the central spot ( which looks the brightest to me) splits into three spots(-1,0,1)  . I am these 3 spots to pass through my  objective to generate a sinusoidal pattern. However, I am not being able to get sharp fringes. Which I believe might be due to unequal power of the (+1 and _1)  

    Now even if I do not project the pattern on the DMD I still see the central spot to be brightest but the spots on the one side are brighter than the other side. which I do not understand why. 

    Since the incident angle is 24, the zeroth order and the since envelope should coincide and I should get maximum power in the zeroth order only. as per the theory so why is that I am not getting equal power in my +1 and -1 order. 

    I hope I have kept my situation clearly. 

  • 0 in reply to Sahil Sharma
    TI__Mastermind 46950 points

    Sahil,

    Since the envelope center is not exact in the middle of 4 orders then 3 of them are brighter.  Please see section 5.2 of the second app note - System Design Considerations Using TI DLP® Technology in UVA (363 – 420 nm) (Rev. A).  This section has the formula you need to calculate the incident angle.  Please send me friend invite if you are still having a difficult time with it. 

    Fizix