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Original question:

DLP6500FLQ: Pulsing at bit depths other than 1

DLP6500FLQ: Lens set up required for collimating and focusing a 808 nm laser

Aaron Liao
Prodigy 20 points
Part Number: DLP6500FLQ
Other Parts Discussed in Thread: DLP650LNIR, DLPR410B, DLPC410

Hi,

I am looking to use this DMD chipset with an 808 nm quasi-CW laser. I am treating each of the pixels on the DMD as a diverging point source that I wish to collimate and focus down to a 1 micron spot (1/7.6 magnification). I am assuming the divergence angle of the beam is 12 deg, and the light engine housing the chip does not allow for mounting optics closer than 30mm away from the DMD. 

This may just be a simple optics question, but can I just place a f = 30mm lens that has a large enough diameter such that the illumination cone of the pixels at the edge are encompassed? Must I account for interference effects since the illumination cones overlap? Anything pointing me in the right direction is appreciated. 

  • 0
    TI__Guru 54320 points

    Hello Aaron,

    Please allow time to consult with the optics team on this.  I believe this will be ok optically, but 808 nm is outside the recommended operating conditions for this device.  For wavelengths greater than 700 nm the power is listed as 10 mW/cm^2 max.

    Fizix

  • 0 in reply to Fizix
    TI__Guru 54320 points

    Aaron,

    I have sent this over to the optics team and hope to hear back from them soon.

    Fizix

  • 0
    Prodigy 20 points

    Thanks! I noticed you wrote a white paper on 'Laser Power Handling for DMD's'. In the meanwhile, I could use some help understanding the powers I can use with the laser. 

    I am still learning what the system can handle, and I am having a hard time drawing a comparison between the model in that paper and the setup that we are using. In figure 7 which is a wavelength v.s. reflectance plot, I see that at 808 nm, the reflectance is at a minimum dropping to around 87%. Is that a big enough difference to drop the recommended irradiance for visible light from 25 W/cm^2 to 10 mW/cm^2?

    Unless the efficiency of the optics in the DMD engine (Wintech PRO 6500) are extremely low, I believe we have been operating the DMD at powers above this threshold for a while now. The laser operates at 100 Hz, and has varying pulse widths between <100μs to >5ms (load dependent) according to the specification sheet. There is an option to set the duty cycle on this controller, but it is not something I have experimented with. The nominal voltage that the controller sends to the diode is 20V, and we have used current values between 15 - 40 A. I imagine that we have reduced the lifetime of this DMD, but for some of the sintering window experiments that this group has performed in the past, we've definitely exceeded what is recommended. That wasn't well explained to me before this project was passed on to me, so I'm trying to figure it out.

    Your help is greatly appreciated! :)

  • 0 in reply to Aaron Liao
    TI__Guru 54320 points

    Hello Aaron,

    I got a response from the optics team.  This configuration would be very challenging.  Since you are working at one wavelength, re-converging the diffraction orders back to an image should not be an issue.

    The real challenge here is the demagnification (7.6 X).  Our optics guys looked at the question and indicate that the aperture at the output of the DMD would likely have to be much smaller (on the order of say F/10) to keep the F/# on the image side manageable.

    For example at F/2.4 on the DMD side the effective F/# on the image side would be around F/0.3!!  Making the F/# larger would make the image side F/# a bit more manageable.

    You could use a 30 mm focal length lens on the output, but then the focal length on the image side would have to be 3.95 mm.  This would require more than adding a simple lens.  A "stepperlens" would likely be needed.  This type of lens is quite complex having around 30 lens elements.

    Regarding the power.  The AR coating is designed for the visible range, so testing has not been done into the NIR, hence the 10 mW/cm^2.

    We do have an NIR chip (DLP650LNIR) that is operated with the DLPC410 paired with a DLPR410B prom.  The DLP650LNIR has a much higher power rating and is designed for the NIR.  You would have to look at it's data sheet.  The targeted max power range is beyond 808 nm, but still 40 W/cm^2 for the range you are using.

    Please look at this page for possible offerings for this DMD:  https://www.ti.com/dlp-chip/advanced-light-control/optics-electronics/optics-electronics.html 

    Note:  I believe that ViALUX recently added offerings for this chip that are not yet reflected in the table.  Also Dli in Austin resells a lot of ViALUX offerings.  

    Fizix

  • 0 in reply to Fizix
    Prodigy 20 points

    Thanks for the response! I guess I was wondering if it was possible to collimate close to the exit of the DMD module, the main goal would just be to have collimated light such that most of it can be captured for maximum intensity. I think you may have answered my question about reconverging the diffraction orders. After collimation, I was thinking of using a beam expander so that the f/# would be easier to achieve for diffraction-limited resolution, instead of it just being a two lens set up.

    This is helpful, and I appreciate your responsiveness on this forum!

  • 0 in reply to Aaron Liao
    TI__Guru 54320 points

    Aaron,

    Thanks for letting us know.  I am marking this as resolved for now.

    Fizix