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Laser profiler for precision focusing
#1
I had asked on YouTube about the exact setup for the laser profiler that you used in order to achieve perfect focus.

What I'm hoping to learn is what I need to replicate your results, my understanding of it is as follows:
  • Lower laser output to minimum and send beam through neutral density attenutators, such as these:  https://www.thorlabs.com/newgrouppage9.c...up_id=5011 to prevent camera sensor damage. Acceptable range would be something like OD6 and attentuator(s) should be close to laser to prevent exceeding W/cm2 damage threshold 
  • Get a cheap camera, determine the distance to the sensor surface from some reference so you can figure out the distance from the laser to focal plane.
  • Remove camera lens element so that there is no additional beam divergence/convergence.
  • Move and focus laser to get the tightest spot, using profiling software to help determine spot diameter.
Is that essentially it or have a missed any key parts?


Thanks!
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#2
That's pretty much it. Technically you can use any camera but for cheaper units I think a CCD sensor is preferable. CCD used to be the defacto standard as they are less prone to flaring but supposedly CMOS technologies rival them now. I suspect that can't be said for all CMOS sensors though, particularly cheap webcams.

I will say one thing in regard to making absolute measurements. Even with the pixel pitch known, using the software I referenced does not give you an exact measurement. To achieve that I believe you would have to reference the alpha component of the image and apply a filter that reflects the light reduction imposed by the attenuators. This also assumes that none of the pixels are saturated.
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#3
Another method is to us anodized Aluminum plates, at a lower power setting step through different focal heights and make lines, some will burn the anodized off and leave a line, others will not. It is a manual process so there is some fudge, but should be able to get pretty close.
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