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I am new in working with an optical setup. I have a small setup to mimic a 3D printer motion stage. So, it consists of a laser hitting on a cube beam splitter. The transmitted one is dumped while the reflected is focused on the target and then the backscaterred light collected via the beam splitter and focused to the CCD camera.

My problem is that on the camera image I got 2 spots (one weaker though). I think its some kind of unwanted secondary reflection from the beam splitter because:

  • When I cut the beam between the laser lens and the beamsplitter with a blade from any direction the spots distort and fade at the same time.

  • If I dump the beam hitting the target I only see the weaker spot

Can someone give some suggestion to get rid of this unwanted spot? Thanks

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  • $\begingroup$ Do you have any apertures in your optic path? If not I'd suggest adding one. $\endgroup$
    – Jon Custer
    Oct 2 '20 at 13:57
  • $\begingroup$ I can't use an aperture at the exit of the beam splitter $\endgroup$
    – student210
    Oct 6 '20 at 12:43
  • $\begingroup$ I would suggest using a lens and putting the aperture at the beam waist of the lens. This would be somewhere else in the optical path than right at the beamsplitter (for example in your section that focuses the reflected light to a CCD camera). $\endgroup$
    – Jon Custer
    Oct 6 '20 at 12:47
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One can apply material to unused surfaces, but you have two used surfaces, at nearly normal angle of incidence, that the beam goes through. These are the first face of the cube and then the 2nd exit face for the beam that undergoes the 90 degree deviation. (Let's ignore for the moment the possible reflection from the beam that goes straight through).

If the cube is coated for use as a broadband cube in the visible, the reflection off of one face might be 0.2 % to as much as 0.5%. If your laser is near the edges of the design wavelength band for the cube, the reflection could be higher. Your eye is very capable of detecting spots of much different intensity, so these could be the spots you see.

One thing you can try is to purposely rotate the cube about the axis that is perpendicular to the plane containing your source, your target, and the cube. If you do this slowly, you might be able to sort out where the reflection is coming from. Basically, you may want to purposely misalign the system in order to have the back reflections go to another location.

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  • $\begingroup$ The BS is mounted on a cage system so I can't really rotate the BS. Would you think that instead of a cube BS is I use a pellicle BS os a plate (with a small wedge) BS would work? $\endgroup$
    – student210
    Oct 6 '20 at 12:45
  • $\begingroup$ A pellicle might help, though I don't think you need the plate and wedge. However, if I understand your setup correctly, you only need light to transmit through three surfaces of the cube BS. I'll call them "entry face", "target face", and "CCD face". The fourth surface might be called the "exit face", where the undeviated beam would exit. I suspect that the unwanted spot of light comes from internal reflection off the "exit face". If your beam power is not too high, you should be able to put a piece of black plastic electrician's tape on the exit face to solve the problem. $\endgroup$
    – S. McGrew
    Oct 6 '20 at 15:29
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It may be due to reflections from the surfaces of the beamsplitter cube. If you paint all unused surfaces black (or equivalently, put black plastic electricians' tape on those surfaces), the unwanted reflections will be absorbed.

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  • $\begingroup$ Painting is not a solution. Would you think that instead of a cube BS is I use a pellicle BS os a plate (with a small wedge) BS would work? $\endgroup$
    – student210
    Oct 6 '20 at 12:45

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