Real world problem for your pleasure.

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Integral

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The team I am a member of is currently in the process of installing a new steering mirrors in our lasers. The mirrors are about 15mm in Diameter and 7mm thick. They are first surface mirrors coated for a 355nm laser beam with 45deg incidence. 355nm is in the UV portion of the spectrum and therefore is invisible, but we can see a spot where the beam reflects from the mirror. The beam diameter along the beam path is about 3.5mm. The reflective surface is indicated by a small arrow on the side of the mirror. Once the mirrors are installed in their holders the mark is no longer visible. Suppose some sleepy graveyard tech installed a mirror back wards. How without removing the mirror could we verify proper orientation? Or must we remove the mirror to do this?

BTW: At full power this is a 7W beam, so please work safely, do not block the beam path with your hands, and take care not to reflect the beam into your eyes!
 
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Bystander

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Do the mirror specs include finishing the back side? You oughta get loads of scattering (vis and UV) from both the surface and the substrate (unless it's laser quality, which it might have to be even for first surface --- hmmm). Course if you're stuck with full power only, I wouldn't want to be the guy finding out I can blow the mirror coating off by hitting it from the back side.
 

Integral

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We do have a CW Alignment beam which is MUCH lower power, and still 355nm.

I also wonder about the backside, coated or not.

The coating consistes of many λ/4 layers. That is λ/4 at 45deg.

I am thinking of installing a used mirror bacwards just to see if my predictions are correct. I think have a solution to this, just curious if someone else with come up with the same one.
 
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