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Is this mode hopping?

okie

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Oct 17, 2019
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This is a PAQ 4 IR laser as seen through a PVS 14. As you can see, it's projecting two distinct dots. There is a small dot on the right hand side of the main beam. It's way more apparent in person, but much quality gets lost when you're taking photos through the PVS with an iphone. In real life it's not blurry and the stray dot is way more defined. If you're on a phone, you might have to zoom in to see what I'm talking about. If you're on a desktop, it's immediately obvious what I'm talking about.

When I type in "laser projecting two dots" I get a lot of hits for something called mode hopping. Could that be what's wrong here?

Note that I've already ruled out glare on the PVS lens. If you rotate the laser module the stray dot rotates with it. If it were just glare on the PVS lens, then the dot would stay fixed relative to the rotation of the module. I.e. no matter how you rotated it, the dot would always be on the right, but that's not the case. The stray dot will rotate with the laser.

I also cleaned the lens really well to make sure there wasn't any dust on it.

 



hwang21

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That looks like a slight misalignment somewhere or a backreflection that is making it all the way back through the aperture. Mode hopping would cause large drops in power as it shifts TEM modes too, so if you aren't seeing major power fluctuations, it's probably not a mode hopping issue

Here's a pic of an Ar laser that had slightly misaligned mirrors:

1571338326626.png

1571338252863.png
 

okie

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@hwang21

I guess that would explain why changing the windage causes it to come and go.

My biggest concern is that it's not going to hold zero. Do you think those fears are justified?
 

jeffreythe00

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That looks like an internal reflection more-so than mode hopping. The largest dot is your primary beam and all others are likely reflections. If the dots move/multiply with windage adjustments that also strongly suggests internal reflections. As far as true accuracy/zero is concerned, its impossible to tell from photos. It's possible the unit had taken a hard impact at one point causing a misalignment. Your best bet for testing the zero would be to go out in the field and test with known distances and known accurate optics/or ask someone with a known working unit to test with theirs. These internal reflections shouldn't affect the accuracy of your main beam, unless your unit suffered a misalignment, then they most certainly would affect your windage zero. If no misalignment then, they would just be "artifacts".

A quick google search indicates this is a military unit and there seems to be other people with the same unit. Maybe ask one of them to do the same tests with their phone to see if they experience the same issue. Worst that happens is nobody responds :), so IMHO you have nothing to lose. In any case, I'd be interested to see how you make out and what info you can find. Good luck!
 
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