Old 11-11-2010, 12:30 PM #1
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Default How "doable" is a 0.4 mrad (or better) laser?

Looking for a .3 mrad or better laser beam. My goal is a visible 25mm dot @ 90meters (aka: ~1"@100yds or 1MOA).
It'd probably have to be an already good laser around 0.8-1.2mrad with a small (maybe 2X-3X) expander.
It just has to fit on a rifle... I think a high precision laser sight would be cool.

Color doesn't matter, it just has to be visible from ~100yds.
I'm thinking 130+mW red or 35+mW green would probably do the trick.
Plus reds and greens generally have goom charactaristics too.
IR would be totally badass, although I'm not sure how I would collimate/align it.

So are compact beam expanders available or easily built (I have a mill & lathe)?
Are 0.8-1.2mrad diodes or modules available?


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Old 11-11-2010, 12:43 PM #2
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Default Re: How "doable" is a 0.4 mrad (or better) laser?

OK, after googling a bit I found the GDLM1000 Green module... but it's >$500
http://www.photoptech.com/main/produ...L%20Module.pdf

50mW, 0.7mrad, and 2.5mm beam... a small 2X-3X expander could easily achieve my goal.
Anything cheaper out there guys?
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Old 11-11-2010, 01:12 PM #3
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Default Re: How "doable" is a 0.4 mrad (or better) laser?

Well, if you don't care about the "internal photodiode stabilized power" of these particular modules, you can get a standard o-like or similar module for a fraction of these $$ ..... and use a 2X or 3X beam expander, or also just buy one yourself, for get a better divergence .....

Is the size important ? ..... if not (also considering that a beam expander still increase the size of a pointer), you can try to take away the front lens of a standard green DPSS module, fit it inside a threaded tube, and use a larger diameter lens as output / focusing element (the expander that is already in front of the crystal acts as first element of a standard beam expander, so, if you use a larger positive lens placed at a higher distance from it, the effect is the same of a standard beam expander added in front of the module ..... only, it's more long)

Or also, for short the assembly, insert a negative element in front of the original expander, and use the larger positive lens more near the assembly, this must work too ..... ofcourse, remember that all these solutions gives you a larger beam at the start (short distance), also the already-made commercial beam expanders ..... anyway, i suppose you can start to experiment with a cheap module, maybe the ones from ebay, and swap it with a better one when you find a good combination of elements that give you the wanted results .....
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Old 11-11-2010, 01:47 PM #4
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Default Re: How "doable" is a 0.4 mrad (or better) laser?

Larger beam at the start is irrelevant. This is for a rifle laser sight so the beam wouldn't be used until at least 10-15 meters away. Even a 10x expanded beam would only be about 30mm in diameter at the aperture... so I'm not worried about the diameter or beam size of a 2X-3X expander.

I think overal size would probably have to rival a typical firearm attachment... 25mm or 30mm diameter and ~150mm long. Even with a Li-ion 18650 and module, there should still be enough room for an expander.

So in a beam expander, the secondary larger lens is what collimates the lens? So if mount the larger lens on a threaded end I should be able to adust for optimum divergence? I was kind of hoping that they just made premade beam expander modules. I don't care about using more or less lenses because any power losses won't affect the use as a laser sight.
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Old 11-11-2010, 03:55 PM #5
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Default Re: How "doable" is a 0.4 mrad (or better) laser?

But a green DPSS module IS a beam expander, already ..... must be enough to change its parameters (i made some tests in the past, but only the "long" version, cause i had no spare negative lenses for try the "short" modification .....

Wait a minute, i try to draw it, i think images are better than words.

EDIT:

basically, the optical structure (very simplified ) of a DPSS module, is this one:



As you see, the internal optic is already a beam expander, cause the crystal itself have a bad divergence, and need to be corrected, but the only way is to use a beam expander structure ..... now, there are 2 possible ways, for modify a module like this, for have a better divergence ..... one is take away the front lens, and substitute it with a bigger one with longer focal distance, and place it far from the expander, so it act like a longer beam expander:



But this need a very long distance ..... i've tried this system, and it work decently, but for get around 8X, i ended with an assembly that was almost 10 times longer than the original module ..... the other system is to align a second expander in front of the first, then use a bigger lens with short FL, at short distance, like this:



I had no lenses good enough for try this solution, but i think it must work decently too, with the right FL lenses ..... also, both these systems draw less power than a standard beam expander (the first one left you the original power, the second one drop only the loss from the second expander ..... where a standard beam expander added on the module loss the drop from both its lenses)
Attached Thumbnails
How "doable" is a 0.4 mrad (or better) laser?-dpss001.jpg   How "doable" is a 0.4 mrad (or better) laser?-dpss002.jpg   How "doable" is a 0.4 mrad (or better) laser?-dpss003.jpg  
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