DPSS Laser Gloat!

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Re: DPSS Laser Gloat!

Postby Jerry » Mon Jul 02, 2012 2:48 pm

Plain aluminum takes a big dive in reflectivity below about 400-450nm. I can vaporize it right off the surface of the glass. There are UV enhanced aluminum coatings but they still only reflect about 91%. Dielectric is about the only way to efficiently move the beam around at about 99.75%. They are only $125 a piece at thorlabs!
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Re: DPSS Laser Gloat!

Postby Doug Coulter » Mon Jul 02, 2012 3:35 pm

Looking for the charts (haven't found them yet) but Wiki says this under aluminum:

Aluminium mirror finish has the highest reflectance of any metal in the 200–400 nm (UV) and the 3,000–10,000 nm (far IR) regions; in the 400–700 nm visible range it is slightly outperformed by tin and silver and in the 700–3000 (near IR) by silver, gold, and copper.[9]


It could be the best and still not be good enough, of course. I know silver has a hole up there somewhere - you can use it for a short-UV pass filter.

And yeah, forget just buying specialized optics at those outfits - Ednumd optical is probably even worse in prices.

Here we go - so mid 90's won't work? Might have to be a solid chunk of Al to carry away the losses - half a watt, about?
Image-Metal-reflectance.png
Image-Metal-reflectance.png (8.78 KiB) Viewed 2673 times
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Re: DPSS Laser Gloat!

Postby Jerry » Wed Jul 04, 2012 5:21 am

By the time you bounce off a few mirrors to get where you are going the mid-90's add up. Second problem is the power density, the beam output on this laser is pretty tiny, about a half mm or less and stays pretty tight. Add that to a pulsed beam that has peak powers in the 10's of kW, that is enough to vaporize the aluminum.

Take a look at the graph on this page. Typical non-coated front surface mirrors suck at these wavelengths.

http://www.ofr.com/oc-12_metal_mirror.htm

But not to worry, today I got some dielectric mirrors for 355nm as well as some various lenses. Makes a huge difference. I would really like to try this with a beam expander. Then I can get the spot size down. There are not too expensive, about $100 to 150 on ebay. I have ones for 532 and 1064.
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Re: DPSS Laser Gloat!

Postby Doug Coulter » Wed Jul 04, 2012 7:18 pm

Yeah, I lecture people all the time about what things like .9^N work out to, but it's interesting that even their mirrors aren't claiming better numbers than just plain Al (as I linked above) or even going back in time, John Strong's numbers, even with the MgF. Funny thing that. I was thinking you need at most two mirrors - one for focus to spot, one for XY movement, but I could be wrong about that. No lenses for certain!
In my idea, you don't need super power - at 3-4w CW, that's still going to expose really fast anyway, and be off most of the time as well (only on where you want the stuff to stay).

I have the same issue with my big Nd:YAG - beam too small and dense to work with as is...I called all the fancy optics guys and they are saying no way, spread it out somehow, then our products can work on it.
But how do you get there from here?
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Re: DPSS Laser Gloat!

Postby Jerry » Sun Jul 08, 2012 6:06 pm

It all depends on how I build the system. If I keep the focus lens still and move the work then I really only need a mirror and beam shaping optics. If I go the more traditional flying optics route and hold the work still then I would normally have about 3 mirrors plus optics. Last option is use galvos and a f-theta lens but that is much more difficult software wise.

Lenses are not a problem. I have UV specific lenses for the 355. You shouldn't have a problem with it on your yag either once you expand the beam. I have access to some beam expanders for 1064 if you are interested. If you had to you could make a beam expander with mirrored optics.
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Re: DPSS Laser Gloat!

Postby Jerry » Sun Jul 15, 2012 2:40 am

Here's what it does to pop cans:

Image
Laser cut pop can by macona, on Flickr
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Re: DPSS Laser Gloat!

Postby Doug Coulter » Sun Jul 15, 2012 3:28 pm

Hmm, well, that wasn't polished Al of course, but that would seem to indicate you need high 90's reflectance even on a solid Al piece to be a good mirror. Nice clean cut, though!

I'm a ways off from trying anything with my YAG. I'm not even dead sure which end the beam comes out of! Frank Sans says it works, I have all the parts, but it's obviously not Q switched at this point, just a big rod in a cavity with polished ends on the rod. Supposed to be 60j out for 1500j in. Keep me in mind on those optics, though. Frank says it will punch a hole in a quarter with one pulse!
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