Baffled as hell - need help from physics nerd

[pseudonomen137]

So here's the deal, in what's very roughly an extension of the plotted project I posted earlier (http://www.laserpointerforums.com/forums/YaBB.pl?num=1213075631), we have a decent 300mW 532nm laser for burning. The smaller the spot, the better, but any improvement over the red laser plotter is a welcome step forward. So far, SEM images have shown marks down to 16 microns out of the red setup.

Now, I got the 532nm module pointed down with a focusing lens on the front. First a relatively weak ~20cm focus, then a stronger ~5cm focus, then I got desperate and used one of those acrylic AixiZ collimating lenses to try focusing. No matter what I do, I always seem to get spot size in the 35-60 micron range.

Admittedly, all I have to adjust my focus is the target on a lab-jack (got a lovely Newport stage for X and Y, but not much for the Z). Normally I'd just blame it on that, but I've tried everything to fine tune that focus, and in any case, I keep getting numbers very close to 40-50 microns. If it were lets say, 70, then 200, then 40, etc. I could blame it on poor focusing, but this just seems too damn much of a coincidence.

Soo... what idiot thing am I doing wrong here? Beam specs seem in line for M^2 <2 at the very worst, so getting it down to a few microns shouldn't be too hard I guess. I just can't figure out how to do it though! Perhaps diffraction, abberation, etc of the crappy lenses I'm using? But could that really account for that big of a problem? Anything else anyone can think of? Or am I just being an idiot about the whole thing? This is driving me crazy!

 

[pseudonomen137]

Re: Baffled as hell - need help from physics junki

Could it be imperfections in the lens?  For example, a small flat spot or imperfect grinding?

No. Its definitely focusing the beam, and I'm catching it as close to the focus as I'm able to with this equipment. Dunno about abberations, astigmatism, etc. but could that really result in a nice perfectly circular spot that has such a large minimum(?) spot size like this?

 

[FrothyChimp]

Have you tried expanding the beam using a collimator then cranking it down with a short focal length lens?

 

[pseudonomen137]

Have you tried expanding the beam using a collimator then cranking it down with a short focal length lens?

Haven't yet broken down to the point of getting the lab to buy a beam expander, though that was a last resort. I focused it through one of those acrylic AixiZ collimating lenses though, so its a pretty damn close focus already. Not to mention the red was focused using the same type of AixiZ lens so it should be getting such significantly better spot sizes anyway... right? :-/

 

[FrothyChimp]

yes, you would expect that to be the case but your red beam diameter is 4-5mm? The green DPSS output is probably on the order of 1-2mm depending on the design. I think you might be chasing the divergence down at the focus size. Expanding the beam and then using a short focal length focusing lens might buy you enough to shrink the spot. That's just an off the cuff thought without knowing much about the rig.

You don't need a fancy expander to at least give you proof of concept. A matched achromatic pair would give you the expansion and the focus to see if there is any difference. Go to something like 6-10mm beam diameter and see where you are at on spot size.

 

[pseudonomen137]

Possible, though it still seems strange to me that I keep getting the same rough diameter. I was thinking about it though, and although both lenses have the same focus (hence two similar, collimated beams would have the same min spot), since the red was initially divergent, I gotta move the lens further out on the diverging beam to get a tighter focus, and its gunna be bringing it to a stronger focus too. So yeah, not beam expanding, but diverging it first may help. Right before I saw your power I started picking out some nice big lenses for the task to bring to work tomorrow. Hopefully this'll work, thanks!

 

[Justin]

It seems to me that you're running into an issue of initial beam diameter vs. divergence. I think that the idea of expanding the beam first and then collimating it down to a spot is a good one, as this should enable a smaller final diameter by reducing the divergence of the beam. This is basically a beam expander setup.