Scanner Mirror Modification

[krazer]

Do you think Macona's Q switched 355nm laser can drill glass?

I doubt this would work with a 355nm laser, 355 is just at the edge of transmission for normal glass and goes right through good quartz. You would need to go to sub-300nm (266nm would be your best bet) to be able to do any decent UV micromachining. Alternatively, a CO2 laser might do the job, ideally one of the big pulsed ones but I think even a normal ~100w should have enough power to vaporize glass if you focus tightly enough.

Another alternative would be to sand blast it, I am not sure if you will get the repeatability/resolution that you need, but if you use a laser cut stencil (ex, sold for PCB solder past application) and start blasting away it seems possible that you could get what you need.

On an unrelated note, is there a particular reason that you want to do the shaping on a finished mirror? It seems like it would be much easier to do it on a blank, polish, and have it aluminized just like you would with a telescope mirror.

[planters]

Krazer,

Would there be a nonlinear threshold where Q switched might work? I thought Coherent was marketing their high repetion 532 lasers for glass/ solar cell trimming.

I think you are right regarding the sand blasting. The problem might be the determination of when to quit in order to reach a consistent depth.

The problem with post-machining polishing is for the most part, practical. If I do this and it works, I will want to test it and run some mirror pairs on various motors. If I can modify existing mirrors then there is an inventory of spare mirrors at EM and Pangolin etc. Figuring my own small collection will be expensive, slow and limited to batches. There is also the concern that as with telescopes when a thinned substrate is figured that the variation in the sub structure will cause the surface to be distorted. On the other hand, stresses within the substrate can be exposed by the drilling processes and if this is the case then post drilling figuring can't be avoided. Only way to know is to test this. My first runs with the drill, if the laser is not the way to go, will be on the sides of microscope slides.

[macona]

Do you think Macona's Q switched 355nm laser can drill glass?

It actually does blast glass to bits when focused. A normal lens will focus the 355nm beam but it is very lossy, the optics get hot pretty quickly and you can see the glass fluorescing in the UV. But when that beam is focused, the glass does not stand a chance. I have drilled about 1/8" into a piece of glass, it kind of ablates it. It also does the same to silicon, but more efficient.

[macona]

Alternatively, a CO2 laser might do the job, ideally one of the big pulsed ones but I think even a normal ~100w should have enough power to vaporize glass if you focus tightly enough.

CO2 lasers cant cut glass, as soon as the beam hits the glass the surface fractures which scatters the beam. This makes it great for engraving. We tried cutting a hole in a test tube with a co2 laser having it go over the same spot over and over. Evenually we made a pin hole but that was it. Thats was with a 45watt laser and maybe 1mm thick glass. And it took quite a while. Even with the bigger 80w CO2 it couldnt go deeper.

[krazer]

That is why you need enough energy to ablate it--very tight focusing and lots of energy. It sounds like with your 355nm system you are getting some plasma generation, which should work decently well enough for hole drilling, out of curiosity did the hole have square edges and a clean bottom (like the cut from a CO2 laser into acrylic?). My experience with 355nm and glass has always been the 'icky shallow' hole that you describe with a co2 laser, although I have never put a good effort in to get a tight focus.
The only problem would be that any light which is not absorbed by the plasma will go strait to melting the the mirror coating on the other side

[planters]

Krazer,

I've got it! Now don't tell anyone, but your femtosecond amp is perfect.

Macona,

When drilling the glass, what stopped you. 1/8" is almost enough. My 14mm wide mirrors will probably be fine with 4mm deep holes. With 13 0f these along each long edge the rotating inertia will be reduced to 65% and the specific stiffness will increase by approx 1.7. I don't know the dimensions of the Pangolin 8mm mirrors ( I'll email them), but odds are 2mm may be enough for these. How precisely can you place these holes. Do these holes taper? A small taper is probably even better than straight.

[macona]

It actually did a pretty good job, especially since I was holding the chunk of glass with my hand and moving it around that way. I have not tried it under my laser cutter now that it is all put together.

The glass cracked after a while, there was still enough heat generated tp create enough of a pressure differential to crack it. With the air assist on the laser cutter it ought to be possible to do it without cracking. I have never tried. Heck, I havnt even turned the thing on in the past couple months.

[planters]

If you're up to it, try the edge of a microscope slide. The slide is made (literally) for inspecting under a microscope for hole profile and for micro cracks. If the cracks appear after some time then it is probably due to some extent from average power rather than the shock from a single shot. How clean is the focus?

[macona]

I *think* I might have a couple slides around here someplace. Not sure though.

It is not the shock from a single pulse, the thing runs at about a 10khz rep rate.

The focus is pretty clean, these Lightwaves have pretty good beam quality to start with, though ESI did use a spatial filter to get a "flat top" beam as well as a beam expander to get the spot size down.

[mixedgas]

I'm suddenly very interested in Glass ablation. I got my hands on three 6800s that are in excellent shape. However the mirrors I bought are too thick, even though they are speced for 6800, they are actually for 6800HP. I'm past the 90 day return date, so I'm stuck.

My choices are dremel tool out the slot in the galvo shaft or thin the mirrors. Or fork over a small fortune for better mirrors. I'd rather thin the mirrors just at the shaft location. Macona, I think you have a possible side business thinning mirrors here. If you can hit a tight enough focus, I can see possibilities.

Oven annealing the small glass parts before cutting is easy, and will reduce the breakage.

I know more then one person who is annoyed that Chinese mirrors are very thick compared to Cambridge mirrors.

May be a wild goose chase, may be worth persuing.

See: http://www.potomac-laser.com/category/gallery/glass/

Steve