[Cathode]

I am building a CO2 laser CNC machine :drowning: and am looking for a mirror suitable for combing the invisible IR laser beam with visible light from a laser pointer. The idea is to pass the IR beam through the the back of the mirror, and to reflect the visible beam at a 45 degree angle off the shinny side, thereby creating a co-axial beam of both visible and invisible light. This is will serve two purposes: to help align my flying optics, and to indicate where my laser will hit the work surface during operation.

Does anyone know the proper name for such a mirror and where they can be procured? Is there another way to implement a co-axial beam?

[Laser.Tech]

You've got the right idea and terminology . The combiner is a plate made of zinc selenide coated to transmit the co2 beam thru with little loss while completely reflecting the red diode or HeNe beam coming in at 90º, therefore combining the two into one beam.
Just do a search for 'znse combiner' and you'll find several sources.

One note, if your on a tight budget ZnSe optics tend to be expensive. A common less expensive technique is to use a small regular mirror mounted to a solenoid actuated shutter that moves in and out of the beam path. The only difference in doing it this way is that you don't get the red dot while the laser is actually working (which is useless anyway!).

Laser.Tech

[Cathode]

Thanks, I did find several sources with that search The cheapest so far is around $45 plus shipping.

I was considering a mechnical solution, but concluded it would take some pretty tight tolerances to get the mirror to move into the exact postion under solinoid control. Sounds like an idea I should revist. Thanks.

[Laser.Tech]

Glad to help.
I would definately give the shutter idea serious thought. The tolerances shouldn't be any real problem, as long as the shutter has a good solid (repeatable) stop. Remember, the diode laser is only giving a 'close' indication of the beam path anyway, regardless of which method you choose. Either method will always have drift and inaccuracies, and initial setup will be also be the same either way.
Personally, I would go with shutter design. Aside from initial cost (low), if anything goes wrong, you lose a ten cent piece of glass instead of a very expensive optic ($45 was unbelievably cheap, if you do use a combiner, better grab it!).
Also, the shutter method is much SAFER! When your doing an allignment of your delivery system using the diode, you can feel secure that theres no high power beam present also (the shutter should be set up to block it).

If you would like to use the shutter method, I would be happy to post a drawing of a typical setup.

Laser.Tech

[Cathode]

Thanks for the info. A drawing of a practical shutter assembly would be helpful, please post it.

I have a few of my own design ideas for one, but would certainly like to see a tried and tested approach. We certainly don't want any failure modes that will crack into the laser tube or get stuck in front of the beam without warning. The simplest thing I can come up with is a pivoting lever that holds the laser diode itself (no mirror needed). The lever would be mounted in an assembely between the CO2 tube and the first reflection mirror. There would be some alignement screws on the diode mount, and an adjustable stop on the solinoid end of the lever. Gravity would pull the diode end of the lever out of the way when the solinoid is not energized, for fail safe operation. During alignment, the solinoid would energise to swing the diode into a postion co-axial with the CO2 tube. A micro switch would disable the laser when the lever is not in the down position. The whole assembly would be build as a module, so I could easily remove it and try different ideas.

This is what I am thinking now for a mechnicial solution. However, the beam combiner is slick, with no moving parts and is still very tempting. This is going to be a tough decision. I think the ZnSe optics are getting cheaper now because of the growing popularity of CNC laser machines. No moving parts is a big plus.

[Laser.Tech]

ricswika, attached is a basic drawing of a typical shutter assembly.
The shutter/mirror is mounted to a rotary solenoid that swings it into position when energized. The laser pointer is then mounted above on a pivot mount. It's not absolutely neccesary, but a polished piece of copper on the back side of the shutter and a hefty chunk of metal to act as a beam dump would make a good safety addition in case of any failure. And a limit switch should definately be mounted to detect when the shutter is open/closed, and tied in to the CO2 laser control.

You make a good point about the cost of the beam combiner, and its ease of use. It's just I know how pass-thru high power optics love to collect dust and film over time, eventually degrading the power and beam quality, so I tend to try to avoid them when possible. But if a suitable combiner is available for $45, thats the way I would go too. It would be a lot less work. You would just want to inspect/clean it on a regular schedule like the rest of your optics.

Out of curiosity, what kind of system are you building?

Laser.Tech

[Cathode]

I am building 24"x48" 60 watt system intended to cut 1/4 inch hardwood, balsa, depron, mylar and probably some veneers. I already did a CNC retrofit on a sherline mill and lathe setup, so making parts isn't a problem. I've come up with a fairly compact design, using common materials, with everything fitting inside of a 3.5'x5'x3' box. However, I'm a little worried about using flameable materials inside the box. I've got some left over ceramic tile to line the inside of the bed. I'm using aluminum drop ceiling grills for my 'honeycomb' bed. For etching thicker objects, I'm planning to adjust the table depth by varing the number of grills.

I'll use a dust collector to evacuate fumes, and a small compressor for an air jet. I'm worried the compressor will cause vibration, so I'm not sure how that will work out. I want the air jet to be strong enough to put out flames.

My laser is scheduled to arrive this Thrusday. :cheers: I should be test firing it this weekend.

I've won a small motorized beam expander/reducer on ebay. It should come next week. I don't know the focal length, but it was reportedly from a medical CO2 laser. I'm hoping there will be some markings on it that will help determine if it is suitable.

I want to put a flow switch on my coolant loop, but can't find anything reasonably priced.

Thanks again for the help. I've got a million more questions. I'll probably start a thread with my project log.

[ImanCarrot]

I've used a HeNe aligning laser along with a CO2 laser interferometer to align optics (similar to what you're doing).

You need a window aligned as a previous poster said so that the CO2 beam goes straigh through (material correctly specified previously as Zinc Selenide) and the aiming laser incident at 90 degrees to the CO2 beam and bouncing off the "front" face of the window.

Note that if the CO2 beam in converging or diverging at the window then it should have a small wedge manufactured in it to avoid astigmatism... not sure if this would matter for cutting lasers, but it does for interferometers.

The optics should be coated for transmission at 10.6um

Note that ZnSe is toxic, don't eat it

[Laser.Tech]

Sounds like your going to end up with a nice machine, I hope you post a log of your build, it would be interesting to see...

For your flow switch, nothing could be simpler (ignore those expensive purpose made ones...). If you have a resevoir, just place your return line pointing down into it. Then under the opening, put a small paddle mounted on a regular limit switch. Voila! instant cheap flow switch. If the flow isn't strong enough to push the paddle, use a small cup with drain hole to act as accumalator.

Any questions you have, don't be afraid to ask. Besides the usual, there's several laser experts on this forum who could help with those obscure technical questions...:rainfro:

Laser.Tech

[Cathode]

My tube did arrive today, but it is broken.

http://www.cnczone.com/forums/showthread.php?t=37333