Strange laser alignment issue

[Mike Lysov]

Hi guys,


I have spent many hours both alone and with a technician trying to align my new laser but no matter what we have done one corner is very bad.
It is the top left corner where beams(I have the laser with two tubes) are traveling the shortest distance from the tubes.

And the other strange thing that the top left corner is the one with the minimum difference in beams paths. I used a special alignment tool supplied with the laser. It is used instead of lenses and it has a cross bar to show where beams will be hitting the lens when it is installed. At the top left corner the thermal paper under the crossbar shows much better result in this corner than anywhere else. So they are combined at their best at this corner and still the kerf at this corner is very thick.

Laser bed leveling is checked and seems to be perfect and before cutting I am focusing at the point where cutting will be done to make sure I have perfect distance.

See attached image. It has been done on 16mm thick MDF.

It shows cutting a square at three points of a 1300x900mm bed. The kerf thickness is not consistent all over the bed but at least everywhere except some area of 250x150mm size at the top left corner I am not getting the kerf thin enough to cut through.

I have run out of the ideas how to fix it. The factory at this point is not helpful as they just not read my comments properly and ask me to check and do things that I have done already.
I do not know, is there some science behind combined laser beams working differently at their shortest path comparing with the longer path beams traveling?

[Scott Shepherd]

Mike, please let us know what brand/model laser you have so we can help you.

[Rodne Gold]

It might have something to do with how the beams are combined..can you post pics of the laser tubes and the combining mechanism....

[Mike Lysov]

I have attached a diagram with mirrors before the brewster window. The mirrors 1(near field) and 2(far field) are used to align the beam path for the tube B. The 3rd one is fixed and cannot be adjusted. The 4th mirro is used to align combined beams path along Y axis. The other two mirrors are after the brewster window and are used to align combined beam path along X and Z axises. The laser came with two cross bars tools. One is used in front of the mirror # 5(not shown on the picture) to make sure path is aligned along Y and then the same tool is used to align X axis when placed in front of the 6th mirror(not shown on the picture) . And the other tool is used on a lens assembly instead of the lens to align path along Z axis.

The tubes polarization was done by the techician when he was installing the second tube. He did it by fixing one tube in place and rotating the second one until maxiumum output was achived on the laser power meter that he brought with him. He just followed instruction from the factory.


I do not think anybody here on SWC owns any laser from this factory but it is LCW 280D model made by LaserLife. It's taiwanese company that sells most of their lasers to Japan.

[Rodne Gold]

Mike , that looks mucho complicated...have you tried just disabling one laser and checking how it works and then disabling the other and doing so again , it might give you a pointer as to which laser is "at fault"...

[George Carlson]

My guess is that the tubes are not is the same plane. The mirrors can only compensate if the beams are slightly out of parallel. But if both tube are not dead on level, the mirrors can only combine the beams at a single point (where the two planes intersect).

[Mike Lysov]

I have tried cutting with one tube at a time. The results are the same and in both cases the kerf is quite thick. The only difference than one tube makes a cut with a kerf slightly thinner than the other. The difference however is minimal and hardly noticeable. Based on that I do not think that the problem is somehow linked with the tubes not being on the same plane.

The one thing which I have not checked yet is that the beams at this corner are not going through the nozzle hole straight. They may hit the nozzle inside and are just being reflected to the nozzle hole. The manual the laser came with mentions it can happen when it describes how to align the nozzle.

That's still may not be the cause of my problem because I have actually checked the beam path along the Z axis and it seems to go almost through the centre of the lens.

[Rodne Gold]

Is the table dead level to the nozzle at all points , you can check by having the nozzle a teeny bit above the table and moving the nozzle around and checking that the gap is constant at all points...

[Mike Lysov]

It is not perfect but it is less than 0.5mm difference. It cannot cause the problem anyway because I focus in the middle of the 130x130mm square before I try to cut it at the top left corner. I have also tried to set up focus distance both shorter and longer than it should be but it does not make any difference.

[Mike Lysov]

I have removed the nozzle and tried to cut without it but it does not fix the problem.
Now I am back to the point where I have no ideas what's wrong with this corner.

[Mike Lysov]

Does anybody know whether it is important to keep the beam centered along one axis or as long as it hits the same spot close to the center both at near and at far point of an axis centering it exactly in the middle it is not important? Or to rephrase, should the beam to go always through the center of the mirrors?

I have another laser(Spirit GX) and as far as I remember its manual says that the beam should be centered.

The manual for the new laser does not mention centering the beam and it describes Y axis alignment at near and far field as

"Compare the two patterns. If they are not consistent and the same direction, adjust screw of #1 mirror assembly to make Y-axis far point be consistent and same direction with Y-axis near point."
(X axis alignment is actually described in the manual in the same way)

I did not get at the beginning what this extra detail in bold meant so based on my experience with Spirit GX I tried to center the beam. Pictures in the manual with burn marks through the crossbar actually show the beam centered and I thought centering was important.

It was hard to do and I tried to confirm it with a tech guy who works for this laser dealer company and has a PHD in Physics. He explained that I should not try to center the beam and all I need to do is to align its path is a such way that it hits exactly the same spot on the mirror at near and far point of one axis.

It makes sense to me now in case mirrors used in lasers are flat.

[Vladimir Suvorin]

Mike, all galvo heads have programmed delays in them of multiple types. For example, there's delay at the each node of polyline, because mirrors need some time to stabilise on ther new course. In your case there seems to be a problem with starting delay, where mirror sits and waits for laser beam to reach its full power. Or there can be one of a mirror motors at fault, please try to draw horisontal and vertical lines separately, to see.

[Mike Lysov]

Vladimir, I am not sure why you assume that I have a galvo head.
It is just a regular head for a regular laser cutter. The only difference between it and my other laser that this one comes with two tubes and I cannot align it.

[Kalle Pihlajasaari]

I hope you have managed to sort out your problem.

Just for interest and for others I thought I would add my few cents worth on the beam combiner and your alignment issue.

Firstly once the beams leave the Brewster window they can not be brought closer together by any later mirror or lens. At this point they must be exactly coincident and parallel if you want them to focus on the same spot.

The tube mount and combiner fixture is no doubt made so that the two tubes will mount with beams parallel and at the same height, correcting it with mirrors 1 and 2 will not likely succeed and they are likely set at the factory with a reversed beam to make sure they match the factory aligned tube mounts.

If it is possible I would try to inspect the beam parallelism at a great distance (across the room if you are brave) as they leave the Brewster window (or the first mirror after it). If the two beams diverge at all anything you do later down the beam will be frustrating.

The Brewster window performs the beam combining task but can only do this with two carefully cross polarised beams. One polarisation wants to pass through and the other wants to reflect. Peak combined power is when the polarisations orient correctly to the window. Polarised laser tubes often incorporate a similar Brewster plate or exit window to preferentially amplify and pass light polarised in one plane.

Now if the exit beams are not parallel to the tube mounts the two beams (in the combined beam) may diverge as you are tuning the beam for best polarisation, there is little you can do for this unless you can adjust the tube mountings somehow and best left to the factory. However at a cost to total power you may be able to turn one of both tubes slightly (wasted power will be dumped at the place the thermal paper is placed in the test position) to find a sweet spot where the two beams are parallel and as close as possible to a common axis. This may allow you to achieve a consistent (perhaps optimal) focal spot size at all distances.

In your case if the beam quality/size/shape is changing depending on the distance from the source (as you move further across the bed) and all other adjustments are sound with the beam centred in the lens all the time and the bed is held evenly at focal distance then you likely have a problem with beam divergence or convergence. The individual beams are highly unlikely to be a problem in a functioning long tube laser so the combined beam seems to be a very likely candidate.

Please note that I am not a specialist, just widely read with an interest in optics.

Kalle
--
Johannesburg, South Africa

Working on my first CO2 laser rebuild.

[Dave Sheldrake]

In your case if the beam quality/size/shape is changing depending on the distance from the source (as you move further across the bed) and all other adjustments are sound with the beam centred in the lens all the time and the bed is held evenly at focal distance then you likely have a problem with beam divergence or convergence. The individual beams are highly unlikely to be a problem in a functioning long tube laser so the combined beam seems to be a very likely candidate.


Please note that I am not a specialist, just widely read with an interest in optics.

Maybe not a specialist Kalle but your explanation is spot on.

I'd agree convergence / divergence after the Brewster window is the problem, the entire system seems to be a very "mechanical" way to go about stacking with rotation of the tube being responsible for cross polarisation. I recall some time ago a few of the Chinese manufacturers tried this method then stopped as adjustment was close to impossible for end users without specialist equipment. I think WuHan are now using U tubes to achieve higher powers in shorter tubes rather than trying to phase stack beams.

Reverse optics would probably be the only way to get both tubes aligned properly as there is a distinct possibility that the beams from each tube are not concentric to the tube bodies so making central mount alignment pointless.


He did it by fixing one tube in place and rotating the second one until maxiumum output was achived on the laser power meter that he brought with him

Effective to a point but a little "off the wall" way of doing it when there is measuring equipment available that will do the job accurately.

cheers

Dave

[Mike Lysov]

Thank you guys.

I should have updated this thread that the problem has been fixed. I have just been trying to do as much tests as possible to make sure it has been fixed before I can be sure and report it's gone.

All these problems came for some dirt(smoke residue) accumulated on a exit glass window of the tube A.
Strangely this dirt had no affect on the beam traveling longer distances to all distant parts of the bed. However at the top left corner where the distance of laser path was the shortest it did affect the quality of the beam. It seemed the beam from the tube A was even split by this residue and it was noticeable on thermal paper when I test-fired the tube A only at the top left corner. That's actually how I have found out about this dirt.
This dirt accumulated on the glass window from testing the tube B firing only while the tube A beam was stopped by placing a piece of a sanding sheet for my palm sander on its way just a few mm away from the tube exit.
Unfortunately that sheet was the only thing I could find to cover the tube A exit as there is no much room to use anything thicker and not reflective. When the tube A was firing into that sheet(back of the sheet actually as the front is for sanding and may reflect the beam back) there was some smoke from the sheet burn that settled on the glass window of the tube exit. I was very lucky that I found all this dirt before that glass could start cracking anytime. It has taken me a while to remove all dirt with alcohol because there is very limited access to it. But it seems quite clean now. The top left corner works now and I can cut there at the same speed as everywhere else on the bed.
However the kerf still looks a bit thicker at the top left corner than everywhere else. I will try to find a way to inspect the window to see if there is still some dirt left on it but I am quite sure it can come from the beam not aligned at its best.