Beam splitting after extensive alignment check

[omgwtfkthx]

Searched through the forum about this beam splitting/halo/ghosting effect but all answers are basically check your alignment.

tldr: why is my laser doing a halo cut when my alignment seems okay?

I'm building my own laser and have been trying to tackle what I assume is an alignment issue for a while now. I sometimes get a double cut halo as if the laser beam is splitting and I can't find the cause. The first image shows that after doing a basic mirror alignment my beam is mostly hitting the same spot at all four corners of my gantry. I also checked the vertical alignment on the last mirror. I used a laser level to ensure my gantry has an even height across both axis. I used the same laser level to ensure the tube itself is also properly horizontal. I used a speed square to ensure the gantry and laser tube are spaced correctly. Checked the lens for deformations but found nothing. Secured mirrors to make sure they aren't vibrating. Ensured the cutting bed was level.

The thing is, I've repeated these steps over a dozen times, aligned/reset/aligned again, fixing things, making my four corner alignment tighter along the way. But this halo cut won't go away. What am I missing?

Removing the nozzle shows the issue more clearly.

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[grawley]

Searched through the forum about this beam splitting/halo/ghosting effect but all answers are basically check your alignment.

tldr: why is my laser doing a halo cut when my alignment seems okay?

I'm building my own laser and have been trying to tackle what I assume is an alignment issue for a while now. I sometimes get a double cut halo as if the laser beam is splitting and I can't find the cause. The first image shows that after doing a basic mirror alignment my beam is mostly hitting the same spot at all four corners of my gantry. I also checked the vertical alignment on the last mirror. I used a laser level to ensure my gantry has an even height across both axis. I used the same laser level to ensure the tube itself is also properly horizontal. I used a speed square to ensure the gantry and laser tube are spaced correctly. Checked the lens for deformations but found nothing. Secured mirrors to make sure they aren't vibrating. Ensured the cutting bed was level.

The thing is, I've repeated these steps over a dozen times, aligned/reset/aligned again, fixing things, making my four corner alignment tighter along the way. But this halo cut won't go away. What am I missing?

Removing the nozzle shows the issue more clearly.

Since you have done the standard checking and alignment, you can progress to other checking, for example:
- Prior to the first mirror, perform a mode burn into acrylic to see the unfocused laser beam shape. It should be gaussian, or like a hill with the top of the hill the deepest part. If you get an inverted hill, or other shape, then likely the gases are depleted and you need a new tube (note, this can be caused by running the laser at max power excessively). To see a mode burn example, look at my youtube demo here. . In the, see the example finished mode burn from a RECI glass tube laser.

- Examine the output coupler optic as shown in the photo above. If cleaning is required try to use a hand-operated bulb air blower to blow dust off it, or check youtube for cleaning this very sensitive optic (it's not like cleaning mirrors and focus lenses, it is much more delicate, so take care when cleaning).

- If the gases are depleted, then check your power supply output amps to make sure that it is properly calibrated to your glass tube. Use a CO2 laser power meter probe to check power, to make sure that you are not prematurely burning up the laser tube. See my instructable here. How to Test and Set a Chinese (China) Made CO2 Laser

[omgwtfkthx]

Since you have done the standard checking and alignment, you can progress to other checking, for example:
- Prior to the first mirror, perform a mode burn into acrylic to see the unfocused laser beam shape.
- Examine the output coupler optic as shown in the photo above.

Thanks for taking the time. I did a spot check using acrylic and the beam seems pretty even to me. I've attached a photo. The coupler looks good with no obvious dirt/damage. I have a mA meter on the machine and never went over the tube's rated power.

I've also attached photos of 100mm square cuts that I did at each corner of the bed. Since it happens more on one side compared to the other I'm still inclined to think I'm somehow messing up my alignment. But since it happens more on the top of the bed (closer to the tube), I'm left scratching my head.

[grawley]

A common method to check how well the laser beam is aligned to the final mirror is move the focus assembly to one side of the laser machine then apply masking tape then momentarily power on the laser enough to mark the beam on the tape. Next, without removing the tape, move the focus assembly to the opposing side of the table (in this case where you have or have no halo effect) then mark the tape again. Next, determine if the amount of movement on the final beam bender is sufficient to cause the halo effect. As a side note: As this is an iterative process, and can be time consuming, I made a CO2 laser beam visualization tool to speed up the process, and that is a faster way to go.

In addition to the test above, you can repeat the same test below the focus lens with masking tape to see if the mirror in the final beam bender is moving the focus beam sufficiently to cause the halo effect. You can also remove the focus lens to run the test. In some cases, the focus lens can have a hairline internal crack that is visible, and causes the problem, so inspect the lens carefully under good lighting.

[omgwtfkthx]

A common method to check how well the laser beam is aligned to the final mirror is move the focus assembly to one side of the laser machine then apply masking tape then momentarily power on the laser enough to mark the beam on the tape. Next, without removing the tape, move the focus assembly to the opposing side of the table (in this case where you have or have no halo effect) then mark the tape again.

Just to make sure I'm not misunderstanding you, isn't this a standard alignment check on the x axis? I'll double check to make sure it's close enough to the center of the mirror, but I also want to make sure I get what you're asking.

In some cases, the focus lens can have a hairline internal crack that is visible, and causes the problem, so inspect the lens carefully under good lighting.

Nothing I could spot visually on the lens. I tried turning the lens 90 degrees to see if the halo shifted at all but no change.

[grawley]

Just to make sure I'm not misunderstanding you, isn't this a standard alignment check on the x axis? I'll double check to make sure it's close enough to the center of the mirror, but I also want to make sure I get what you're asking.

Yes - standard procedure

Nothing I could spot visually on the lens. I tried turning the lens 90 degrees to see if the halo shifted at all but no change.

It may not be visible. I had a customer in Brooklyn with a high power laser and nothing was visible on the lens, but changing the lens solved the problem.

[omgwtfkthx]

Since you have done the standard checking and alignment, you can progress to other checking...

Thank you for helping me brain storm but it turns out the problem was my alignment all along. My beam was hitting the center of the guide ring but not actually the center of the second mirror. I've attached some photos to help explain. Realigned the first and second mirror mounts and that solved the problem.

[grawley]

Thank you for helping me brain storm but it turns out the problem was my alignment all along. My beam was hitting the center of the guide ring but not actually the center of the second mirror. I've attached some photos to help explain. Realigned the first and second mirror mounts and that solved the problem.

By far, the number one reason for lost profit, machine downtime, shoddy laser cutting/engraving, ruined materials, head scratching beside the laser machine, is CO2 laser beam alignment problems. The CO2 laser beam visualization tool by Mahoney (that I make), helps to reduce the beam alignment issues. In summary, the path taken to solve the problem for the beam alignment, often requires checking the other components, and learn for next time in case they have an issue.