Beam seems to change at various power

[Robertmr]

Hi all,

We're running into a strange issue with our diy lasercutter that has been running near perfectly for the past 2 years.

All of a sudden, our laser output seems to very depending on the power output. When we cut a line at 50% power and above, the line is nice and crisp. If we go below that 50% threshold, the line gets "wider".
The center of the wider line is still equally thin, but there is "ramping" on the edge of the line.
After noticing this, I also checked the "raw" output of the laser tube at different powers and noticed quite a difference in TEM modes. The >50% gives a nice TEM00, while everything below seems to give a TEM10.

Here are some images for clarification:

55.png (57.11 KiB) Viewed 2110 times

note: I think the 10% version is actually only looking ok because it's lacking the power to make the outer circle of the TEM10 visible. Anything below 10% doesn't give us any output at all.

54.png (66.73 KiB) Viewed 2110 times

note: TEM00 at 50% and TEM10 at anything below that.

Am I right to think the TEM10 is the culprit for the wider lines at lower powers? What could be the cause of this sudden change of TEM mode?
I'll try cleaning the tube output mirror, see if that changes anything. Could the laser PSU change anything? Software?

Tech specs:
AWC608 controller with LaserCad, all latest patches en software versions
100W Reci Laser (W4) from 2013.

Any help would be great!

[Manuelbus]

Hi,It is normal for Reci 100W or 130W. For those tube, it need 8~10mA to get fully activated mode (stable). If you drive the tube with less than 8mA, the beam is not stable and not solid. Plus, the beam spot size is much bigger than you can imagine. No surprise to get 15mm in diameter spot right before the laser head intake hole!

That is one of the reason we try not to use higher power laser tube for engraving because of higher output current is required. Also, the trigger delay is worst than a smaller tube like the 60W or so.

For a good LPSU, there are potentiometer (VR) built-in; one for 50% setup and another one is for 100% setup. By doing so, you may get better linearity power control. Not sure what kind of LPSU you have, you may want to test the output current at 10%,20%,30%..... 100%. Note, do not driver more than 30mA for Reci 100W (suggest 28mA is max)