K40 Laser Beam Focus Issues

Hi All,
My K40 was starting to reduce its cutting performance the last 2 days. I decided to do a beam alignment just in case things were a bit out of wack. I didn’t adjust anything yet, but instead of getting a nice fine pinhole when I press test I got what looks like 2 lips about 4mm wide.

The is even from tape on the first mirror next to the laser so the issue is before the mirrors (i.e. the laser tube).

Any ideas? Does the laser tube have an internal lens that may have gone astray or is it on the way out?

Thanks in advance.

Looks to me like the tube is on its way out.

This is typical of a tube that has shifted to a higher-order transverse mode. TEM01 is the most common mode to shift to; it’s sometimes called the “donut mode” and in theory is donut-shaped but in practice seems commonly to look like the two dots you see here. [Edit: TEM₀₁* (donut) is different from TEM₀₁, which looks like two dots.]

Poor cooling control can lead to this happening earlier, but as far as I can tell it’s a common failure mode for even well-cared-for tubes that are just used up.

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Thanks for the reply.
Not the answer I was after but spidey senses were saying this to be the result. Its not an old K40, having done less than 50 hours but they are made to a cost not quality I guess. The water temp has never exceeded 25 deg Celcius when cutting, but I might lower the water temp more with some freezer bricks in the water container.

25⁰ is probably too hot, even if you got a high-quality tube. Once the tube shifts mode, it doesn’t shift back; the damage has been done.

There’s a lot of good information in the K40 Laser: Getting Started category, worth reading even if you’ve already gotten started. :relaxed:

If the tube have the strongest possibility of failure, You should also have a look to the lens (& mirrors) for a possible crack, that happen to me as I reported many years ago.

Looks like that’s one of the first things @Mitch looked for! :slight_smile: