TEM mode questions

So, I’ve parsed through enough of the posts on TEM mode shift to diagnose the two-dot burn pattern i found coming out of a laser tube. I haven’t previously worked with this machine and so I don’t know if this tube was always firing in this shape, or if it was a recent development.

I generally see that the advice is something along the lines of “your tube is firing in TEM01, it is failing, you need to replace it.” That said, I have a few standing questions about the signifiance of a laser’s TEM mode that hopefully someone can help me out with.

  1. Are some tubes manufactured to shoot in higher TEM modes, or is any mode above TEM00 always a result of tube failure?

  2. Does a shift in TEM mode always result in a noticable loss of power, or is it still possible to align and focus these lasers effectively?

  3. I’ve noticed some users offhandedly mentioning that certain tubes, when fired at lower power (say, under 30%) will produce a beam at lower TEM modes. The discussion on this is sparse and doesn’t seem connected to any solution. Does anyone have more information on this phenomenon or a link to an expanded conversation about it?


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Hi David and welcome to the Forums!

To the best of my knowledge CO2 laser tubes are always manufactured to operate in TEM00. This is mainly because other TEM patterns result is a larger and distorted beam shape which is harder to focus. Also some TEMs have a lower power output. Usually changes in TEM occur at the end of a tubes lifetime and exacerbate an already lowered power output.

I’m not aware of any discussion about TEM modes as a function of applied tube power. But it’s certainly a possibility depending on the change of the tube parameters that has caused the TEM shift.

Again, to the best of my knowledge there is no practical way to recover a tube with a shifted TEM.


A source I use for laser theory:

CO2 laser Optics, CO2 replacement optics for industrial high power lasers by Ophir Optronics.

I never dug into the physics much but know that over temp can cause an irreversible shift in mode.
My crude understanding is that the TEM mode is achieved by tuning a combination of excitation, dimensions, (wavelength) and the optics.
When a fixed (not tunable) tube shifts modes it is a result of change in those opto/dimensional parameters such as may be caused by excessive heat.
I assumed it cannot be fixed because its not tune-able.


Thanks Ned,

I was mixed up about about where I was finding discussiong about power level - it was actually in a few different locations and not this forum. Here’s one instance on another forum.

The image the user there attaches to show the difference in laser shape between 10% and 30% isn’t totally convincing because the burn marks appear to be a similar diameter. I think the higher power beam could have just singed the material to the point where the TEM01 pattern was no longer recognizable?

I was recently given similar information from a customer support conversation, though I think my question and the company’s answer may have been suffering from a game of telephone (from me, to the distributor of the laser, to the US branch of its parent company, to the company in China and then back.) They wrote that a “tube normally fires the laser with a crescent burn at lower power percentages (10-30%) then it begins to create a circle burn as the beam combines better at higher power percentages.”

My suspicion is that the tube ultimately needs to be replaced, but I’ll run some test burns when I’m back at the laser in question and share what I find.

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I might buy that the lasers beam is “wonky” below 10% (4ma) because that is at/below the ionization level but not between 10 and 30%.