CO2 laser with four mirrors

You don’t lose much at all from quality gold-over-silicon mirrors. American Photonics claims 99.6%. That’s so trivial, don’t worry about it.

Molybdenum mirrors are listed as 98%. However, they are nearly indestructible due to high thermal conductivity. I like to use molybdenum on the #3 mirror, the one most likely to get dirty from smoke, at our makerspace because this machine is 225W and a dirty silicon mirror will get so hot it bubbles off the gold plating. And, being a makerspace, most users are casual and don’t inspect and clean so this does happen.

Molybdenum mirrors get hot in operation due to the losses, but they can absorb a lot of wattage without overheating the front face. It’s literally a polished, uncoated face of a solid molybdenum puck, and molybdenum doesn’t oxidize.

I did have some lenses explode from the power density. Like the beam bored a hole through the zinc selenide and the user reports it was cutting poorly… like, yeah, no lens. The #3 mirror was moly but had such a baked-on coating of brown crud I couldn’t clean it. But, the part of the surface I could get the brown crud to come off of appeared undamaged.

In the 4 mirror setup I have seen, it was an HPC*, I think, you keep mirror 2 next to mirror 1 and mirror 3 is on the ‘top’ rail, next to the tube, shorter beam path.

Set up is trickier with 4 mirrors but losses are minimal.

Benefit (theoretical?) is mainly that you have a much shorter unsupported rail so less vibration at higher speeds.

*almost identical to a K40 until you spotted the extra mirror.

more complexity, more 4 letter expletives during alignment, more dirty mirrors to clean…

‘What would it add?’

One mirror, a slightly longer alignment but, if you are running your tube along the long edge, a shorter unsupported axis.

As alignment is just a one step at a time process it will add just one simple step.
Never understood how complex many people make mirror alignment.