You’re looking at how CO₂ laser tubes behave at less than full power. The nifty little peaks you’re seeing correspond to tube current spikes, like these at 10% PWM from a claimed 60 W tube:
The regular edge ridges may be due to speed variations from the belt teeth passing over a smooth idler pulley. St. Sadler has a whole rack-and-pinon drive conversion project that seems to improve the situation, although finding it amid hundreds of hours of videos lies beyond my enthusiasm.
This is his creation called the Russ Spec machine, that used to be available from Cloud Ray… It has some neat updates, but most of his creations are on this machine. He goes through them rather quickly. It’s a good overview.
The variations would be equal to your belts teeth if this is curtains, as explained by Russ here about 9:17 into the video…
