Full disclosure, I don't run CNC lasers for a living anymore. But I did for about ten years.

The laser is generated in a cabinet the size of a large refrigerator. Inside that cabinet is a bunch of stuff, but what we are concerned with today are the tubes and turbine. There are glass tubes with mirrors at each end, and they are filled with a mixture of helium, CO2, and nitrogen. These tubes have an electrode in the side of them where high-voltage DC (around 40k volts) is used to "pump" the laser. Some lasers are RF pumped which is nice, because you don't have to put an electrode through the tube, and they are less prone to leaking. In addition to the laser tubes, you have at least one turbine circulating the laser gas through a heat exchanger, because lasers are only 10%-15% efficient and the extra heat has to go somewhere. There will be a chiller, a machine that makes cold water, which circulates water through the heat exchanger and the mirrors.

Once you have that juice being pumped into the laser gas, the tubes will look light purple or pink. But the beam itself is invisible, in the infrared spectrum. There is typically a small red laser, like a laser pointer, which is aimed through the optics to show where the beam is pointed.

To cut metal, you need a cutting gas in addition to the laser. For carbon steel, low-pressure oxygen is used, around 35-60 torr. (so .04 to .05 bar) To cut the steel, the laser is focused on the top surface of the material, and a nozzle is held about .035" (~0.9mm) above the material. The laser pulses at first, say around 2200 watts and 1/2 Hz, for a second if the material is more than 9mm or so thick. Once the laser has pierced the metal, the beam will switch to either continuous wave (full "on") at 2000-2500 watts, or it will run at a frequency which can be chosen by the operator, and a duty cycle which the operator also chooses. In this case the beam will look like it's "on" but it's really flashing too fast to see. While the laser is running, the aforementioned low pressure oxygen is blown at the hot spot it makes, which burns the steel out of the cut.

If you are cutting aluminum or stainless steel, you need nitrogen instead of oxygen, and it needs to be higher pressure. Like 120 psi/8.2 bar. And you have to use more power, because there isn't oxygen to help with the cutting, and because aluminum is a good heat conductor. So you run at 3600-4000 watts, and this is important, your focal point needs to be about 2/3 of the way through the material. This produces a cut that is shiny and fairly smooth on stainless, and fairly neat and clean on aluminum. A good machine with clean optics can cut steel and stainless steel with no burrs, and aluminum with a slight burr which can be easily knocked off with a file.

A 4kw laser can handle carbon steel 3/4"/19mm thick, and stainless or aluminum 1/2"/13mm thick. Speeds vary, but I could generally get any machine to cut 1/4" carbon steel (6mm) at 90-120 inches per minute, which is 2286-3050 mm per minute.

And now, everywhere I go, I spot bad laser cuts on stuff. Nothing like going to the gym and seeing focus lines in the equipment.