During the scene in the office/control room of the lab, Colson (Dana) describes the laser to Mac after Mac asks the question "infrared or gas discharge" as "Gas, CO2, 10,000 watts". Colson is referring to a carbon dioxide laser, which can indeed set fire to things or cause very serious burns. Mac takes Colsons packet of smokes with the intention (unknown to the viewer at this time) of exposing said laser using cigarette smoke particles. CO2 lasers are in fact, deep infrared. No amount of smoke would expose the location of an ir laser
In the scene in the elevator shaft, Mac kicks the grate down the shaft into the deadly laser, producing a nice fireworks display. The resultant smoke from the grate exploding exposes the location of the laser, a pretty blue beam. Looks suspiciously like an argon gas laser. Its output power is probably a watt at most - just enough to light a cigarette (and then turned down to keep our Mac from being blinded by any reflections when he is "toying with it").
The laser would be visible almost as soon as Mac lights the first cigarette, yet he has to take 3 or 4 puffs on 4 cigarettes before the beams become visible. In fact I'd bet that elevator shaft isn't a clean room and the laser would be slightly visible all the time due to dust particles, not just when someone throws something into it, or fills the room with cigarette smoke.
A 10kW laser would burn through a cigarette in less than a second (and a small mirror like what Mac used later in the scene) in at most a few seconds. The strongest argon laser is a few 10's of watts. Even that power would do the same to the cigarette (but not the mirror). Macs vision would have been ruined when he lit the cigarette if it was a 10kW laser, and I hope the crew gave him laser glasses for that stunt, along with the initial catching of the beam with the binocular part, as any laser strong enough to light a cigarette is a serious eye hazard
. In fact even a half watt laser can set fire to paper, light cigarettes, burn holes in clothes, burst balloons and make you blind
in less than an instant.
You cannot disable a gas laser by sending the beam back to its source. In some cases it will make the laser even stronger if the mirror is perfectly aligned with the HR mirror at the other end of the laser tube.
I still love this episode, its the thing that got me into lasers in the first place.