I think your laser was 40mW. A 40W laser can engrave steel.

Judging by the fact that they mentioned being in grad school at the time and that the laser was infrared, I imagine engraving steel isn't too far off from what they were using it for.

But they mentioned it would slowly burn black painted objects.

It's all about absorption spectrum. 40W at 10600nm will burn clean through a few mm of wood but could only maybe lightly etch brass and be utterly useless on aluminum. 40W of energy is being delivered, what happens to the thing it's being delivered to is entirely dependent on absorptive potential.

Agree on the 3 orders of magnitude difference. 40W soldering iron will inflate post-it notes, 40W laser steel/stone engraving/cutting area.

Depends highly on wavelength and pulse length. IIRC the vast majority of laser engraving lasers are pulsed (the cheaper ones are probably qswitched) so 40W actually gives you peak powers much higher than 40W.

Igniting something is actually quite different from cutting or engraving. Lasers are often so good at cutting because they don't deliver enough energy to set things on fire, but enough inatantaneous intensity to rip molecules apart (have a look at comparisons between femtosecond and nanosecond laser cutting for example).

Beg your pardon, but I think I would know (PhD in experimental AMO physics).

It was one of these: https://www.ipgphotonics.com/en/products/lasers/low-power-cw...

Guy's name is literally rubidium!

Could a fiber break in the wall ignite the sheath? What if the Sheath broke as well and it is up against the cardboard backing of drywall? Worst case, cellulose insulation?

Cellulose insulation is in fact very flame retardent. But to your question yes if the broken fibre was stuck against something flammable it could slowly ignite it.