Agreed. A 40W 1550nm laser I used in grad school would burn post it notes readily and slowly burn black painted objects. It gave me a little burn once too (Was like touching a hot stove). 1.5W focused to 100 um would be a zinger, but at 4 mm would be not super dangerous thermally.

The main risk is you couldn’t see it so no blink reflex to help from even specular reflections.

Regardless, it would be reckless to expose this to people without eye protection.

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.

"it is probably highly divergent out of the fiber"

Those of us in the styropyro discord have found that to almost never be the case. WEAR EYE PROTECTION AT ALL TIMES.

What types of fibers are you talking about?

Single-mode, multi-mode, etc. All fibers. Let's use my LASER marker at work as an example. Divergence in that fiber happens AFTER two things happen - first the photons need to leave the fiber,then they have to cross-over their 'hip' (where the collimated beam is tightest) THEN they diverge.

That's how fiber LASER markers work. I'm the operator of them where I work (I do everything outside of SMT - x-rays, LASER marking, high-voltage testing, etc.) Every LASER coming out of that fiber hits focal point about 2 feet past the fiber, where it begins to diverge.

Do not trust a LASER coming out of a fiber to be unfocused or not have enough power density to blind you. Period.

That's fine safety guidance but on the facts this is not correct.

For single and multimode fibers if it has a smaller focal spot some distance the fiber than at its tip you are talking about an assembly that includes the fiber plus an optic.

This is unambiguously true for single mode fibers. Multi mode one will diverge less

I could see there being some exception for a strange fiber which has a multi-emitter output, was nanostructured, tapered, etc..

"For single and multimode fibers if it has a smaller focal spot some distance the fiber than at its tip you are talking about an assembly that includes the fiber plus an optic."

A properly-shaped and polished fiber tip is already the optic. That's literally how it's fed into the galvo in a fiber-marking LASER. I'm also responsible for the maintenance on all equipment in the facility so I've had those machines apart more than once.

When you say most people, are there in fact people that can see this at low intensity?

Most (all?) IR lasers leak visible light too. Look into any 1310nm SFP module and you'll see red light even though 1310nm is well outside the visible range.

And no, this won't burn your eyes out. Despite what the clipboard warriors claim, a laser that's designed to couple to fiber will not magically focus itself at the exact distance where your eyes are located. Instead the beam will diverge as if the 1mW laser was a 1mW LED.

I can see 780 nm (rubidium laser cooling light) specularly reflected from an index card at maybe 100 mW, if memory serves.