[htgb]

I know things kind of derailed downthread from here, but I just have to add that your claim is either confusing or simply untrue. I visited a manufacturer of industrial laser welds [1], and they use 16 kWh lasers over reasonably small fibers. I don't know the exact dimensions, but the overall cable looked like… a reasonable cable. Somewhere like 10 to 25 mm perhaps?

[1] https://www.permanova.se/en/our-products/laser-source/

[pi-e-sigma]

The cables look reasonably thin because they are water-cooled. That means that the power loss in the cables is so great that without active cooling the fiber would just melt. Take a look at this page https://www.photontec-berlin.com/power_delivery_fiber_cable.... which contains actual specs of the cables, for instance https://www.photontec-berlin.com/pdf/ld80.pdf or https://www.photontec-berlin.com/pdf/ld80-water.pdf