Why laser output power decreases with increasing temperature

[DariusP]

Why laser output power decreases with increasing temperature? By increasing temperature, I mean increasing temperature of the active medium.

I think that thermal lensing alone should not have effect on output power? Just different focusing (focal length).

What happens with energy levels in materials?

 

[DariusP]

Also, could I get an explanation as to why when we introduce a dopant (Neodymium (Nd) for example) to a material (Yttrium aluminium garnet (YAG) for example) then the energy levels "widen"? I know I've read this but I'm not sure if I understand it.