Why is there a temperature gradient in lasers causing thermal lensing?

[DariusP]

Hello, I wanted to ask about thermal lensing. Thermal lensing is when a temperature gradient from the center of the medium to the outermost part of the medium is created. This causes a varying refractive index throughout the medium.

What I want to ask is: Why is the core of the rod(or any other active medium) always hotter and the outermost part - cooler?

 

[sophiecentaur]

Why is the core of the rod(or any other active medium) always hotter and the outermost part - cooler?

Same reason as the penguins at the centre of a colony in a storm are warmer than those on the outside (or why there is a temperature gradient across any cooling body). Each part of the laser is transferring the same amount of input power per unit volume into heat, the outer parts have a higher net loss than the inner parts so the equilibrium temperatures will be different.

 

[anorlunda]

penguins at the centre of a colony

Thanks, that's a marvelous metaphor.

Water always flows downhill. Heat always flows from warmer to colder. Consider just 3 penguins; center, middle, outer. For heat to flow, temperature at center must be hotter than middle, and temperature at middle must be hotter than outer.

 

[sophiecentaur]

Thanks, that's a marvelous metaphor.

Cheers. I saw some Emperor Penguins on the TV last night and the metaphor just thrust itself into my mind this morning.

 

[DariusP]

Same reason as the penguins at the centre of a colony in a storm are warmer than those on the outside (or why there is a temperature gradient across any cooling body). Each part of the laser is transferring the same amount of input power per unit volume into heat, the outer parts have a higher net loss than the inner parts so the equilibrium temperatures will be different.

So simple now when I think about it (net gains and net losses). Thanks.