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

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Discussion Overview

The discussion revolves around the phenomenon of thermal lensing in lasers, specifically addressing the temperature gradient that occurs within the active medium of a laser. Participants explore the reasons behind the core of the medium being hotter than the outer regions, examining the implications of heat transfer and energy loss.

Discussion Character

  • Exploratory, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant describes thermal lensing as a result of a temperature gradient from the center to the outer part of the medium, leading to a varying refractive index.
  • Another participant draws an analogy to penguins in a colony, suggesting that the core is warmer due to differences in net heat loss, with the outer parts experiencing higher losses.
  • A further comment emphasizes the necessity of a temperature difference for heat flow, reiterating the analogy of penguins to illustrate the concept of heat transfer.
  • Participants express appreciation for the metaphor used to explain the temperature gradient, indicating a shared understanding of the concept.

Areas of Agreement / Disagreement

Participants generally agree on the analogy of penguins to explain the temperature gradient, but the discussion remains open regarding the detailed mechanisms of heat transfer and the implications for thermal lensing.

Contextual Notes

The discussion does not delve into specific mathematical models or detailed physical principles governing heat transfer, leaving some assumptions and mechanisms unexplored.

DariusP
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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?
 
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DariusP said:
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.
 
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sophiecentaur said:
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.
 
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anorlunda said:
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.
 
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sophiecentaur said:
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.
 

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