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I Thermal lensing in lasers

  1. Dec 1, 2018 #1
    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?
  2. jcsd
  3. Dec 2, 2018 #2


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    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.
  4. Dec 2, 2018 #3


    Staff: Mentor

    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.
  5. Dec 2, 2018 #4


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    Cheers. I saw some Emperor Penguins on the TV last night and the metaphor just thrust itself into my mind this morning.
  6. Dec 2, 2018 #5
    So simple now when I think about it (net gains and net losses). Thanks.
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