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LASERs: Why is population inversion required for amplification?

  1. Feb 17, 2016 #1

    I'm doing a module on quantum optics and lasers. Most of the texts and online source I have read simply state that 'population inversion is essential for amplification in laser operation'.

    My question is, why? Amplification/Gain is essentially the result of stimulated emission.
    Can't stimulated emission occur without population inversion?

    Wouldn't a population of atoms in a higher state that is equal to or less than 50% (but still sufficient) be enough to have a high enough chance of stimulated emission if an adequate photon comes along?
    Theoretically, couldn't one atom in a higher state be enough to produce stimulated emission if the correct photon interacts with that atom?

    I feel I might not be understanding something about the rate of absorption (for ground state atoms to reach a meta-stable higher energy level) and the rate of spontaneous decay back to ground state.

    Could someone please clarify this?
    Thanks for your time.

  2. jcsd
  3. Feb 17, 2016 #2


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    Without population inversion, you have more absorption than stimulated emission, so your gain is less than one.
  4. Feb 18, 2016 #3


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    Yes, it's enough. But how would you bring this atom to the excited state if not by absorption?
  5. Feb 22, 2016 #4


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    There are actually a number of ways to produce a population inversion. The first laser, ruby laser used optical pumping and three levels. The dopant in ruby was pumped to a highest level, which decayed to a second level creating an inversion between the second level and the ground level.
    He-Ne laser the energy comes from an electrical discharge which excites He atoms that transfer energy to Ne atoms creating inversion there.
    Diode laser use carrier injection in a p-n junction to create inversion between electrons in the conduction and valence band. There are also chemical lasers a a few others.
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