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Source of the first Photon in the LASER

  1. Aug 13, 2013 #1
    what is the source of the first photon in the laser ?
    i understand there is a burst of light or electricity that excites up the medium , but what is the source of the first photon ? is it the light source or is it one atom of the medium itself releasing photon as it gets de-excited * if thats even a word * ?
    also i would really like to know in layman terms why does an excited atom release a photon of the same wavelength of a photon that passes near it without actually interacting with it or absorbing it
  2. jcsd
  3. Aug 13, 2013 #2


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    You always have spontaneous emission in your laser transition, so getting a "first" photon is not an issue.

    There is no way to absorb it, this does not lead to an allowed energy.
    Think of light as an alternating electromagnetic field. It "shakes" the electron - and sometimes, this is sufficient for the electron to release the energy it had before, and it emits an additional photon, synchronized with that "shaking".
    That is not an accurate description of the quantum-mechanical process, but it is easier to understand than that.
  4. Aug 13, 2013 #3
    can you please describe alternating ?
  5. Aug 13, 2013 #4


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  6. Aug 14, 2013 #5
    Actually the first photon in highly stabilized lasers comes from a seed laser also, although spontaneous emission is the most common technique, but often to increase the monochromaticity, directionality seed lasers are used.
    Yes de-excited is a common word used in this regard.
    The question you are asking is how does stimulated emission occur?

    these are taken from this forum only I guess it will help you

    Quote by Firestorm000
    how does the incoming photon control the emission of the second photon?
    Think of it as a driven oscillation. The incident EM field causes the electric dipoles to oscillate at the same frequency of the incident field. This, in turn, causes the dipoles to emit at the same frequency, direction, phase and polarisation of the incident field.

    Quote by Firestorm000
    Does the first photon have to be absorbed by the atom, or only get "near" it?
    As the photon is already excited, absorption will not occur. The photon needs to get near it in the sense that the atom needs to interact with the EM field that the photon is associated with.

    Quote by Firestorm000
    Can Stimulated Emission only happen when the incoming photon has exactly the same energy as the difference in potential energy between the current and lower electron shell?
    Yes, after all, it is still emission.

    this is the thread link https://www.physicsforums.com/showthread.php?t=522919
  7. Aug 14, 2013 #6
    okay i get it , thanks alot
  8. Aug 15, 2013 #7
    B4ssHunter, Happy That I could be useful. On a lighter note, you could just click the thanks too on my reply, for giving a thanks!!!!
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