I understand that when a photon of specific properties (phase, wavelength..) comes near an excited atom with sufficient energy, the atom will most probably release a photon that has the same properties as the first photon and will fall to a lower energy state. I understand this is how light is amplified in a laser device after population inversion is achieved. I just don't understand one thing. Where does the photon that causes stimulated emission in the device come from in the first place? I tried to think of an explanation taking spontaneous emissions as a factor and using brewster's window for polarization but that seemed really unintuitve for some reason. Is there an external entity that increases the probability of the spontaneously emitted photons to be in a specific phase? It would be a great help if someone could explain this. Thanks!


1 Answer 1


Every laser medium as far as I know supports not just stimulated emission, but also spontaneous emission. When you start pumping the gain medium to its inverted state, spontaneous emission will happen naturally. Photons generated by spontaneous emission will then begin to trigger stimulated emission.

These photons don't have to be in any particular phase to produce stimulated emission. The stimulated emission photons produced will naturally match phase with whichever photon triggers the emission.

They do typically have to be at some particular frequency/wavelength in order to match the resonant frequency of the laser cavity. Simply put, the number of spontaneous emission photons will typically be enough to produce what appears as a continuous emission spectrum, and so some of them will be at a frequency close enough to the cavity resonance, and trigger stimulated emission at that frequency, so that a self-sustaining oscillation can begin.

  • $\begingroup$ Would it be correct to say that "naturally" in the following phrase, "The stimulated emission photons produced will naturally match phase with whichever photon triggers the emission", actually refers to the Bose-Einstein distribution for bosons? If not, could you please clarify what "naturally" actually implies here? $\endgroup$
    – safesphere
    Jan 10, 2018 at 2:33
  • $\begingroup$ I mean that it's simply the nature of stimulated emission that the emitted photon is in phase with the original photon. $\endgroup$
    – The Photon
    Jan 10, 2018 at 3:09
  • $\begingroup$ Understood. However, I believe, this nature is not an unexplained experimental fact, but is rooted in the BE distribution and does not hold for fermions. For example, there is no way of creating a coherrent strream of fermions, unless they are paired into bosons like in superconductivity. $\endgroup$
    – safesphere
    Jan 10, 2018 at 3:14
  • $\begingroup$ @ThePhoton Is it possible for the photons to travel in a direction different than that of the cavity? I mean if laser beam exits in horizontial direction can the photons from stimulated emission travel in the vertical direction? $\endgroup$ May 13, 2020 at 16:41
  • $\begingroup$ @AntoniosSarikas, I believe there's some subtlety to the answer to that question, beyond my ability to explain. From an engineering point of view we often assume that stimulated emission photons will enter the same modes as the triggering photon. But I don't know what physics justifies that assumption. $\endgroup$
    – The Photon
    May 13, 2020 at 16:56

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