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I know in a 3-level laser system for example, electrons get excited from the ground level to the pump level from which they spontaneously decay quickly to the upper laser level. The upper laser level has a long spontaneous lifetime, so the atoms tend to accumulate there, creating a population inversion between that level and the ground state. Then lasing takes place by stimulated emission of photons by de-exciting electrons from the upper laser level to the ground level. My question is how stimulated emission started if initially there was no photon having an energy equals the difference between the upper laser level and the ground level? Was this photon found by spontaneous decay of some electrons from the upper laser level to the ground level?

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The electrons will eventually spontaneously emit photons, which in turn can stimulate the emission of further photons. The spontaneously emitted photons are emitted in all directions, however, if these spontaneously emitted photons are emitted along the laser cavity, the laser will start to lase.

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  • $\begingroup$ Can you please explain a small question. As you said, the spontaneously emitted photon are emitted in all directions, so they are incoherent. These incoherent photons will produce coherent photons due to stimulated emission. So photon A produced from spontaneous emission will form a number of coherent photon to itself due to stimulated emission, however photon B which is not coherent to photon A formed from spontaneous emission will produce another set of coherent photons via stimulated emissions. However the incoherence still remains. How do we form a laser then? $\endgroup$
    – user471651
    Aug 16, 2023 at 4:55

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