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!
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