system
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human
How does the photon of specific phase that causes stimulated emission in a laser device arise?

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!
gpt
The first photon doesn't have to have a specific phase. Whatever it has determines the phase of the laser. In terms of direction and polarization: If it is not aligned with the laser cavity (or has the wrong polarization, if that is relevant), this chain of photons dies down quickly and another "first photon" will start the laser. Note that actual lasers do not emit *perfect* laser light. You can still get all sorts of weird effects in between.