The principle of operation of the laser is based on stimulated emission concept. In short, if a suitable energy photon hits the electron of an atom, in which the electron is in the excited state, there is a probability that the electron returns to the ground state. The photon emitted has the exact same phase of the photon that has stimulated. This appears to be an acquired principle, but I find no rigorous proof of why the two photons must be in phase with each other. A "heuristic" reasoning might be as follows. The electron should absorb the incident photon and jump to the higher level. Since there's a not higher level, you could say that the electron absorbs "virtually" the incident photon and then emit it instantly. At this point, just as instantly decay by emitting at the fundamental level coupled with the first the second photons. It is as if emits two photons simultaneously in which one is the exact copy of the other. Can this reasoning, although descriptive have a foundation.