Population inversion atoms in the ground and excited states are equal, the rate of stimulated emission is equal to the rate of absorption for a given radiation density. The critical detail of stimulated emission is that the induced photon has the same frequency and phase as the incident photon. In other words, the two photons are coherent. It is this property that allows optical amplification, and the production of a laser system. During the operation of a laser, all three light-matter interactions described above are taking place. Initially, atoms are energized from the ground state to the excited state by a process
Stimulated emission to the number of atoms in the lower state, "N". Einstein showed that the coefficient for this transition must be identical to that for stimulated emission: Thus absorption and stimulated emission are reverse processes proceeding at somewhat different rates. Another way of viewing this is to look at the "net" stimulated emission or absorption viewing it as a single process. The net rate of transitions from "E" to "E" due to this combined process can be found by adding their respective rates, given above: Thus a net power is released into the electric field equal to the photon energy "hν"