Sound amplification by stimulated emission of radiation mechanism into the active medium is the existence of superlattice layers that reflect the phonons back and force them to bounce repeatedly to amplify sound. Therefore, to proceed to an understanding of a SASER design we need to imagine it in analogy with a laser device. In a laser, the active medium is placed between two mirror surfaces (reflectors)of a Fabry–Pérot interferometer. A spontaneously emitted photon inside this interferometer can force excited atoms to decay a photon of same frequency, same momentum, same polarization and same phase. Because the momentum (as a vector) of the photon is nearly parallel to
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"