II.

Principle of Operation

As in lasers, amplification of radiation in masers is obtained by stimulated emission. This occurs when a photon induces an excited atom or molecule to fall to a lower energy state while emitting a photon of the same frequency as the incoming photon. The emitted photon travels in the same direction and in phase with the incoming photon, which is not absorbed during the interaction. The amplitudes of the two waves add up, and amplification of the incoming wave has taken place. Masers make use of those transitions in molecules or crystals that correspond to the energies of microwave or radio frequencies.