Optical Amplification
Under certain conditions, stimulated emission can provide a physical mechanism for optical amplification. An external source of energy stimulates atoms in the ground state to transition to the excited state, creating what is called a population inversion. When light of the appropriate frequency passes through the inverted medium, the photons stimulate the excited atoms to emit additional photons of the same frequency, phase, and direction, resulting in an amplification of the input intensity.
The population inversion, in units of atoms per cubic meter, is
where g1 and g2 are the degeneracies of energy levels 1 and 2, respectively.
Read more about this topic: Stimulated Emission
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