= Three-level laser
A <three-level laser> pumps emitters out of their <ground state> into a higher pump state, which rapidly relaxes into the upper lasing state. The lower lasing state is the <ground state> itself. <Photon absorption> on the lasing transition therefore competes with <stimulated emission> until enough emitters have been removed from the ground state. This is the <ground state> depletion problem implicit in <population inversion>.
The model below treats nondegenerate lasing <energy levels> with equal absorption and emission cross-sections. Its inversion condition is $N_2>N_1$. Real materials require the relevant sublevel populations, degeneracies and transition strengths; the simple half-population condition should not be imposed on every optical transition. Extra levels let the pump and the emitted <light> act on different transitions, avoiding the steady two-level saturation limit discussed in <population inversion>.
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