A laser combines pumping, an amplifying medium and optical feedback. Stimulated emission adds photons to an occupied optical mode; net amplification requires population inversion between the relevant levels after accounting for their degeneracies. Pumping through additional levels and rapid lower-level relaxation can sustain inversion; a closed two-level system under steady incoherent resonant pumping cannot sustain it.
An optical resonator selects modes and feeds light back through the gain medium. For intensity gain coefficient , internal intensity loss , length and mirror intensity reflectivities , the round-trip laser threshold condition is
(1)
Above threshold, saturation limits gain and a partially transmitting mirror provides output. Typical properties are strong directionality, substantial coherence and narrow bandwidth; multiple modes and pulsed operation are possible, so perfect monochromaticity is not universal.[ref]
Figure 1.
Pumping sustains gain while a resonator supplies feedback and output coupling
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