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How does stimulated emission create the coherent light of a laser?

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energy levels of a luminescent centre — luminescence Figure 1: Energy levels of a luminescent centre (see text).

Stimulated emission is the process by which an excited atom or molecule is forced to release its stored energy as a photon when it interacts with an incoming photon of the same energy.12

For this to create the coherent light of a laser, a condition known as population inversion must first be established, where more atoms occupy a higher energy state than a lower one. Under normal conditions, atoms are more likely to absorb incoming light than to emit it; however, by using an external energy source to "pump" the system, a cascade of stimulated emissions is triggered.123

When a passing photon stimulates an excited atom, the resulting emitted photon is identical to the incoming one in energy, phase, and direction. This creates a coherent light wave. An optical resonator, typically consisting of two mirrors, reflects this light back and forth through the medium, amplifying the intensity as it stimulates further emissions until a focused, monochromatic beam is released.214