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How does stimulated emission make laser light possible?

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Diagram of a tree leaf illustrating photosynthesis, with arrows showing sunlight, water, and carbon dioxide entering the leaf and sugars and oxygen being produced. A person is shown beside the tree to demonstrate the exchange of oxygen and carbon dioxide between plants and animals. — electromagnetic radiation Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.

Stimulated emission is the fundamental process that makes laser light possible. It occurs when an incoming photon with appropriate energy interacts with an atom already in an excited state, triggering that atom to release a second photon.123

This emitted photon is identical to the stimulating photon in energy, phase, direction, and polarization. To sustain this process, a population inversion—a state where more atoms occupy a higher energy level than a lower one—must be maintained by an external energy source, or "pump".234

Without this inversion, incoming photons would be absorbed rather than multiplied. Once achieved, the stimulated emission triggers a chain reaction, and an optical resonator (typically a pair of mirrors) reflects the light back and forth through the medium. This amplifies the light intensity, resulting in a beam that is highly coherent, monochromatic, and directional.2513