How does stimulated emission produce more light in a laser?

Stimulated emission is the process by which an excited atom is triggered to release a photon by an incoming photon of identical energy. This interaction produces a new photon that is identical to the stimulating one in phase, direction, and polarization, effectively amplifying the light.123

For this to result in a sustained increase in light intensity, the system must first achieve a population inversion, a state where more atoms occupy a higher energy level than a lower one. This is typically accomplished by "pumping" the system with an external energy source, such as an electric current or light.1453

Once a population inversion is established, the initial emission triggers a chain reaction: each emitted photon stimulates further excited atoms to release additional photons. To build this into a powerful beam, a laser uses an optical resonator—a pair of mirrors that reflect the light back and forth through the medium. With each pass, the light intensity increases as it stimulates more atoms, eventually escaping through a partially transparent mirror as a concentrated, coherent laser beam.135