Stimulated emission

In a gas laser helium and neon are confined between parallel mirrors of very high reflectivity. Excited helium atoms transfer energy to excite many neon atoms (referred to as popultion inversion). If an excited neon atom transitions to the ground state emitting a photon parallel to the axis, laser operation becomes possible by stimulated emission.

The photon travels parallel to the axis and reflects. On the way it passes close to another excited atom that it stimulates to emit a photon in the same direction with the same phase. In a real case, with millions upon millions of excess excited atoms, stimulated emission produces a coherent standing wave of very high intensity between the parallel mirrors. Coherent light leaks through the mirrors and we have a laser.

Solid state lasers have a crystal between parallel mirrors. Population inversion and stimulated emission build a standing wave in the same way. The original laser (1960) was a ruby crystal between highly reflective mirrors. Population inversion was achieved by flashing it with an intense light. The ruby laser emitted pulses of radiation.

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