Population Inversion

This article compares the operating principles of two-level, three-level, and four-level laser systems. Two-level systems cannot achieve population inversion through optical pumping, while three-level systems require high pump intensity to establish inversion between the upper laser level and the ground state. Four-level systems, represented by Nd:YAG lasers, are far more efficient because their lower laser level is rapidly depopulated, allowing population inversion and laser amplification to be achieved with much lower pump power.

This article explains the fundamental mechanism of laser amplification, focusing on the role of the gain medium, spontaneous emission, stimulated emission, and population inversion. Stimulated emission enables incident photons to trigger the emission of identical photons, resulting in optical amplification. To achieve net gain, a population inversion must be established, ensuring that more atoms occupy the excited state than the ground state, which is the essential condition for laser operation.