Question

Explain lasing action in a three level system.

Answer 1

Inversion by optical pumping becomes possible when using a three-level system. Pumplight with a shorter wavelength (higher photon energy) can transfer atoms from the ground state to the highest level. From there, spontaneous emission or a nonradiative process (e.g., involving phonons in a laser crystal) transfers atoms to an intermediate level, called the upper laser level. From that level down to the ground state, the laser transition withstimulated emission can occur.