Concept of 3 and 4 level Laser
Optical amplification in the gain medium of a laser or laser amplifier arises from stimulated emission, where the input light induces transitions of laser-active ions from some excited state to a lower state.
Three & Four Level Systems
In a three-level system, the laser transition ends on the ground state. The unpumped gain medium exhibits strong absorption on the laser transition. A population inversion and consequently net laser gain result only when more than half of the ions (or atoms) are pumped into the upper laser level; the threshold pump power is thus fairly high.