Three level and four level laser systems

In three level laser, the laser emission occurs when the atoms in the metastable state are stimulated to undergo transition to the ground state. Very high pump power required here because the terminal level of the laser transition is the ground state. Thus efficiency is lower. Continuous wave mode operation is not possible here. Eg:- Ruby laser.In four level laser, laser transition occurs between two intermediate levels rather than the ground state and higher excited state. Here population inversion can easily achieved with lower pumping requirements. Thus efficiency is greater. Continuous wave mode operation is possible here Eg:- He-Ne laser