Figure 31.1 Illustration of the principles involved in laser action a) A 3 – level Laser and b) A 4 – level Laser.
In a four level Laser, the steps upto the population of M are the same as in the case of a 3 level laser.
If there is a level G' above the ground state G into which the state M can be stimulated to decay, then the “population inversion” is readily accomplished because the population of G' is initially zero! If the transition from G' to G are rapid, then the laser action with frequency can be obtained on a nearly continuous basis.
can be obtained on a nearly continuous basis. 
