Before the stimulation photon is incident on the metastable state, there are five excited molecules. Once the stimulating photon induces another deexcitation and this process repeats until all other excited molecules are deexcited, and six photons come out of the system. It is like one person shouts “FIRE” and enters a room and all the persons in the room also shout fire and the entering person and all the persons in the room rush out of the room!

Lasers can be classified in many ways. One way is to consider the number of levels involved in them, such as 3 – level lasers, 4 level lasers and so on. Another way is to use the classification as gas – lasers, solid state lasers, tunable lasers and so on. We shall use the later classification.