Why four level laser systems are more efficient as compared to three level lasers?
We know that the laser action is initiated only when the population inversion condition is achieved i.e. N2 > N1 where N1 and N2 are the population of the two levels involved in laser action. Further higher is the probability of stimulated emission as compared to absorption, if the difference N2 – N1 is large. In case of three level laser systems, the laser action is initiated only when the excited atoms in level 2 are significantly higher than the number of atoms in ground state at level 1. Since the lifetime of level 3 is of the order of nanoseconds, the excited atoms emit spontaneously and come to a metastable level 2, which has a higher lifetime. In other words, more than half of the atoms should be shifted to level 2 via level 3 to initiate laser action. This requires very strong pumping source. On the other hand, in case of four level lasers, laser action is achieved between levels 3 and 2; both of them are completely empty to start with. So if pumping were able to excite even a
