91Ó°ÊÓ

The lasing transition occurs from '1' to the ground level '0. ' In this three-level type of laser operation the pump field is used to raise the atoms from the ground level '0' to the pump level '2,' from which there occurs a rapid transition

Figure 27 depicts the subsequent transition of electrons in an atomic gas for a four-level laser. Here,$E_2$ is designated as the metastable state. After a continuous round of optical pumping, all energy levels except$E_2$ depopulate. Because of the slow decay from$E_2$ , the number of electrons in the metastable state is now larger than the $E_1$state. This condition then initiates population inversion and the subsequent cascade of stimulated emissions. Four this four-level laser, the downward transition $E_2→E_1$is the lasing transition.

Now suppose that $E_1$is metastable. In this case, the system will STILL FUNCTION as a laser. In fact, such a configuration now corresponds to a three-level laser, where the lasing transition now occurs from$E_1$ to$E_0$ . Make$E_1$ metastable, and$E_2$non-metastable however, this reduces the efficiency of the lasing process since more optical pumping is required to completely exhaust the ground state.