Rami Arieli: "The Laser Adventure" Chapter 2.12 page 1
2.12 Four Level Laser

The schematic energy level diagram of a four level laser is shown in figure 2.7.

Compared to the equivalent diagram of a three level laser, there is an extra energy level above the ground state. This extra energy level has a very short lifetime.

Figure 2.7: Energy level diagram in a four level laser

 The pumping operation of a four level laser is similar to the pumping of a three level laser. This is done by a rapid population of the upper laser level (E3), through the higher energy level (E4).

The advantage of the four level laser is the low population of the lower laser energy level (E2).
To create population inversion, there is no need to pump more than 50% of the atoms to the upper laser level.
The population of the lower laser level (N2(t)) is decaying rapidly to the ground state, so practically it is empty. Thus, a continuous operation of the four level laser is possible even if 99% of the atoms remain in the ground state (!)