Concept of 3 and 4 level Laser
Optical amplification in the gain medium of a laser or laser amplifier arises from stimulated emission, where the input light induces transitions of laser-active ions from some excited state to a lower state.
Three & Four Level laser
In a three-level system, the laser transition ends on the ground state.
The unpumped gain medium exhibits strong absorption on the laser transition.
A population inversion and consequently net laser gain result only when more than half of the ions (or atoms) are pumped into the upper laser level, the threshold pump power is thus fairly high.
(Three & Four Level Systems)
As compared to the three level laser diagram there is an extra energy level above the ground state.
This extra energy level has a very short lifetime.
Four Level laser
The pumping operation in a four level system is similar to the pumping of a three level laser.
by using atoms that have two relatively stable levels between the ground state and excited state(higher-energy) a sustained laser beam can be achieved.
As in the three-level laser, the atoms first drop to a long-lived metastable state where they can be stimulated to emit excess energy.
So that, instead of moving to the ground state, they stop in some other state above the ground state, from where they can be more easily excited back to a higher metastable state, thereby maintaining the population inversion required for continuous laser operation.
(Four Level Systems)
Level 1 is the ground level.
Levels 2, 3, and 4 are excited levels of the system.
Level 3 is a metastable level having a long lifetime.
Atoms from the level 1 are excited by a pump to the level 4 and from level 4 atoms decay very rapidly through some nonradiative ransition to level 3.