Laser Physics: Saturation, Two-Level System & Pump Field

[KFC]

What does saturation mean in laser physics? I am reading some material on two-level system, in which it mentions something call "saturating pump field", what does this field mean?

 

[Andy Resnick]

In the two-state approximation (ground state and excited state), a pump field can move atoms from the ground state to excited state. When the efficiency of pumping goes to zero (which is reached at a 50-50 population distribution, IIRC), the laser medium is said to be 'saturated'- no additional atoms can be moved into the excited state without some returning to the ground state.

Population inversion (lasing with gain) requires at least three states.