Saturation in laser physics refers to the condition where the efficiency of pumping atoms into an excited state reaches zero, typically occurring at a 50-50 population distribution between the ground and excited states. In the context of a two-level system, a saturating pump field is essential for moving atoms from the ground state to the excited state. Once saturation is achieved, no additional atoms can be excited without some returning to the ground state. It is important to note that achieving population inversion, which is necessary for lasing with gain, requires at least three energy states.
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