Under normal conditions, most of the electron population of a material (such as a rare-earth metal) will exist in the ground state. These electrons can move to an excited state if energized by an external source e.g. pump light. Population inversion is achieved if the majority of the electrons are excited from their ground state to an excited state.
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Related Terms: Absorption, Erbium, Erbium Doped Fiber (EDF), Excited State, Excited State Absorption (ESA), Neodymium, Ytterbium