Lasing without inversion: The double scheme
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abstract
We determine the conditions under which a four-level medium in a resonant cavity can start lasing when there is no population inversion between the lasing levels. The mechanism which is used is the creation of low-frequency coherence by a pair of external pump fields. it is shown that lasing with frequency upconversion requires population inversion at (low-frequency) nonlasing transitions. 1990.