Lasers without inversion: Interference of lifetime-broadened resonances

摘要: While it is commonly believed that population inversion a requirement for obtaining laser amplification, this not so. We show if two upper levels of four-level system are purely lifetime broadened and decay to an identical continuum, there will be interference in the absorption profile lower level atoms, absent from stimulated emission atoms. It is, therefore, possible have substantial gain cross section at frequencies where zero obtain amplification conditions greatly exceeds population.