Abstract

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

© 1989 Optical Society of America