Suppression of unwanted lasing in laser isotope separation

The invention provides a system for suppressing unwanted lasing in an excited column of particles to acceptably low intensity levels. The system permits production plant utilization of laser enrichment, such as uranium enrichment, by limiting self lasing of the enriched medium. For production scale utilization, the enrichment medium is illuminated in a long column by a beam of exciting laser radiation. The column of irradiated material is typically, though not necessarily, a vapor phase material containing particles in either atomic or molecular form. Filter means providing a frequency selective transmission or reflexion, are interposed in specific locations in the column of laser radiation to suppress frequencies at which self lasing occur. (J.R.)