Suppression of unwanted lasing in laser isotope separation

Frequency selective means, particularly for use in uranium enrichment by laser-induced isotopically selective ionization to suppress unwanted lasing of the excited uranium medium is described. The frequency selective means such as filters, are placed at intervals in a column of laser illuminated uranium vapor and are transmissive to the illuminating laser frequencies but nontransmissive to frequencies at which population inversions exist in the excited uranium. The filters limit the path length and correspondingly the gain in the excited uranium to prevent high intensity lasing by the uranium itself