Trace-level isotope concentration-ratio measurement /sup 12/CO/sub 2/-/sup 13/CO/sub 2/ laser system

For high sensitivity measurements of isotope concentration-ratios of trace-level gas mixtures, particularly including carbon dioxide isotopes, we tested a /sup 12/CO/sub 2/-/sup 13/CO/sub 2/ composite laser system containing intracavity absorption-cells. Sensitivity enhancement obtainable by the intracavity modulation absorption-cell with synchronous detection scheme was examined theoretically. /sup 13/CO/sub 2/ in atmospheric air was readily detected and minimum detectable concentration by the present system was estimated to be 70 ppb.