MEASUREMENT OF THE CARBON ISOTOPIC COMPOSITION OF METHANE USING HELICOIDAL LASER EIGENSTATES

The spatially generalized Jones matrix formalism is used to design a l aser cavity to make intracavity measurements of the carbon isotopic co mposition of methane. The method is based on a double optical lever ef fect for helicoidally polarized eigenstates, permitting to measure suc cessively the (CH4)-C-12 and (CH4)-C-13 concentrations. To choose the probed isotope, one simply tunes the frequency of the laser by Zeeman effect. The experiment exhibits a good agreement with the predictions and permits to measure the (CH4)-C-13/(12)CH4 composition ratio of met hane with an uncertainty of the order of +/-0.07% for a sample contain ing only 6 x 10(-9) mole of methane.