OPTICAL FEEDBACK STABILIZED LASER CAVITY RING DOWN SPECTROSCOPY: FROM SATURATED SPECTROSCOPY TO ISOTOPIC RATIO.

Optical feedback frequency stabilized cavity ring-down spectroscopy is a very high resolution (sub kHz) and sensitive technique (2$\times$ 10$^{-12}$ \wn limit of detection). Both aspects will be emphasized through the two followingh applications: To illustrate the high resolution, Doppler-free saturated-absorption Lamb dips were measured at sub-Pa pressures on rovibrational lines of H$_{2}$O near 7180 \wn, The saturation of the considered lines was so high that at the early stage of the ring down, the cavity loss rate remained unaffected by the absorption. By referencing the laser source to an optical frequency comb, transition frequencies were determined down to 100 Hz precision and kHz accuracy. To highlight the high precision and stability of the instrument, we will present the first optical absorption measurements of O-17 anomalies in CO$_{2}$ with a precision better than 10 ppm, matching the requirements for paleo-environmental applications