Laser absorption techniques are increasingly used for isotope ratio measurements. These methods offer specific advantages such as the ability to discriminate between isobaric isotopologues (e.g., 16O-13C-16O vs 16O-12C-17O). Their precision, however, still lags behind that of modern mass spectrometers. This project aims to develop an integrated dual-inlet laser spectrometer capable of measuring isotopologue ratios in CO2 with an internal precision on the order of 0.01 permil, allowing measurements of subtle but significant isotopic anomalies in the oxygen isotope ratios (18O/16O and 17O/16O) of atmospheric CO2 and/or carbonate minerals. The corresponding “O-17 excess” (Delta-17O) is an important tracer of atmospheric chemistry and paleo-hydrology, but its measurement in CO2 (as opposed to O2 or H2O) remains challenging because of isobaric interference. Development of this new instrument will be based on close collaboration between stable isotope geochemists and laser spectroscopists from LSCE (Laboratoire des Sciences du Climat et de l'Environnement) and LIPhy (Laboratoire Interdisciplinaire de Physique). Building on our previous work and proof-of-concept experiments, we propose to build the first of a new generation of ultra-precise cavity ring-down spectrometers and to combine it with a custom dual-inlet system derived from existing IRMS devices. In order to attain the extreme precision levels quoted above, we plan to lock a DFB laser near 1.6~µm using optical feedback from a specially designed, ultra-stable "source cavity", resulting in a source with a very narrow linewidth around a highly stable center frequency, which will be injected in one or more ring-down cavities containing sample or reference gases. Design, assembly and initial testing will be performed at LIPhy, and the laser spectrometer will then be transferred to LSCE for accuracy tests, further optimizations, and initial geochemical applications. This process will take place in the context of a new PhD project which will include both instrumental development and early scientific applications, under dual LIPhy/LSCE supervision. As an added benefit, this will ensure a good transfer of theoretical and practical expertise from one lab to the other. By the end of this work, the instrument should be fully operational at LSCE, providing important new observations for paleo-climate and atmospheric studies. Technical advances from this project will provide a foundation for future implementations of this technique, and will certainly benefit a wide range of other laser spectrometric applications.