This paper describes the development of a robust and sensitive spectrometer based on the long path-length absorption technique of cavity ring-down spectroscopy (CRDS). The prototype demonstrated a sensitivity (characterized by a noise-equivalent absorption coefficient of 3.7×10⁻⁹ cm⁻¹ Hz^−1/2) in the near-infrared (NIR) region of the spectrum that is not much more than one order of magnitude worse than that of commercial CRDS devices with a more than ten times larger form factor, while being more than one order of magnitude better than that of the Tunable Diode Laser spectrometer on board the Mars rover Curiosity. Part of its success is due to the implementation of a novel technique to suppress optical interference fringes that tend to distort the baseline of the recorded spectra and therewith limit the capacity to accurately retrieve molecular concentrations. Instead of precisely modulating only the optical path between laser and optical cavity, as was done in a previous study by Burkart and Kassi (Appl. Phys. B 119(1), 97-109 (2015)), here the optical paths before and after the cavity were modulated by a symmetric extension at the sub-micrometer scale of the cavity itself. Spectra recorded at selected different pathlengths were then combined to reduce baseline artifacts. This has the advantage of also suppressing fringes generated by the optical path between cavity and detector, in addition to those associated with the path between source and cavity. The prototype was used to make preliminary water isotope ratio measurements in the NIR, as well as methane concentration measurements in the mid-IR after a change of optical components (laser, high-reflectivity mirrors, and detector). As such, the basic design can target a number of different molecular species of interest to planetary science. Standardization promises to enable re-use of the design and to facilitate space qualification for different planetary science missions.

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