*Article* **Direct Comb Vernier Spectroscopy for Fractional Isotopic Ratio Determinations**

**Mario Siciliani de Cumis 1,2,3,\*,†, Roberto Eramo 2,3,†, Jie Jiang 4, Martin E. Fermann <sup>4</sup> and Pablo Cancio Pastor 2,3,†**


**Abstract:** Accurate isotopic composition analysis of the greenhouse-gasses emitted in the atmosphere is an important step to mitigate global climate warnings. Optical frequency comb–based spectroscopic techniques have shown ideal performance to accomplish the simultaneous monitoring of the different isotope substituted species of such gases. The capabilities of one such technique, namely, direct comb Vernier spectroscopy, to determine the fractional isotopic ratio composition are discussed. This technique combines interferometric filtering of the comb source in a Fabry–Perot that contains the sample gas, with a high resolution dispersion spectrometer to resolve the spectral content of each interacting frequency inside of the Fabry–Perot. Following this methodology, simultaneous spectra of ro-vibrational transitions of 12C16O2 and 13C16O2 molecules are recorded and analyzed with an accurate fitting procedure. Fractional isotopic ratio 13C/12C at 3% of precision is measured for a sample of CO2 gas, showing the potentialities of the technique for all isotopic-related applications of this important pollutant.

**Keywords:** isotopic ratio; frequency comb; Vernier spectroscopy
