Isotope ratio dual-comb spectrometer

We demonstrate the use of dual-comb spectroscopy for isotope ratio measurements. We show that the analysis spectral range of a free-running near-infrared dual-comb spectrometer can be extended to the midinfrared by difference frequency generation to target specific spectral regions suitable for such measurements and especially the relative isotopic ratio $\delta {}^{13}\mathrm{C}$. The measurements performed present very good repeatability over several days with a standard deviation below 2‰ for a recording time of a few tens of seconds, and the results are compatible with measurements obtained using an isotope ratio mass spectrometer. Our setup also shows the possibility to target several chemical species without any major modification, which can be used to measure other isotopic ratios. Further improvements could decrease the uncertainties of the measurements, and the spectrometer could thus compete with isotope ratio spectrometers currently available on the market.

Figure 1

Schematic showing the MIR dual-comb spectrometer based on the difference frequency conversion of combs generated by electro-optic modulators around $1.55\mu \mathrm{m}$. CW, continuous wave; EPG, electrical pulse generator; EDFA, erbium doped fiber amplifier; AOM, acousto-optic modulator; IM, intensity modulator; HNLF, highly nonlinear fiber.

Figure 2

Comparison of the RF combs obtained by the beating of the near-infrared (blue) and MIR (red) combs. The bandwidth difference is due to the finite spectral acceptance of the crystal used for difference frequency generation in the MIR.

Figure 3

(Top) Absorption spectrum of a ${\mathrm{CO}}_2$ gas sample made of 99% of ${}^{13}\mathrm{CO}_2$ recorded around 4511 nm at a pressure of 173 mbar using a 10-s-long interferogram. (Middle) Fitted absorption spectrum using line parameters coming from the HITRAN database. (Bottom) Difference between the experimental and fitted spectra.

Figure 4

(Top) Absorption spectrum of a 2.5% ${\mathrm{CO}}_2$ and 97.5% air gas mixture recorded around 4364 nm at a pressure of 800 mbar using a 20-s-long interferogram. (Middle) Fitted absorption spectrum using line parameters coming from the HITRAN database. (Bottom) Difference between the experimental and the fitted spectra.

Figure 5

Graph showing $\delta {}^{13}\mathrm{C}$ measurements and their error bars at a confidence level of 95% (2 standard deviations) of gas samples coming a gas bottle under characterization. The dashed line represents a $\delta {}^{13}\mathrm{C}$ measurement of the same bottle obtained using an isotope ratio mass spectrometer.

Figure 6

(Top) Absorption spectrum of a 10% ${\mathrm{N}}_2\mathrm{O}$ and 90% air gas mixture recorded around 4628 nm at a total pressure of 455 mbar using a 60-s-long interferogram. (Middle) Fitted absorption spectrum using line parameters coming from the HITRAN database. (Bottom) Difference between the experimental and fitted spectra.