Accurate optical number density measurement of ¹²CO₂ and ¹³CO₂ with direct frequency comb spectroscopy

Abstract

We report on the use of direct frequency comb spectroscopy to accurately measure the concentrations of ¹²C¹⁶O₂ and ¹³C¹⁶O₂ isotopologs in a gas sample. We demonstrate an accuracy of 0.5% and 12% for concentration measurements of ¹²C¹⁶O₂ and ¹³C¹⁶O₂, respectively, with the measured isotopic ratio in excellent agreement with that expected from their natural abundances. The precision of the concentration measurements is also high, at 0.03% and 1.24% for ¹²C¹⁶O₂ and ¹³C¹⁶O₂, respectively. The measurement technique is verified to be highly linear for concentrations ranging over 3 orders of magnitude. Direct frequency comb spectroscopy can be applied to numerous molecular species, and is therefore a promising technique for measurements in environmental monitoring and biomedical sciences.