High-resolution molecular spectroscopy and biological applications

The rapid measurement of gas concentrations is beneficial to a broad range of applications, including pollution monitoring, high-precision spectroscopy, and medical breath analysis. Optical frequency combs (OFCs) are a near-ideal optical interrogation source for molecular spectroscopy, combining broadband light with high frequency-precision and dense spectral sampling, characteristics which also allow short measurement times. A spectrometer based on a virtually imaged phased array (VIPA) is used to unravel the comb into a high-resolution transmission spectrum, with the returned spectra rapidly fitted with a detailed spectral model for concentration extraction of CO2. We demonstrate an accuracy of 0.5% and 12% for concentration measurements of (CO2)-C-12-O-16 and (CO2)-C-13-O-16 respectively, with the measured isotopic ratio in excellent agreement with that expected from their natural abundances. Precision of the concentration measurements is also high, at 0.03% and 1.24% for (CO2)-C-12-O-16 and (CO2)-C-13-O-16 respectively. The measurement technique is also verified to be highly linear for concentrations ranging over three orders of magnitude. The spatial and temporal coherence of OFCs additionally enables the use of the full range of performance-enhancing optical techniques including resonant enhancement. Here we demonstrate the use of a multi-pass Herriott cell to enhance the effective measurement path length, which allows the continuous optical measurement of CO2 production during the respiration of baker's yeast in a closed system. Results from the yeast experiments agree well with existing literature.