Modulated Ringdown Comb Interferometry for next-generation high complexity trace gas sensing

Abstract: Gas samples relevant to health and environment typically contain a plethora of molecular species that span a huge concentration dynamic range. High-concentration molecules impose a strong absorption background that hinders robust identification of low-concentration species. While mid-infrared frequency comb spectroscopy with high-finesse cavity enhancement has realized many of the most sensitive multi-species trace gas detection to date, its robust performance requires gas samples to contain only weak absorption features to avoid dispersing cavity resonances from the comb line frequencies. Here we introduce a new technique that is free from this restriction, thus enabling the development of next-generation multi-species trace gas sensing with broad applicability to complex and dynamic molecular compositions. The principle of Modulated Ringdown Comb Interferometry is to resolve ringdown dynamics carried by massively parallel comb lines transmitted through a length-modulated cavity. This method leverages both periodicity of the field dynamics and Doppler frequency shifts introduced from a Michelson interferometer. Scalable enhancement of both spectral coverage and cavity finesse is enabled with dispersion immune and high-efficiency data collection. Built upon this platform, we realize in the mid-infrared a product of finesse and spectral coverage that is orders of magnitude better than all prior experiments. We demonstrate the power of this technique by measuring highly dispersive exhaled human breath samples over a vastly expanded spectral coverage of 1,010 cm-1 and with cavity finesse of 23,000. This allows for the first time simultaneous quantification of 20 distinct molecular species at > 1 part-per-trillion sensitivity with their concentrations varying by 7 orders of magnitude.