  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.
