Modulated ringdown comb interferometry for sensing of highly complex gases

Qizhong Liang (), Apoorva Bisht, Andrew Scheck, Peter G. Schunemann and Jun Ye ()
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Qizhong Liang: National Institute of Standards and Technology and University of Colorado
Apoorva Bisht: National Institute of Standards and Technology and University of Colorado
Andrew Scheck: National Institute of Standards and Technology and University of Colorado
Peter G. Schunemann: BAE Systems
Jun Ye: National Institute of Standards and Technology and University of Colorado

Nature, 2025, vol. 638, issue 8052, 941-948

Abstract: Abstract Gas samples relevant to health1–3 and the environment4–6 typically contain many molecular species that span a huge concentration dynamic range. Mid-infrared frequency comb spectroscopy with high-finesse cavity enhancement has allowed the most sensitive multispecies trace-gas detections so far2,7–13. However, the robust performance of this technique depends critically on ensuring absorption-path-length enhancement over a broad spectral coverage, which is severely limited by comb–cavity frequency mismatch if strongly absorbing compounds are present. Here we introduce modulated ringdown comb interferometry, a technique that resolves the vulnerability of comb–cavity enhancement to strong intracavity absorption or dispersion. This technique works by measuring ringdown dynamics carried by massively parallel comb lines transmitted through a length-modulated cavity, making use of both the periodicity of the field dynamics and the Doppler frequency shifts introduced from a Michelson interferometer. As a demonstration, we measure highly dispersive exhaled human breath samples and ambient air in the mid-infrared with finesse improved to 23,000 and coverage to 1,010 cm−1. Such a product of finesse and spectral coverage is orders of magnitude better than all previous demonstrations2,7–20, enabling us to simultaneously quantify 20 distinct molecular species at above 1-part-per-trillion sensitivity varying in concentrations by seven orders of magnitude. This technique unlocks next-generation sensing performance for complex and dynamic molecular compositions, with scalable improvement to both finesse and spectral coverage.

Date: 2025
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DOI: 10.1038/s41586-024-08534-2

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