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Modulated ringdown comb interferometry for sensing of highly complex gases

Nature volume 638, pages 941–948 (2025)Cite this article

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Abstract

Gas samples relevant to health1,2,3 and the environment4,5,6 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 far2,7,8,9,10,11,12,13. 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 demonstrations2,7,8,9,10,11,12,13,14,15,16,17,18,19,20, 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.

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Fig. 1: MRCI.
Fig. 2: Extreme-performance multispecies trace-gas sensing.
Fig. 3: Apparatus and survey ringdown spectra collected with MRCI.
Fig. 4: Molecular spectroscopy from the 5-μm cavity.
Fig. 5: Molecular spectroscopy from the 3-μm cavity.
Fig. 6: Molecular concentrations summary.

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Data availability

All data and supporting materials have been deposited to Zenodo and can be accessed at https://doi.org/10.5281/zenodo.14254339 (ref. 80).

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Acknowledgements

We thank G. B. Rieker, S. A. Diddams, T. Q. Bui, O. H. Heckl, A. J. Fleisher, P. B. Changala and D. J. Nesbitt for helpful discussions. This work is supported by AFOSR FA9550-19-1-0148, NIST, NSF QLCI OMA–2016244 and NSF PHY-2317149.

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Author notes

  1. Peter G. Schunemann

    Present address: Onsemi, Hudson, NH, USA

Authors and Affiliations

  1. JILA, National Institute of Standards and Technology and University of Colorado, Boulder, CO, USA

    Qizhong Liang, Apoorva Bisht, Andrew Scheck & Jun Ye

  2. BAE Systems, Nashua, NH, USA

    Peter G. Schunemann

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  1. Qizhong Liang
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  2. Apoorva Bisht
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  4. Peter G. Schunemann
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  5. Jun Ye
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Contributions

All authors contributed to the manuscript writing and results interpretation. Q.L. and J.Y. conceived the modulated ringdown comb interferometry technique. Q.L., A.B. and A.S. designed and constructed the spectroscopy setup and collected and analysed the spectroscopy data. The 5-µm optical parametric oscillator comb was designed and constructed by Q.L. using the zinc germanium phosphide nonlinear optical crystal provided by P.G.S. The research work was supervised by J.Y.

Corresponding authors

Correspondence to Qizhong Liang or Jun Ye.

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A patent has been submitted for the technique of modulated ringdown comb interferometry by Q.L. and J.Y., both affiliated with JILA. The other authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Cavity swept lock on comb-cavity frequency detuning.

For this figure, intracavity dispersion from molecules and mirrors and piezo hysteresis associated with the cavity length modulation are omitted for ease of discussion. a, Servo schematic. Error signals generated from demodulating the observed cavity transmission bursts at the even and odd harmonics of the cavity length modulation frequency ωm are used for ensuring that comb frequencies (frep and fceo) and cavity FSR can all be precisely stabilized to each other. Through further locking the frep to an external frequency reference, the absolute frequencies for frep, fceo and FSR can be fixed. To explain why the transmission bursts can be demodulated at even or odd harmonics of ωm for respectively locking either frep–FSR frequency detuning or absolute frequency drift in fceo, consider three different cases shown in b–d. The triangle waves in dotted lines represent the cavity length periodic sweep. Red, green and blue curves represent transmission bursts generated from three different comb lines. Thick black curve takes the sum of the three comb signals and represents the time-dependent signal measured by a photodiode placed at the cavity transmission side. For each case, the intensity spectrum from Fourier transform of the black curve time signal is shown to the right. Comparing b with c, when frep–FSR frequency detuning is non-zero, the black curve observed from one consecutive cavity upsweep and downsweep form a base pattern repeating at rate ωm. This leads to non-zero intensity measured at odd harmonics of ωm. Comparing b with d, the slow drift in fceo can result in comb lines appearing on cavity resonances sequentially. The black curve becomes wider in shape and its Fourier decomposition requires less intense high-frequency components. Thus, the even harmonics decrease in intensity (excluding the zeroth order, which will not change).

Extended Data Fig. 2 Cavity swept lock on molecular and mirror dispersion.

a, A simulation of ambient air absorption spectrum over 2,700–3,330 cm−1, considering only the three most intense absorbers, HDO, CH4 and H2O, at concentrations of 1.6 ppm, 2.1 ppm and 0.2%, respectively, matched to our measured data presented in the main text. b, The corresponding intracavity refractive index spectrum calculated from a based on the Kramers–Kronig relations. It is non-rigorously assumed that molecular absorption is zero outside the plotted spectral range. Variation in the refractive index of more than 4 × 10−7 can be seen. c, Phase delay per reflection from a mirror surface. The data are obtained from the vendor for the 3-μm cavity used in this work. We arbitrarily vertically offset the entire curve by a constant so that the phase delay is zero at the rightmost wavenumber (3,330 cm−1). d, Cavity resonance frequency distribution calculated on the basis of the molecular dispersion data in b and mirror dispersion data in c, using equation (2) shown in Methods. For our comb-cavity main resonance at cavity length equal to 0.548737 m, which allows all comb lines on cavity resonance simultaneously if intracavity dispersion is absent, detuning the cavity length by about λ/2 allows one to go to the side resonance, in which λ is the laser wavelength. Owing to the presence of intracavity dispersion, neither the main nor the side cavity resonances are a vertical straight line that will allow the comb instantaneous bandwidth to be coupled into the cavity simultaneously. In our presented data, molecular dispersion spreads the resonance cavity length either left or right every time a strong absorption line is scanned across, whereas mirror dispersion shifts the resonance cavity length to the left, which is slowly increasing with the decrease in wavenumber. At this level of dispersion, a swept lock must be used: a particular comb line is guaranteed to be coupled through the cavity if the modulation depth is no less than λ/2. For example, when modulating around the main cavity resonance, the modulation depth should be set sufficiently larger than the variation in the resonance cavity length caused by dispersion, but less than λ/2 to avoid comb on cavity resonance from the side resonance. This is indicated by the purple shaded box. In actual experiments, it is not necessarily required to engage cavity swept lock to the main resonance. Side resonances can be used as well. This can be used as a strategy to counter the mirror dispersion effect to reduce the variation in resonance cavity length.

Extended Data Fig. 3 A complete data processing workflow of MRCI.

The step-by-step workflow of MRCI for the processing of the raw interferogram data to obtain broadband cavity ringdown values. See Methods for details.

Extended Data Fig. 4 Interferogram data.

a, Experimental interferogram raw data collected as a function of time from the two photodetectors measuring the cavity transmitted comb light (red and blue). The black trace is the balanced output obtained from data post-processing. b, Zoom-in of a into ±2 ms. c, Fourier-transformed spectra for data in a over the time span of 20 ms. Signal carriers at nωm ± ωi and non-cosine modulated terms at nωm are labelled. For these data, ωm = 13 kHz and ωi ≈ 34 kHz. All traces in a–c are vertically offset for clarity.

Extended Data Fig. 5 The thulium–zinc germanium phosphide OPO comb.

a, The OPO cavity geometry. Cavity mirrors are labelled M1–M6. Pump light is injected from M1 and idler light is out-coupled from M4. Coarse and fine cavity length control are achieved with a picomotor and a piezo mounted on M5 and M6, respectively. b, Power dependence measured for the idler on the pump, when the idler is tuned to 2,040 cm−1. The highest idler average power of 214 mW is measured at the pump power of 1,530 mW. The threshold pump power is 340(50) mW. Slope power efficiency is 19(1)% and photon conversion efficiency is 49(2)%. c, Table summarizing the crystal and mirrors specifications. AOI, angle of incidence; ROC, radius of curvature.

Extended Data Fig. 6 Cavity ringdown at comb-mode resolution.

a, Spectroscopy data collected at instrument resolution of 0.20 GHz for resolving the cavity transmitted comb lines spaced at 0.27 GHz frequency interval. Measurement is performed for calibrated 1.04(5) ppm CH4 diluted in nitrogen at 100 Torr and 20 °C. Rovibrational absorption lines have a median width at about 0.61 GHz calculated on the basis of HITRAN data, which is considerably larger than the comb spacing. Spectroscopy data are an average from a total of 256 interferograms collected at 7.5 s each. Zero padding is used to oversample at one-tenth of the instrument resolution to accurately locate the comb line frequencies. Only the leading five Fourier harmonics are plotted for clarity. The right panel is the zoom-in to near 3,018.22 cm−1. b, Ringdown absorption data determined from a. Stem lines in blue highlight the absorption coefficients measured by different comb lines. The data spanning 2 cm−1 coverage uniformly sampled by 219 comb lines is globally fitted to the HITRAN database plotted in orange. Fitted concentration yields 0.97(5) ppm. We further fit each rovibrational absorption line to a Voigt profile (black). For this analysis, Gaussian linewidths are fixed at the calculated values based on HITRAN data at 20 °C. Fitted concentration is used as the true methane concentration. c, Molecular parameters for the Lorentzian half width at half maximum (HWHM) widths and line intensities are determined for individual rovibrational absorption lines. Experimental data in black are compared with HITRAN data in orange. Uncertainties in the HITRAN database for which linewidths reported at about 10% and line intensities at about 5% are indicated. Vertical orange dashed lines are a guide to the eye aligning extracted molecular parameters to their corresponding rovibrational absorption lines. d, Spectroscopy data collected at the modest instrument resolution of 0.80 GHz. In contrast to the measured data at comb-mode resolution in b, the two close-lying rovibrational absorption lines near 3,018.2 cm−1 are less robustly resolved owing to the larger spectral resolution. Global fit to the HITRAN database yields 1.00(5) ppm fitted concentration for methane. To summarize, our data confirm: (1) both 0.80 GHz modest resolution or comb-mode resolution yield measured concentrations in good agreement with the calibrated value and (2) at comb-mode resolution, molecular parameters further extracted for absorption linewidths and line intensities are also in good agreement with the reported values from the HITRAN database.

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Liang, Q., Bisht, A., Scheck, A. et al. Modulated ringdown comb interferometry for sensing of highly complex gases. Nature 638, 941–948 (2025). https://doi.org/10.1038/s41586-024-08534-2

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