Environmental Monitoring with Dual-Comb Spectroscopy: Precision Sensing for Trace Gases

By Ankit SinghReviewed by Lexie CornerMay 13 2025

Trace gases such as methane, carbon dioxide, nitrous oxide, and volatile organic compounds (VOCs) contribute significantly to air pollution. Monitoring these gases is essential for quantifying emissions, evaluating mitigation efforts, and informing regulatory decisions.

Traditional spectroscopic methods, like Fourier-transform infrared (FTIR) spectroscopy and gas chromatography, often fall short in terms of resolution, speed, and adaptability. Dual-comb spectroscopy (DCS) offers ultrahigh spectral resolution, fast data acquisition, and portable deployment, making it well-suited for real-time gas detection in complex environments. 

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How Dual-Comb Spectroscopy Works

DCS uses two optical frequency combs (OFCs)—lasers that emit a spectrum of equally spaced frequencies—to perform broadband, high-resolution spectral measurements. By setting the two combs to slightly different repetition rates (denoted as f_r1​ and f_r2), the system generates a series of beat notes.

These beats form a radio-frequency (RF) comb, which down-converts the optical spectrum into a measurable signal. This process enables the rapid and scanless acquisition of high-resolution spectra across broad bandwidths.^1,2

One key enabler of this technique is the use of tunable, gain-switched Fabry-Perot laser diodes operating at low repetition rates (~100 MHz), which generate stable combs across approximately 40 nm in the near-infrared (NIR).

Coherence between the combs is maintained through optical injection locking, ensuring the precision required to resolve fine molecular absorption features.¹

The resulting interference signal is captured using heterodyne detection, where the two combs beat on a photodetector to produce an RF comb.

The spacing between RF tones corresponds to the difference in the repetition rates of the two combs (Δf_r​=f_r2​−f_r1 ) effectively mapping the optical spectrum into a more manageable frequency domain. This configuration enables spectral data to be captured in milliseconds, orders of magnitude faster than traditional scanning-based techniques.³

DCS systems also achieve broad spectral coverage, with recent configurations spanning over 2 THz in the mid-infrared (MIR). This range is particularly valuable for detecting gases like methane and ethane, whose spectral features often overlap in the MIR.^4,5

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Why Use DCS for Environmental Monitoring

Compared to established techniques such as FTIR and cavity ring-down spectroscopy (CRDS), DCS offers substantial performance gains. It delivers spectral resolution down to 100 MHz, enabling the identification of subtle molecular signatures and dynamic gas interactions.^1,3

The method can detect more than 20 gas species simultaneously, even at parts-per-trillion (ppt) concentrations. This multi-species capability makes it efficient for comprehensive air composition analysis, particularly in environments where multiple emission sources are active.^.6

DCS is suited to field deployment. Open-path configurations enable measurements over distances of hundreds of meters to several kilometers without the need for physical sample collection, making it ideal for monitoring large industrial sites, agricultural areas, and remote or inaccessible regions.⁵

Technological Innovations Advancing DCS

Several recent innovations are expanding the capabilities of dual-comb spectroscopy, improving its sensitivity, resolution, and adaptability for real-world gas sensing.

Integration with Cavity Optomechanics

One approach—dual-comb optomechanical spectroscopy (DCOS)—combines DCS with photoacoustic detection. In this method, gas molecules absorb light from dual-comb excitation, generating ultrasound waves that are captured by a nanomechanical membrane.

This setup has demonstrated a noise-equivalent absorption coefficient approximately 1,000 times lower than conventional photoacoustic techniques. The membrane’s broadband mechanical response, covering frequencies from 10 kHz to 1.1 MHz, supports real-time detection of complex gas mixtures, such as those found in industrial emissions.⁴

Modulated Ringdown Comb Interferometry

To address limitations in cavity-enhanced DCS, where strong absorption compromises path length enhancement, researchers developed modulated ringdown comb interferometry.

By modulating the cavity length and analyzing Doppler-shifted comb lines, this technique achieves a finesse of 23,000 and spectral coverage of 1,010 cm⁻¹. As a result, it enables ppt-level detection of more than 20 species in both exhaled breath and ambient air.⁶

Tunable and Cost-Effective Systems

Advances in gain-switched laser diodes and low-cost electronics have democratized DCS. For example, systems using step-recovery diodes (SRDs) and software-defined radios reduce costs while maintaining 40 GHz bandwidths and 400 comb lines.

Spectral agility has also improved through tunable optical injection techniques, enabling DCS systems to target specific absorption bands without hardware reconfiguration. This flexibility is particularly useful for monitoring reactive or species-specific gases such as hydrogen sulfide or ammonia.^1,3

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Applications in Environmental Monitoring

Methane Emissions from Oil and Gas Operations

Methane, a potent greenhouse gas, presents substantial monitoring challenges due to its spatially and temporally variable sources.

In a field study published in Frontiers in Chemistry, a mid-infrared DCS system was deployed at an unconventional oil well in Colorado. The system enabled simultaneous detection of methane, ethane, and propane over open paths of 250 to 500 meters.

This system successfully identified emission hotspots during drilling and hydraulic fracturing. The measured ethane-to-methane ratios provided valuable source attribution, distinguishing fossil fuel emissions from biogenic methane.⁵

Urban Air Quality and VOC Tracking

In Freiburg, Germany, researchers applied DCS to monitor nitrous oxide and carbon monoxide in urban air. The technique addressed spectral interference at 4.6 µm, where absorption lines overlap significantly. Operating at a spectral acquisition rate of 10 Hz, the system achieved a measurement precision of 30 ppm for CO and 50 ppm for nitrous oxide, facilitating real-time pollution mapping.³

Agricultural and Industrial Emissions

Open-path DCS has also been applied to measure ammonia emissions from livestock farms and carbon dioxide from cement manufacturing. A study conducted in Northern Colorado demonstrated the ability to quantify fluxes with an uncertainty of less than 5 %, which is essential for compliance with Environmental Protection Agency (EPA) regulations.⁵

Current Limitations and Emerging Solutions

Despite its capabilities, DCS still faces several technical and practical barriers. The use of high-finesse optical cavities and stabilized frequency combs increases system complexity and cost, limiting accessibility beyond specialist laboratories.

Field deployments also generate terabyte-scale datasets, requiring robust algorithms and computational resources for real-time processing and interpretation. In addition, environmental conditions—particularly humidity and temperature fluctuations—can distort absorption spectra, necessitating reliable calibration strategies to maintain accuracy.^3,5,6

Looking ahead, ongoing miniaturization efforts and integration with Internet of Things (IoT) technologies are expected to broaden the usability of DCS. Photonic integrated circuits (PICs) and quantum cascade lasers (QCLs) are enabling the development of compact, portable DCS systems. Combined with artificial intelligence (AI)-driven analytics, these platforms could support wireless sensor networks in smart cities.^7,8

Efforts to expand DCS technology into the ultraviolet (UV) and THz ranges could open up new applications, including detecting ozone, heavy metals, or other species with characteristic features outside the traditional mid-infrared region. On the regulatory side, initiatives like the EU’s Industrial Emissions Directive are accelerating demand for standardized, high-accuracy monitoring tools.^5,6,8

Through its ability to deliver precise, multi-species gas measurements at high resolution, DCS offers a strong candidate for next-generation environmental sensing, particularly as system costs decrease and deployment becomes more scalable.

To explore more about cutting-edge environmental monitoring technologies, visit:

• The Role of Modern Spectroscopy in Environmental Monitoring
• Nanosensors For Environmental Monitoring and Protection
• How are Sensors Used in Air Pollution Monitoring?
• Advancing Water Quality Monitoring with Photonic Sensing Technologies
• Novel Laser Technology for Accurate Climate Monitoring

For a closer look at how spectroscopy is applied in real-world environmental analysis, this short video from Bruker demonstrates ASTM D7575 oil-in-water testing using FT-IR spectroscopy:

References and Further Reading

1. Monroy, L. et al. (2025). Multi-gas dual-comb spectroscopy with tunable gain-switched laser diodes. Scientific Reports, 15(1), 1-11. DOI:10.1038/s41598-025-90108-x. https://www.nature.com/articles/s41598-025-90108-x
2. How Dual-Comb Spectroscopy Detection of Trace Gases Works. NIST. https://www.nist.gov/image/how-dual-comb-spectroscopy-detection-trace-gases-works
3. Nitzsche, L. et al. (2021). Two-component gas sensing with MIR dual comb spectroscopy. tm - Technisches Messen, 89(1), 50–59. DOI:10.1515/teme-2021-0107. https://www.degruyterbrill.com/document/doi/10.1515/teme-2021-0107/html
4. Ren, X. et al. (2023). Dual-comb optomechanical spectroscopy. Nature Communications, 14(1), 1-8. DOI:10.1038/s41467-023-40771-3. https://www.nature.com/articles/s41467-023-40771-3
5. Mead, G. J. et al. (2023). Open-path dual-comb spectroscopy of methane and VOC emissions from an unconventional oil well development in Northern Colorado. Frontiers in Chemistry, 11, 1202255. DOI:10.3389/fchem.2023.1202255. https://www.frontiersin.org/journals/chemistry/articles/10.3389/fchem.2023.1202255/full
6. Liang, Q. et al. (2025). Modulated ringdown comb interferometry for sensing of highly complex gases. Nature, 638(8052), 941-948. DOI:10.1038/s41586-024-08534-2. https://www.nature.com/articles/s41586-024-08534-2
7. Process Spectroscopy Market Size, Share & Trends Analysis Report. Grand View Research. https://www.grandviewresearch.com/industry-analysis/process-spectroscopy-market
8. Environmental Monitoring Market Size, Share & Trends Analysis Report. Grand View Research. https://www.grandviewresearch.com/industry-analysis/environmental-monitoring-market

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