HIGHLIGHTS
• Cavity enhanced frequency comb spectroscopy is used to measure components in a gas sample at extremely low levels but is not that effective due to the frequencies of laser light not matching to the resonances in the cavity containing a specific sample. 
• A new approach, known as modulated ringdown comb interferometry, can quantify different substances in a gas sample.
• This technique was found to identify substances collected from the nasal and oral respiratory airways of individuals from ambient air.

As sustainability becomes more prominent, the need to find better strategies for determining emissions in order to assess the carbon footprint involved in the manufacturing and use of a specific lubricant is required. This process may involve detecting such gases as carbon dioxide, methane and nitrogen oxides (NOx). 

Cavity enhanced frequency comb spectroscopy is one of the techniques for measuring components in a gas sample at extremely low levels. Qizhong Liang, a graduate student at JILA, a joint research institute between the University of Colorado Boulder and the National Institute of Standards and Technology (NIST) in Boulder, Colo., says, “Different molecules generate their unique absorption fingerprint to the laser light based on how their unique absorption fingerprint to the laser light based on how their molecular bonds stretch and bend. The mid-infrared spectroscopic region spans in frequency approximately from 500 to 4,000 cm^-1 and is particularly useful for trace gas detection.”

The combs used in this technique represent a special kind of laser that emits many narrow-linewidth optical lines at evenly spaced frequencies. They can be coupled into a cavity containing a specific gas sample. To identify molecular components, the spectral intensity dynamics for light transmitted out of the cavity is measured. The technique is known as cavity ringdown spectroscopy. 

Liang says, “The laser light bounces back and forth between the cavity mirrors by about 50 centimeters in length. As the light travels across the gas samples multiple times, compositional information encoded to the laser light dynamics via optical absorption. The sensitivity of the measurement increases as the light continues to cycle through the cavity. A mirror that is 99.99% can amplify the absorption signals by about 10,000 times.”

However, the need to concurrently match the frequencies of the laser light to the cavity resonances has so far reduced the method’s effectiveness if the gas samples contain complex compositions. Liang says, “A laser light must form a standing wave to be coupled through the cavity. If strongly absorbing compounds are present, they can alter the coupling condition and result in only a limited range of comb light can be coupled through.” 

Liang and his colleagues have now developed a new analytical technique that overcomes the issues with previous methods and enables compounds in a gas to be detected at the parts per trillion range. 

Modulated ringdown comb spectroscopy
The researchers devised a new approach that allows for the coupling of the entire spectrum of laser light by continuously modulating the length of the cavity. Sensitivity enhancement from the use of high-reflectivity mirrors can now be robustly deployed for measuring gas samples containing arbitrary molecular compositions. The technique is known as modulated ringdown comb interferometry (MRCI). 

Liang says, “MRCI has the potential to be used in real-world application because it is a robust technique that can be use over a long-term and can identify and quantify different substances present in a gas sample. This technique is highly automated and can collect data efficiently.”

The experimental setup for MRCI is shown in Figure 2.


Figure 2. The experimental setup for the new technique known as modulated ringdown comb interferometry can be used to detect and quantify substances at low concentrations in a gas. Figure courtesy of Patrick Campbell/ University of Colorado.

To demonstrate the potential for MRCI, the researchers measured the contents of exhaled breath collected from the nasal and oral respiratory airways of individuals and from ambient air. Data can be collected with two seconds of acquisition time. Among the substances identified and quantified were carbon dioxide, carbon monoxide, nitric oxide, nitrous oxide, formaldehyde, methane and methanol. 

Since MRCI has been used to detect compounds in ambient air, Liang mentioned that this technique is well suited for measuring greenhouse gas (GHG) emissions. He says, “MRCI will be able to detect GHG emissions because many of them give intense absorption to the laser light. Our research demonstrated the detection of many of these compounds in ambient air.”

Future work will entail using MRCI for medical research studies. Liang says, “We are collaborating with medical researchers to better understand how detection of specific compounds in a patient’s breath may assist doctors with diagnosing specific medical conditions. The second aspect of our next steps is to build a MRCI prototype that can be used in commercial applications.”

MRCI has the potential to not only identify specific compounds related to emissions but could be used in manufacturing plant applications to monitor air quality and be a tool that can identify a problem with leakage of hazardous components before this becomes a health and safety problem. 

Additional information on this research can be found in a recent article.¹
REFERENCES
1. Liang, Q., Bisht, A., Scheck, A., Schunemann, P. and Ye, J. (2025), “Modulated ringdown comb interferometry for sensing of highly complex gases,” Nature, 638, pp. 941-948.