Modulated ringdown comb interferometry for sensing of highly complex gases

Area of Science:

• Spectroscopy
• Analytical Chemistry
• Environmental Science

Background:

• Gas analysis for health and environmental monitoring requires detecting many species across wide concentration ranges.
• Mid-infrared frequency comb spectroscopy with cavity enhancement offers high sensitivity but is limited by strong absorption and dispersion.
• Comb-cavity frequency mismatch hinders robust performance in complex gas samples.

Purpose of the Study:

• To introduce a novel technique overcoming limitations of current cavity-enhanced spectroscopy.
• To improve sensitivity and spectral coverage for multispecies trace gas detection.
• To enable robust quantification of diverse molecular compositions in real-world samples.

Main Methods:

• Developed modulated ringdown comb interferometry.
• Utilized a length-modulated high-finesse cavity.
• Employed a Michelson interferometer to introduce Doppler frequency shifts.
• Measured ringdown dynamics of transmitted comb lines.

Main Results:

• Achieved a finesse of 23,000 and spectral coverage of 1,010 cm⁻¹ in the mid-infrared.
• Demonstrated simultaneous quantification of 20 molecular species.
• Reached above 1-part-per-trillion sensitivity for gases with concentrations spanning seven orders of magnitude.
• Successfully analyzed exhaled human breath and ambient air samples.

Conclusions:

• Modulated ringdown comb interferometry resolves the vulnerability of cavity enhancement to strong intracavity absorption or dispersion.
• The technique offers a significant advancement in the product of finesse and spectral coverage.
• This method enables next-generation sensing performance for complex and dynamic molecular compositions.