Laser absorption with non-resonance cavities and Faraday rotation spectroscopy for gas sensing applications, towards laboratory detection of OH and HO2 radicals.
Laser absorption spectroscopy is a fundamental technique widely used in trace gas sensing, with versatile applications in atmospheric chemistry, environmental monitoring, biomedical diagnostics, and molecular astrophysics. By quantifying the attenuation of light passing through a gas sample, this non-invasive method enables in-situ and absolute quantification of sample concentration. The utilization of optical cavities can significantly increase the light-sample interaction length, thereby enhancing detection limits according to the Beer-Lambert law. Among laser-based cavity techniques, off-axis Cavity-Enhanced Absorption Spectroscopy (off-axis CEAS) stands out as a robust approach for trace gas detection. A key feature of this approach involves eliminating the cavity mode structure through the un-resonance coupling of a narrow bandwidth laser source into a cavity, facilitating continuous scanning of laser frequency for rapid spectral measurements. This strategy offers simple optical alignment and circumvents the need for high-bandwidth devices or laser frequency-locking, ensuring robustness against misalignment and mechanical vibration.
In this work, we present the developments and applications of non-resonance cavities for the direct quantification of OH and HO2 radicals, key species in atmospheric chemistry. Furthermore, the enhancement of optical effective path length not only amplifies the absorption of the target molecules but also extends to common atmospheric species such as water vapor and CO2. The occurrence of spectral interference, resulting from the overlapping absorptions of various species, poses challenges in accurately determining the concentration of the species of interest. To address these challenges, we introduce Faraday Rotation Spectroscopy, leveraging the magneto-optic effect observed only for paramagnetic species (e.g., NO, NO2, OH, HO2) to facilitate interference-free measurements. The combination of the optical cavity and Faraday rotation technique enhances both sensitivity and selectivity for gas concentration determination.