Cavity-enhanced Faraday rotation spectroscopy for interference-free measurement of OH radical at 2.8 μm

• Published in: Sensors and actuators. B, Chemical Chemical, 2025-02, Vol.424, p.136901, Article 136901
• Main Authors: Ngo, Minh Nhut, Nguyen-Ba, Tong, Houzel, Nicolas, Coeur, Cécile, Dewaele, Dorothée, Cazier, Fabrice, Zhao, Weixiong, Chen, Weidong
• Format: Article
• Language: English
• Subjects: Balanced detection; Faraday rotation spectroscopy; OH radicals; Optical cavity; Pulsed microwave discharge
• ISSN: 0925-4005
• DOI: 10.1016/j.snb.2024.136901

An instrument based on cavity-enhanced Faraday rotation spectroscopy (CE-FRS) operating at 2.8 μm has been developed for interference-free measurement of OH radicals in the laboratory. By off-axis coupling of a continuous-wave laser into a high finesse optical cavity, FRS signal is obtained from balanced detection of time-integrated light intensity leaking out of the cavity in the presence of magnetic field. Radio-frequency white noise (5–520 MHz) was injected into laser current which reduced intensity fluctuations in cavity transmission, thus improved the signal-to-noise ratio of the spectroscopic signal by a factor of 2. The setup provides a simple and robust spectroscopic instrument for in-situ and highly-selective detection of paramagnetic species. We demonstrated the instrument’s capabilities using OH radical with concentration in the range of 1012 molecule.cm−3, generated by microwave discharge of water vapor at low pressure. The CE-FRS instrument exhibited a limit of detection of ∼ 1010 molecule.cm−3 in an integration time of 20 s, which is enhanced by a factor of 2.5 compared to cavity-enhanced wavelength modulation spectroscopy involving an off-axis integrated cavity output spectroscopy approach. A time-resolved FRS signal was recorded in a pulsed microwave discharge regime, giving a millisecond time resolution for the measurement of OH concentration profile. The developed instrument provides a potential analytical tool for the measurement of OH concentration for chemical kinetic study in reactor cells. •Demonstration of off-axis cavity-enhanced Faraday rotation spectroscopy (CE-FRS).•Balanced FRS detection of time-integrated cavity output.•Radio-frequency White noise injection to reduce laser intensity fluctuations.•Real-time interference-free in situ measurement of OH radicals using CE-FRS.•Capacity of monitoring OH concentration profile in millisecond time resolution.