Laser-driven stabilized cavity-enhanced absorption spectroscopy for HO2 detection near 1506 nm 

Cavity-Enhanced Absorption Spectroscopy (CEAS) [1-3], stands out as a cavity-based approach for sensitive measurement of sample absorption, without the need of complex optical setup and fast electronic devices for the used optical cavity compared to Cavity Ring-Down Spectroscopy (CDRS). CEAS offers continuous data acquisition within rapid scanning of laser frequency. The absorption spectrum is derived from the measured intensity outputs from the cavity with and without sample inside. While in tunable diode laser spectroscopy with wavelength modulation, the baseline is constructed of a high background signal, resulted from direct modulation of the laser power, superimposed with a small gas absorption signal. Achieving precise absorption assessments demands thus overcoming challenges in baseline structure, due to the laser intensity variation during laser frequency scan. Therefore, stabilization of laser intensity to maintain the baseline constant could improve the instrument's stability as well as measurement accuracy and precision. This work introduces the development and application of a laser Intensity-Stabilized Cavity-Enhanced Spectroscopy (IS-CEAS) operating near 1506 nm. The key innovation lies in employing an acousto-optic modulator (AOM) as an external power actuator for laser intensity stabilization. This strategy resulted in a remarkable 5-fold reduction in the laser intensity noise at lower frequency ranges where the integrated signal is detected. The fluctuations of cavity output intensity during laser scans were effectively eliminated, offering a baseline-free measurement of the absorption. This advancement notably bolstered system stability, enabling nearly 10 times longer integration than the conventional CEAS approach. The developed IS-CEAS system was employed to quantify the concentration of HO2 radicals produced in laboratory, yielding a bandwidth-normalized (1σ) limit of detection for HO2 at 1.6×108 molecule.cm-3.Hz-1/2. This value is comparable to the detection limits achieved by CRDS systems operating at the same wavelength. The experimental detail and the preliminary results will be presented and discussed. Acknowledgments The authors thank the financial support from the French ANR Foundation : ICAR-HO2 project (ANR-20-CE04-0003). This work has been partly supported by the EU H2020-ATMOS project, the ANR LABEX CaPPA (ANR-10-LABX-005) project and the regional CPER ECRIN program.   References [1] M. Mazurenka, A. J. Orr-Ewing, R. Peverall, G. A. D. Ritchie. Annu. Reports Prog. Chem. - Sect. C 2005, 101, 100–142. [2] W. Chen, A. A. Kosterev, F. K. Tittel, X. Gao, W. Zhao. Appl. Phys. B 2008, 90, 311–315. [3] M. Ngo, T. Nguyen-Ba, D. Dewaele, F. Cazier, W. Zhao, L. Nähle, W. Chen. Sensors & Actuators: A. Physical 2023, 362, 114654