Off-axis integrated cavity output spectroscopy enhanced Faraday rotation techniques for OH detection at 2.8 µm

Faraday Rotation Spectroscopy (FRS) is well known as a useful technique for sensitive quantification of paramagnetic trace gases (O₂, OH, HO₂, NO₂, etc) within shorter optical paths compared to direct absorption techniques [1,2]. Combination of long path absorption approach [3,4] with balanced detection based FRS (BD-FRS) [4] allows further enhancement of the measurement sensitivity.

We report in this paper the development of an off-axis integrated cavity output spectroscopy (OA-ICOS) [5] enhanced BD-FRS instrument for OH radical measurements. OH radicals with concentration of ~10¹² molecules/cm³ were generated by continuous microwave discharge at 2.45 GHz of water vapor at low pressure (~ 0.6 mbar) and were used for performance characterization of the developed instrument. A distributed feedback (DFB) interband cascade laser (ICL) operating at 2.8 µm was employed for probing the Q (1.5e) and Q (1.5f) double-line transitions of the ²Π_3/2state at 3568.52382 and 3568.41693 cm^-1, respectively. OA-ICOS method was used for determination of OH concentration. OA-ICOS was coupled to BD-Faraday rotation technique (OA-ICOS FRS) to enhance the sensitivity of OH monitoring. A 1s detection limit of ~ 9.3×10⁹ cm^-3 was obtained for an averaging time of 20 s, which is 7 times better than that obtained by OA-ICOS approach. Moreover, the OA-ICOS FRS approach exhibits the specific advantage of interference-free of close-by (non-paramagnetic) water vapor absorption.

The experimental detail and the preliminary results will be presented and discussed.

Acknowledgments. The authors thank the financial supports from the EU H2020-ATMOS project, the ANR ICAR-HO₂ (ANR-20-CE04-0003), the CPER CLIMIBIO program and the Labex CaPPA project (ANR-10-LABX005).

References

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[2]  Zhao W, Wysocki G, Chen W, Fertein E, Le Coq D, Petitprez D, and Zhang W, Sensitive and Selective Detection of OH Free Radical using Faraday Rotation Spectroscopy at 2.8 µm, Opt. Express 19 (2011) 2493-2501

[3]  Minh N. Ngo, Tong N. Ba, Denis Petitprez, Fabrice Cazier, Weixiong Zhao, and Weidong Chen, Measurement of OH radicals using off-axis integrated output spectroscopy (OA-ICOS) at 2.8 μm, EGU21-16416, EGU General Assembly 2021

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