Off-axis integrated cavity output spectroscopy enhanced Faraday rotation techniques for OH detection at 2.8 &#181;m
Authors: Ngo Minh Nhut , Nguyen-Ba Tong , Zhao Weixiong , Chen Weidong 
Published: March 2022

&lt;p&gt;Faraday Rotation Spectroscopy (FRS) is well known as a useful technique for sensitive quantification of paramagnetic trace gases (O&lt;sub&gt;2&lt;/sub&gt;, OH, HO&lt;sub&gt;2&lt;/sub&gt;, NO&lt;sub&gt;2&lt;/sub&gt;, 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.&lt;/p&gt;&lt;p&gt;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 ~10&lt;sup&gt;12&lt;/sup&gt; molecules/cm&lt;sup&gt;3&lt;/sup&gt; 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 &amp;#181;m was employed for probing the Q (1.5e) and Q (1.5f) double-line transitions of the &lt;sup&gt;2&lt;/sup&gt;&amp;#928;&lt;sub&gt;3/2&lt;/sub&gt;&lt;sub&gt;&lt;/sub&gt;state at 3568.52382 and 3568.41693 cm&lt;sup&gt;-&lt;/sup&gt;&lt;sup&gt;1&lt;/sup&gt;, 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&amp;#215;10&lt;sup&gt;9&lt;/sup&gt; cm&lt;sup&gt;-3&lt;/sup&gt; 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.&lt;/p&gt;&lt;p&gt;The experimental detail and the preliminary results will be presented and discussed.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Acknowledgments. &lt;/strong&gt;The authors thank the financial supports from the EU H2020-ATMOS project, the ANR ICAR-HO&lt;sub&gt;2&lt;/sub&gt; (ANR-20-CE04-0003),&lt;strong&gt; &lt;/strong&gt;the CPER CLIMIBIO program and the Labex CaPPA project (ANR-10-LABX005).&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[1]&amp;#160; So SG, Jeng E, Wysocki G. VCSEL based Faraday rotation spectroscopy with a modulated and static magnetic field for trace molecular oxygen detection. Appl Phys B 2011;&lt;strong&gt;102&lt;/strong&gt;:279-291.&lt;/p&gt;&lt;p&gt;[2]&amp;#160; 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 &amp;#181;m, Opt. Express &lt;strong&gt;19&lt;/strong&gt; (2011) 2493-2501&lt;/p&gt;&lt;p&gt;[3]&amp;#160; 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 &amp;#956;m, EGU21-16416, EGU General Assembly 2021&lt;/p&gt;&lt;p&gt;[4]&amp;#160; Chang C-Y, Shy J-T. Cavity-enhanced Faraday rotation measurement with auto-balanced photodetection. Appl Opt 2015;&lt;strong&gt;54&lt;/strong&gt;:8526-8530.&lt;/p&gt;&lt;p&gt;[5]&amp;#160; Chen W, Kosterev AA, Tittel FK, Gao X, Zhao W. H&lt;sub&gt;2&lt;/sub&gt;S trace concentration measurements using off-axis integrated cavity output spectroscopy in the near-infrared. Appl Phys B 2008;&lt;strong&gt;90&lt;/strong&gt;:311-315&lt;/p&gt;