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Carbon dioxide (CO2) is an atmospheric trace gas, and its accurate sensing is therefore of great interest. Optical sensors exploiting the mid-infrared (mid-IR) light absorption of CO2 provide high sensitivity and are widely used in medical diagnostics, atmospheric monitoring, remote sensing, and industrial applications. In this work, we demonstrated an accurate CO2 gas sensing at 4.2 µm wavelength. In addition, detecting the weak mid-IR molecular absorption bands of gases at low concentrations requires increasing optical path lengths to be used. The most obvious method that can expand the potential beam path in a spectroscopic system is to use a longer linear gas cell, which in some situations may be adequate; however, space and volume requirements need to be considered. In this work, a circular multi reflection (CMR) cell was used to reflect the radiation back and forth through the sample medium multiple times, thus greatly reducing the size footprint compared to a linear cell of equivalent optical path length. A CMR cell was designed and constructed to allow multi-reflections within the cell. The optical alignment of the cell and the convenience of changing the optical path length by adjusting its position with respect to the incident light beam were also used to maximize the advantages of the device. This work will be used as the groundwork for designing an instrument for the high-resolution measurement of CO2 gas in planetary atmospheres.
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