High resolution gas mid-infrared spectroscopy using circular multi-reflection (CMR) cell

Mid-infrared (IR) laser spectroscopy is broadly used to study trace gas species in medical diagnostics, atmospheric monitoring, remote sensing, and industrial applications. Its capability to measure fundamental rovibrational bands due to the chemical functional groups in the most relevant gas molecules allows for high instrumental sensitivity. In this work, we used a target mid-IR wavelength laser diode to measure the concentration of CO2 gas. In addition, detecting the weak mid-IR molecular absorption bands of gases like CO2 at low concentrations requires increasing optical path lengths to be used. There are a number of methods that can potentially be used to lengthen the beam path in a spectroscopic system; the most obvious being to use a longer linear gas cell, which in some situations may suffice; however, space and volume requirements need to be considered. In this work, we used a circular multi-reflection (CMR) cell, which reflects the radiation back and forth through the sample medium multiple times greatly reducing the footprint size compared to a linear cell of equivalent path length. A CMR cell is designed and constructed so that it allows 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 entering light beam are key advantages. This work will be used as the groundwork for designing an instrument for high-resolution measurement of gas abundances in planetary atmospheres.