RESUMO
Absorption cells with circular geometry are a class of multipass reflection cells consisting of a single, circular mirror. They can be particularly favorable for trace gas measurements because of their mechanical robustness, simplicity, and their optical versatility. In this article, we present detailed theoretical considerations and ray tracing simulations for the optimization of the optical design of circular multipass reflection cells. A parabolic mirror shape in a confocal arrangement is found to be most suitable for long optical paths in a small volume. We experimentally demonstrate more than 12 m optical path in a 14.5 cm diameter gas cell and NO2 concentration measurements in ambient air with a measurement precision better than 0.1 ppb.
RESUMO
High precision mobile sensing of multi-species gases is greatly demanded in a wide range of applications. Although quantum cascade laser absorption spectroscopy demonstrates excellent field-deployment capabilities for gas sensing, the implementation of this measurement technique into sensor-like portable instrumentation still remains challenging. In this paper, two crucial elements, the laser driving and data acquisition electronics, are addressed. Therefore, we exploit the benefits of the time-division multiplexed intermittent continuous wave driving concept and the real-time signal pre-processing capabilities of a commercial System-on-Chip (SoC, Red Pitaya). We describe a re-designed current driver that offers a universal solution for operating a wide range of multi-wavelength quantum cascade laser device types and allows stacking for the purpose of multiple laser configurations. Its adaptation to the various driving situations is enabled by numerous field programmable gate array (FPGA) functionalities that were developed on the SoC, such as flexible generation of a large variety of synchronized trigger signals and digital inputs/outputs (DIOs). The same SoC is used to sample the spectroscopic signal at rates up to 125 MS/s with 14-bit resolution. Additional FPGA functionalities were implemented to enable on-board averaging of consecutive spectral scans in real-time, resulting in optimized memory bandwidth and hardware resource utilisation and autonomous system operation. Thus, we demonstrate how a cost-effective, compact, and commercial SoC can successfully be adapted to obtain a fully operational research-grade laser spectrometer. The overall system performance was examined in a spectroscopic setup by analyzing low pressure absorption features of CO2 at 4.3 µm.