Ppb-level CO2 detection with miniaturized gas absorption cell based on low-loss flexible infrared hollow waveguide

The flexible mid-infrared hollow waveguide is used as the absorption cell for spectroscopic gas sensing. The low-loss bending shape is proposed to achieve a limit of detection (LOD) of ppb level. Simulation and experimentation demonstrate that the spring-shaped waveguide cell, characterized by a single rotation axis, exhibits reduced loss compared to the bending shapes with variable axes. The loss properties were comprehensively analyzed, and the loss spectra were measured using a Fourier transform infrared spectrometer (FTIR) system. To assess the practical application of the proposed spring-shaped waveguide cell, a CO2 sensing system was established employing a low-cost broad-spectrum source and Fabry-Pérot (FP) pyroelectric detector. The fabricated waveguide cell has an inner diameter (ID) of 2 mm and a length of 119 cm. Experimental results show that the limit of detection (LOD) as low as 349 ppb is achieved, and further improvement can be obtained by increasing the ID. With a weight of merely 2.24 g and a radius of 5 cm, the spring-shaped hollow waveguide represents the most compact and portable waveguide gas cell to our best knowledge. The robustness of the spring-shaped waveguide cell to pitch allows for flexible changes in length and curvature radius, thereby facilitating easy adjustment of sensitivity and size without requiring the custom-made frames. Moreover, the hollow nature of the spring design significantly overcomes the space limitation, further enhancing the practicality of the waveguide cell.