Highly sensitive TDLAS gas sensor based on a dual-curvature Herriott-type multi-pass gas cell

Tunable diode laser absorption spectroscopy (TDLAS) gas sensors are highly suitable for rapid and accurate gas detection. The challenge for the TDLAS gas sensors is to convert the interaction between light and gas into a sensitive sensor signal within a confined space. One effective method to address this challenge has come primarily from the design of miniaturized multi-pass gas cells (MPGCs) to extend effective optical path length (EOPL). In the conventional Herriott-type MPGC design, the utilization rate of the mirror surface and the integrity of the light spot are both limited due to the use of a single-curvature mirror. Here, we propose a dual-curvature mirror design strategy for the Herriott-type MPGCs, featuring a unique three-concentric-circle spot pattern. The ray-tracing simulations were conducted based on an optical transmission model that quantitatively relates mirror spacing, incident beam parameters, and the resulting spot distribution. The optimal MPGC prototype exhibited an EOPL of 32.05 m within a volume of 155.13 mL, thereby achieving a high ratio of optical path length to volume of 20.66 cm⁻². Through a combination of wavelength modulation spectroscopy, we have developed a methane TDLAS gas sensor with a detection limit as low as 33.78 ppb. The dual-curvature Herriott-type MPGCs offer a new degree of freedom for the miniaturization of highly sensitive TDLAS gas sensors.