Off-axis integrated cavity absorption spectroscopy technology is widely used in atmospheric molecular detection, environmental detection, and industrial production monitoring because of its advantages of high sensitivity, fast response, and strong anti-interference. These characteristics have attracted considerable research attention. Currently, research on off-axis integrated cavity absorption spectroscopy primarily focuses on the performance improvement of lasers, detectors, and other components and signal processing methods; this increases the complexity of the device. The detection sensitivity of the off-axis integrated cavity is primarily determined by the optical path length. Theoretically, the longer the length of the cavity, the longer the total effective optical path. However, the longer the cavity, the larger the volume of the device, and the portability and robustness are weakened. Therefore, by improving the cavity structure, this study designed an off-axis integrated cavity device with a V-shaped structure, which makes the device more compact and highly stable.

LightTools software was used to perform the optical path tracking simulation for the V-shaped cavity structure, and the optimal parameters of the V-shaped cavity were determined by analyzing the spot distribution on the surface of the highly reflective mirrors. When the arm lengths of the two arms were 25 cm, the angle between the two arms and the optimal off-axis incidence angle were confirmed by simulation, and the V-shaped cavity was designed and machined. In the experiment, a set of V-shaped structure off-axis integrated cavity absorption spectroscopy (V-OA-ICOS) measuring devices in the 2 μm band was developed, and the related measurement experiments were performed. The performance of the V-OA-ICOS device was tested with standard CO₂ gas, and the effective absorption path length and detection limit of the device were determined. Furthermore, the absorption signal of indoor NH₃ was measured at 4986.9955 cm^-1.

The simulation results (Figs. 2 and 3) indicate that the spot distribution on the surface of the highly reflective mirrors in the V-shaped cavity exhibits a uniform square distribution (Fig. 4). When the distance between the centers of lenses M₁ and M₂ exceeds 10 cm, the spot distribution no longer changes significantly. The larger the off-axis incidence angle of the laser, the more uniform the spot distribution on the mirror surface. However, an incidence angle that is excessively large would also result in a larger spot distribution range beyond the mirror surface. Therefore, the V-shaped cavity has two symmetrical arms; one side is 25 cm, and the included angle is 23.06° (Fig. 5). A V-OA-ICOS experimental device was built with this V-shaped cavity (Fig. 6), and a series of experiments was carried out on the device with CO₂ standard gas with a volume fraction of 400×10^-6. When the pressure in the cavity is 12 kPa, a CO₂ absorption signal is obtained at 4991.258 cm^-1 (Fig. 8). According to the calculations, the reflectivity of the cavity mirror is 99.947%. The device was measured for a long time of 50 min, and the device exhibits high stability. The Allan variance indicates that the detection limit of the device is 0.35×10^-6 (Fig. 9) when the average time is 375 s. To further verify the weak detection ability of the device, indoor NH₃ was measured at 4986.9955 cm^-1 (Fig. 10). The Savitzky-Golay algorithm, with a fitting order of 2 and a window width of 60, was selected to smooth the obtained NH₃ signal (Fig. 11), and the signal-to-noise ratio increases to 46.06 (Fig. 12).

In this study, a new V-OA-ICOS device was developed. LightTools software was used to simulate and optimize the optical path, and the distribution of spots on the surface of the highly reflective mirrors in the V-shaped cavity is uniform square, which makes the surface utilization of mirrors higher. Moreover, the optimal angle of the cavity is 23.06° when the V-shaped structure is symmetrical, the length of one arm is 25 cm, and the V-OA-ICOS experimental device was developed. The CO₂ standard gas with a volume fraction of 400×10^-6 was used to test the device, and the reflectivity of the mirrors was calibrated to 99.947%. Subsequently, the stability of the device was tested for 50 min, and the volume fraction fluctuation is within ±10×10^-6. The Allan variance results show that the detection limit of the device can reach 0.35×10^-6 when the average detection time is 375 s. The indoor NH₃ absorption signal was measured using the fabricated device. The Savitzky-Golay algorithm was used to smoothen it, and the signal-to-noise ratio improves from 23.96 to 46.06. Compared with the traditional OA-ICOS device, the V-OA-ICOS device has higher stability and compactness, exhibiting excellent detection ability, which provides a choice for the development and application of trace gas sensors in different scenarios.