Carbon dioxide (CO₂) and methane (CH₄) are major atmospheric greenhouse gases. In recent years, due to the continuous development of human activities and industrial production, the global greenhouse effect caused by the rising levels of CO₂ and CH₄ in the atmospheric environment has seriously affected human life and health. Therefore, accurate concentration detection is of significance for both environmental monitoring and human health. The off-axis integrating cavity output spectrum technology (OA-ICOS), has been widely concerned for its simple experimental operation, strong anti-interference ability, high sensitivity, and in-situ real-time measurement. It is often adopted in atmospheric and environmental science, medical diagnosis, industrial production engineering, and other research fields. Even when the incident light is fully off-axis, OA-ICOS still has some residual cavity modes that cannot be eliminated and become the main noise of the system. Injecting a radio frequency (RF) noise source into a laser current is a new method to suppress cavity modes. We optimize the CO₂ and CH₄ dual gas sensing system of OA-ICOS using an RF signal source.

We study the influence of different power RF noise sources on CH₄ absorption signals and select the best RF noise source power. In realizing simultaneous measurement of CO₂ and CH₄, the output lights of the two DFB lasers are combined into a beam through a fiber coupler and coupled into the cavity. However, the time-division multiplexed scanning signal is designed by software to realize multiplex signal transmission and avoid interference between the signals during measurement. The RF noise source is injected into the near-infrared (NIR) distributed feedback laser, and the time-division multiplexing method is employed to collect dual gas signals at the same time for maximizing the signal-to-noise ratio (SNR) of the signals and the detection limit of the system. Meanwhile, we establish an OA-ICOS sensing system combined with TDM-DAS.

As shown in Table 1, the influence of different RF noise power values on the CH₄ absorption spectrum is studied by analyzing SNR and absorption line width of the CH₄ absorption spectrum. Considering the influence of SNR and broadening on absorption lines, -20 dBm is chosen as the white noise power of the system. The OA-ICOS systems without a noise source and with a -20 dBm RF noise source are utilized to measure CO₂ and CH₄ continuously for a long time. The stability and measurement accuracy of the two OA-ICOS systems are evaluated according to the experimental results of the two groups. As shown in the left side of Fig. 5(a) and Fig. 6(a), the average volume fraction of CO₂ concentration is 5.8157×10^-4 and 5.9895×10^-4 in the system without a noise source and with an RF noise source. The average volume fraction of CH₄ is 2.24×10^-6 and 2.25×10^-6. As shown in the right side of Fig. 5(a) and Fig. 6(a), the measurement accuracy of CO₂ is 40.5200×10^-6 and 14.7500×10^-6 in the system without a noise source and with an RF noise source, respectively. The measurement accuracy of CH₄ is 0.2716×10^-6 and 0.0997×10^-6 in the system without a noise source and with an RF noise source, respectively. Compared with the system without an RF noise source, the system measurement accuracy of the system with an RF noise source is increased by 2.74 times. Figs. 5 (b) and 6(b) show the analysis of Allan variance results. In the systems without a noise source and with an RF noise source, the CO₂ detection limits at 1000 s are 1.85×10^-6 and 5.50×10^-7, with the detection limits of CH₄ at 1000 s being 1.61×10^-8 and 5.78×10^-9. The Allan variance values of CH₄ and CO₂ are always lower for OA-ICOS with an RF noise source than OA-ICOS without a noise source, and the system detection limit is at least three times higher. Adding an RF noise source can improve the stability and detection limit of the OA-ICOS system. Under the average time of 5 s, the NEAS of CH₄ and CO₂ in the system without adding a noise source is 4.98×10^-9 cm^-1·Hz^-1/2 and 2.14×10^-9 cm^-1·Hz^-1/2 respectively. By adding an RF noise source, the NEAS in the system for CH₄ and CO₂ is 1.70×10^-9 cm^-1·Hz^-1/2 and 1.07×10^-9 cm^-1·Hz^-1/2 respectively.

We present a near-infrared OA-ICOS dual gas detection sensing system for continuous and real-time CO₂ and CH₄ detection. By adding an RF noise source to the laser drive current, the cavity mode noise is suppressed, and the SNR, accuracy, and measurement sensitivity of the OA-ICOS system are enhanced. The results show that the measurement accuracy of the OA-ICOS system with an RF noise source is improved by a factor of 2.74 relative to that of the system without a noise source. According to the analysis of Allan variance results, in OA-ICOS systems with an RF noise source, the Allan variance values of CO₂ and CH₄ are always better than those without noise sources, and the detection limits of CO₂and CH₄ at 1000 s are 5.50×10^-7 and 5.78×10^-9. The system detection limit is at least three times higher than that without noise sources. Under the average time of 5 s, the noise equivalent sensitivities of CH₄ and CO₂ in the system with an RF noise source are 1.70×10^-9 cm^-1·Hz^-1/2 and 1.07×10^-9 cm^-1·Hz^-1/2 respectively. Additionally, the CH₄ and CO₂ concentrations in the atmosphere are continuously monitored for four days to verify the stability and reliability of this system.