Theoretical study of <sup>14</sup>CO<sub>2</sub> spectra based on sub-Doppler wm-NICE-OHMS
Authors: LI Yong  , ZHOU Xiaobin  , ZHAO Gang  , YING Runtao  , YANG Jiaqi  , YAN Xiaojuan  , MA Weiguang  ,   1 State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan 030006, China;,   2 Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: January 2025

The massive emission of greenhouse gases, particularly CO<sub>2</sub>, has led to severe damage to the Earth's ecological environment and poses a threat to human health. Many countries have therefore proposed policies to endeavor to curb the greenhouse effect. Carbon monitoring is a critical prerequisite for realizing these goals, and performed under the simulated experimental conditions. The results of the simulation show that the Doppler broadened spectral signal of <sup>14</sup>CO<sub>2</sub> is covered by the other gases' signals with a very low amplitude, which is adverse to the detection of <sup>14</sup>CO<sub>2</sub>. To eliminate the linear slope of the Doppler broadened signal and to further improve the signal-to-noise ratio, we perform <sup>14</sup>CO<sub>2</sub> spectral measurements using wavelength-modulated NICE-OHMS (wm-NICE-OHMS). The results of the simulation show that the spectral lines are effectively separated, and the detection accuracy of the <sup>14</sup>CO<sub>2</sub> ratio is greatly improved. Finally, we analyzed the effects of pressure and modulation index on the <sup>14</sup>CO<sub>2</sub> wm-NICE-OHMS signal. The results show that when the pressure is 42 mTorr and the modulation index is 1.07, the signal amplitude of wm-NICE-OHMS reaches its maximum. This work establishes a theoretical foundation for the high precision detection of <sup>14</sup>CO<sub>2</sub> in real-time environmental monitoring. The potential for large-scale application of wm-NICE-OHMS in carbon emission tracking is highlighted, providing a more cost-effective alternative to traditional detection methods. Furthermore, the technology is able to suppress spectral interference from other gases and achieve high resolution in <sup>14</sup>CO<sub>2</sub> measurements, which will greatly help in monitoring and reducing greenhouse gas emissions. tracking carbon emission sources can support the precise implementation and advancement of related policies more effectively. The contribution of fossil fuel combustion to greenhouse gas emissions can be inferred by detecting the abundance of <sup>14</sup>C in carbon dioxide in a specific region. Conventional <sup>14</sup>CO<sub>2</sub> detection methods have significant drawbacks, including complicated operation, high cost and large equipment size. Laser absorption spectroscopy (LAS) offers advantages such as real-time, online in-situ measurement and simple operation, making it suitable for the online detection of isotopes. Among the various LAS techniques, Noise Immunity Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS) stands out as the most sensitive. This methods integrates Frequency Modulation Spectroscopy (FMS) into Cavity Enhanced Spectroscopy (CES), suppresses the low-frequency noise and at the same time increases the absorption paths, providing a minimal detectable absorption coefficient as low as 10<sup>-13</sup>. Additionally, the buildup of high intracavity laser power in NICE-OHMS can stimulate saturation absorption, which has a narrow spectral width that can mitigate spectral overlap. In this work, we modeled the <sup>14</sup>CO<sub>2</sub> spectral signals at different locations and selected the transition line of <sup>14</sup>CO<sub>2</sub> at 2209.108 cm<sup>-1</sup> as the optimal measurement target based on the principles of high-intensity and well-resolution. The theoretical analysis of the NICE-OHMS technique was then carried out and theoretical simulations of a mixed sample of <sup>14</sup>CO<sub>2</sub> and its nearby interfering gases (<sup>13</sup>CO<sub>2</sub>, <sup>12</sup>CO<sub>2</sub>, and N<sub>2</sub>O), were