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The cavity mirror in the resonant cavity is the core device of the system, defects and contamination of the cavity mirror will directly affect the accuracy of the instrument. With the use of limited cavity length in cavity ring-down spectroscopy (CRDS) technology, which is targeted to achieve tens of thousands of meters of absorption path length, is widely used in atomic absorption spectroscopy, medical diagnosis, life support systems in the space and other fields. CRDS in the use of passive resonant cavity, is more vulnerable to the scattering impacts of defects. In the measurement process, due to the accumulation of contaminants in the gas and the damage of the laser beam to the high reflectance film of the reflector, the optical path characteristics of the cavity will be affected by the scattered stray light, which will reduce the ring-down time, decrease the absorption path length in the resonant cavity, and affect the measurement accuracy of the instrument. In this paper, the impact of defect from micrometer to submicrometer magnitudes on CRDS accuracy is analyzed. The electric field characteristics in the defective resonant cavity was simulated using Comsol Multiphysics software, and it was concluded that the scattering of pits and particles shifts the cavity polarization and disrupts the cavity resonance. A standard particle deposition system, PDS, and a laser damage pit were utilized in the experiments to simulate particles and pits, respectively. The different defect categories of the cavities were calibrated by a high-precision 3D measurement system. Cavity mirrors in different defect magnitudes are sequentially combined to form a resonant cavity. By analyzing the correspondence between CRDS measurement accuracy and defects magnitude from micron to submicron, it is concluded that when the pollution particles and pit defects of the cavity mirror are larger than the critical value, the reduction of ring down time will seriously affect the accuracy of the instrument. The study of scattering correlation model of cavity defects establishes a quantitative relationship between cavity defects and CRDS measurement accuracy, which lays a foundation for the improvement of instrumental measurement accuracy.
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