Modeling the optical field in off-axis integrated-cavity-output spectroscopy using the decentered Gaussian beam model

Off-axis integrated-cavity-output spectroscopy (OA-ICOS) is widely used in trace gas detection and isotopic analysis for its enhanced sensitivity as well as robustness against optical instability. However, its sensitivity is ultimately limited by the spurious coupling noise formed in the cavity, and much of the design and optimization process relies on empirical iterations while quantitative analysis is lacking. In this paper, we develop a method to model the optical field in OA-ICOS based on the decentered Gaussian beam model, which is a generalization for large tilting angles as compared with previously developed models. From the optical field, the cavity transmission spectrum for different cavity configurations or input beam conditions can be calculated, and the fringe noise level can be derived. Results show that an optimum combination of input laser beam and off-axis alignment exists to fully suppress the interference fringes. Factors affecting the fringe noise level, including a mismatch between the input beam and the cavity, optical alignment conditions, and deviation from the re-entrant condition, are studied thoroughly. The developed method can serve to guide the design and optimization of OA-ICOS systems.