Ring-down cavity adjustment based on the circumscribed ellipse criterion of the cavity transmission spot

Abstract

In the application of cavity ring-down (CRD) high reflectivity measurement, precise adjustment of ring-down cavity (RDC) is of great importance. The transmission spot shape monitoring, which contains the judgment of the mode order and nodal direction of intracavity transverse mode, is an important method for cavity adjustment. In this paper, several criterions of circumscribed spot shape are compared. After spot binarization, the circumscribed circle, circumscribed rectangle, and circumscribed ellipse of transmission spot is analyzed, respectively. The theoretical comparison shows that the circumscribed ellipse criterion can effectively distinguish the mode order and nodal direction simultaneously. The spot can be recognized as the fundamental transverse mode when the area ratio of the spot to its circumscribed ellipse is in the range of 0.9 to 1.0. This method is tested by an experimental setup. It is found to be an efficient cavity adjustment method for the CRD technique.