Minimizing the influence of higher-order transverse modes on the precision of cavity ring-down measurements. (February 2023) - Wu, Meiyu
Highlights: It is experimentally found that the excitation of higher-order transverse modes degrades the precision of cavity ring-down (CRD) measurements. Simple measures such as mode-matching optics, aperture, and trigger threshold are used to minimize the influence of higher-order mode excitation on CRD measurement precision. Less than 0.2ppm uncertainty in reflectance measurement with CRD was obtained. Abstract: In cavity ring-down (CRD) measurements, the excitation of higher-order transverse modes and their random occurrence together with the fundamental mode inside the ring-down cavity (RDC) inevitably induce measurement errors of the decay time constant, which lead to a reduction in the measurement precision of the CRD instrument. To analyze and minimize the influence of higher-order transverse mode excitations on the CRD measurement precision, in this paper we report on the experimental observations of the impact of mode matching lens, aperture inside the cavity and trigger threshold on the excitation probability of fundamental and higher-order transverse mode excitations. The theoretical analysis and experimental results showed that when the optical misalignment is negligibly small, the occurrence probability of higher-order transverse mode excitations can be effectively suppressed by using a single lens between the laser and RDC to match preferably the fundamental model of the laser beam into the RDC, or inserting an aperture of an appropriate diameter inside the RDC, or using a high trigger threshold to record only CRD events with high amplitudes, or the combination of these approaches. For example, when the mode-matching lens is located at the optimal experimental position, and combined with the 2.2 mm diameter aperture, a 100% suppression of the excitation of high-order transverse modes can be achieved. Our results demonstrated that these methods effectively eliminated the decay time constant determination errors caused by the higher-order transverse mode excitations and therefore improved the precision of CRD experiments for high reflectance measurements. In practice the combination of mode-matching and appropriate trigger threshold setting is preferable for highly precise and yet fast CRD measurements.