Study On Cavity Adjustment Method Of Cavity Ring-down High Reflectivity Measurement

In recent years,with the continuous improvement of high-reflectivity?reflectivity>99.9%?coating technology,the application fields of high-reflectivity optical components have become more and more extensive.How to accurately measure the reflectivity of high-reflectivity optical components has become the focus of researchers.Optical cavity ring-down method is currently the only technical method that can accurately measure reflectivity higher than 99.99%.In the process of measuring reflectivity by the optical cavity ring-down method,due to the conversion between the initial cavity and the test cavity,it inevitably leads to relative misalignment of the cavity parameters.The existing process of adjusting the ring-down cavity relies heavily on manual cavity adjustment,not only the low efficiency,and the evaluation index of cavity adjustment is not uniform,the process of cavity adjustment is not standardized,and there are human subjective effects,making the cavity adjustment problem a technical bottleneck for the automation and instrumentation of the current optical cavity ring-down high reflectivity measurement technology.This paper is based on overcoming the problem of relative misalignment of cavity parameters caused by the conversion of the initial cavity and the test cavity structure,to improve the efficiency of cavity adjustment,and to achieve fast and precise cavity adjustment as the technical goal.The research on the cavity adjustment method for optical cavity ring-down high reflectivity measurement is carried out.In this paper,various phenomena caused by cavity misalignment are systematically described,including the effects of cavity misalignment on the peak value of the transmitted signal of the ring-down cavity,the fitting curve,the mode of the transmitted spot of the ring-down cavity,and the ring-down time.The reasons for the different sensitivity of the spot changes caused by adjusting M₁ and M₂ are analyzed.It is proposed that the aspect ratio of the spot and its area ratio to the circumscribed rectangle are used as the basis for cavity adjustment.Combining simulation and experiment proves the feasibility of the aspect ratio and the area ratio as the indicators to determine the shape of the spot.A optical cavity ring-down high reflectivity measurement experimental device was established.According to the different sensitivity of M₁ and M₂ to the change of spot morphology,a detailed cavity adjustment strategy combining coarse and fine adjustments was formulated,and the quantitative criterion for judging the spot as the fundamental transverse mode is determined.The two-dimensional adjustable frame is used to adjust the angle of the cavity mirrors to achieve the suppression of high-order transverse mode excitation,so that the transmitted spots of the initial cavity and the test cavity are kept in the fundamental transverse mode state,so as to ensure that the losses of the initial cavity and the test cavity are as uniform as possible.The effectiveness of this method is proved by measuring the ring-down time constant under different cavity lengths and the average reflectivity of the cavity mirror.Compared with other cavity adjustment methods,this method reduces the influence of human factors,the repeatability error of the reflectivity measurement results is low,and has the advantages of simplicity and high efficiency.It plays a certain role in promoting the development of instrumentation for optical cavity ring-down high reflectivity measurement technology.