Study On Precise And Accurate Measurement Technique Of High Reflectivity

Cavity ring-down(CRD) technique which is based on high finesse optical resonant cavity is one of high precision and high sensivity optical test techniques. Since 1970 s, the CRD technique has been adopted in several research fields, especially in the measurement of high reflectivity. It is the only method which can achieve high precision and accuracy measurement of high reflectivity so far. This thesis focuses on the engineering and instrumentation of cavity ring-down high reflectivity measurement technique, and studies the extraction of cavity decay time constant and adjustment of ring-down cavity.The extraction of cavity decay time in ideal Gaussian noise is analyzed in this thesis. At first, the analyses and comparisons of typical extraction methods are presented, where the extraction precision, accuracy, and computation cost are focused. Results show that the nonlinear least square(NLS) fitting method has the best precision and accuracy, but its computation cost is a little high. Therefore, we improve the weighted least square(WLS) fitting method to optimize the extraction performance further. Here a data point selection method in supplemented to the WLS method. As a result, fine precision and fast cavity decay time extraction is achieved, meanwhile the extraction error is quite small. Then, a signal preprocessing method which based on the spatial filter smoothing in digital image processing technique is introduced to decrease the extraction error when signal-to-noise ratio(SNR) is not high enough.Furthermore, the modulation effects of system parameters such as system response time, background noise distribution, and sampling depth of data acquiring card are considered. System response time will bring additional extraction error. However, this error can compensate the theoretical error of the WLS method after proper data point truncation. Thus the precise and accurate decay time extraction can be achieved. The distribution form of background noise is quite complex actually. There is Gaussian noise and Poisson distributed noise at least. After simulations, we find that the distribution form of background noise has little effect in decay signal analyses. If only the SNR is ensured, the extraction results are almost the same under different distributed noise. The sampling depth can effect the decay time extraction to some degree. However, we find that this effect is quite acceptable if the sampling depth is larger than 10 bits.For the adjustment of ring-down cavity, this thesis analyzes the mode coupling caused by the misadjustment of ring-down cavity. According to the mode coupling thorey, multi-transverse modes will be excited if the transverse mode of laser source does not match the ring-down cavity. So the multi-transverse mode running is analyzed by the angular propagation theory. We do not find considerable difference in fitting results of the cases of multi-transverse mode running and the fundamental transverse mode running. We conclude that the change of cavity total loss when misadjustment is not caused by the mode coupling. Then we introduce current cavity adjustment standards and explore the cavity adjustment standard proper for instrumentation and automation of cavity ring-down technique for high reflectivity measurement. In addition, we recommend the configure that based on plane-concave steady cavity for simpler cavity adjustment.All in all, this thesis focuses on the cavity decay time extraction and simply study the adjustment of ring-down cavity. It is helpful for not only the engineering and instrumentation of cavity ring-down high reflectivity measurement technique, but also the others application fields of cavity ring-down technique.