只考虑腔长失调因素下建立了反射率模拟测量的理论模型.根据高斯光束传输规律分析了腔长失调对衰荡腔模式耦合的影响,推导了腔长失调与谐振腔输出脉冲信号、衰荡信号与反射率之间的关系,模拟了腔长失调在±10mm范围内的光脉冲衰荡现象.结果表明:对于光敏面直径为ф0.2mm的高速探测器,为了保证10^-6的测量精度,腔长的失调量应控制在±1mm之间.在光路调节中采用具有对数变换功能的示波器和动态范围较大的探测器,可以提高测量精度.

According to experimental parameters, a theory model is made up to simulate measurement of reflectivity for length misadjustment of ring-down cavity. According to propagation equation of Gaussian beam, the influence of length misadjustment of ring-down cavity on its mode coupling is investigated, and the equation is given for relation between length misadjustment and output power, then the relation between ring-down signal and reflectivity is deduced from output characteristic of optical cavity. Applying this model, it is simulated that ring-down signal changes with length misadjustment of -10～10mm. The relation between metrical precision and length misadjustment is also numerically simulated. The analysis shows that the metrical precision drops rapidly with length misadjustment. To achieve metrical precision of 10^-6, length misadjustment should be restricted within -1～1mm for those fast detectors, whose sensitive area diameter is only 0.2mm. It is suggested that logarithm transform of oscillograph and rge dynamic range of detector should be applied to adjust the optical cavity for improving metrical precision.