本文从测试腔长与初始腔长不一致、探测器耦合方式分析了连续光腔衰荡技术测量超高反射率元件损耗的误差。测量结果表明，在635 nm 波长，常规1 000 级超净实验室空气损耗系数约10 ppm/m，当测试腔长比初始腔长长20 cm 时，导致测试样品损耗测量结果偏大约2 ppm。另外，当采用交流(AC)耦合方式探测CRD 信号时将使衰荡信号波形失真，通过实验对比直流(DC)耦合和AC 耦合结果，发现采用AC 耦合将使损耗测量结果偏低约2 ppm。实验结果与分析表明测量超高反射率元件损耗时应保证测试腔腔长与初始腔腔长一致，并且采用足够带宽的直流耦合探测方式。

The loss measurement error caused by difference between the test cavity length and initial cavity length and by detection coupling scheme are analyzed. Measurement results show that even in class 1 000 clean-room laboratory, at 635 nm wavelength, the air loss coefficient is approximately 10 ppm/m. When the length of the test cavity is approximately 20 cm longer than that of the initial cavity, the air loss caused approximately 2 ppm error to the measurement of the optical loss of the high-reflectivity optics. For CRD signal detection, the use of Alternating Current (AC) coupling scheme distorts the measured ring-down signal waveform, results in loss measurement error of approximately 2 ppm as compared to the Direct Current (DC) coupling scheme. The analysis and measurement results show that for the measurement of the optical loss of ultra-high reflectivity optics with CRD technique, the same lengths for the initial cavity and test cavity should be used and the DC coupling detection scheme with sufficient bandwidth should be employed.