Using cavity ring-down spectroscopy technique, the precise measurement device for the high reflectivity at 1.315μm has been built. Tested reflectivity of high-reflecting mirror is as high as 99.979%. The precision of measurement is at the 10<sup>-5</sup> level. For further investigation, some factors such as the instability of laser source, the cavity length, the diaphragm aperture and detector position, the mode match, the reflectivity of cavity mirror, which may affect the precision of measurement are studied. Our study will provide a theoretical and experimental reference for further improvement of the measuring accuracy.
Cavity Ring-Down Spectroscopy (CRDS) is a highly sensitive loss measurement technique. It was used for quantitative diagnostics of molecular species. In our experiment CRDS has been applied for measuring the reflectivities of optical mirrors. An experimental system has been established. The reflectivities of a pair of cavity mirrors and of a 22.5° optical mirror have been measured. The reflectivities of the cavity mirrors are (99.925±0.001) % and that of the 22.5° mirror is (99.992±0.003) %<sub>°</sub> The measurement error was as low as 0.001%.