中红外激光领域广泛使用高性能高反射光学元件，高反射率高精度测试技术是制备高性能反射光学元件的基础。针对2.7～3.0 μm波段光学元件高反射率测量的实际需求，基于量子级联激光器建立了连续光腔衰荡反射率测试实验装置，通过优选2.7～3.0 μm波段反射带内水汽吸收较弱的测试波长，分析空气中水汽吸收对衰荡时间和反射率测量的影响，并比较空气和氮气环境下反射率测量结果，实现了2.7～3.0 μm波段高反镜反射率的准确测量，在反射率约99.95%时绝对测量精度优于2×10⁻⁵。实验结果显示，采用测试波长2.9 μm并在测量时保证初始腔和测试腔腔长相同，无需使用氮气环境，直接在实验室空气环境可实现高反射率的精确测量。

The highly reflective (HR) mirrors with high-performance are widely employed in mid-infrared (mid-IR) laser systems. The manufacturing of mid-IR HR mirrors with high reflectivity requires techniques to precisely measure their high reflectivity. In this paper, a continuous-wave cavity ring-down (CRD) experimental apparatus in the 2.7-3.0 μm spectral range is established based on a quantum-cascade laser for high reflectivity measurement. By precisely optimizing the laser wavelength within the reflection band of the mid-IR HR mirrors, analyzing the influence of water vapor absorption on the ring-down time and reflectivity measurements, and comparing the reflectivity results measured under ambient air in clean-room laboratory and under nitrogen purging, the accurate measurement of high reflectivity is achieved at the 2.7-3.0 μm spectral band with an absolute reflectivity measurement accuracy of below 2×10^-5 for about 99.95% reflectivity. The experimental results demonstrate that by setting the laser wavelength precisely to 2.9 μm and employing equal lengths of initial and test ring-down cavities (RDC) to avoid the influence of water vapor’s absorption lines, the reflectivity measurement for the 2.7-3.0 μm spectral band can be performed under normal clean-room laboratory air, without the need of nitrogen purging.