Reflectivity measurement of highly reflective mirrors at spectral band of 2.7−3.0 μm
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摘要: 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。Abstract: 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.
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Key words:
- mid-infrared /
- highly reflective mirror /
- reflectivity /
- cavity ring-down /
- absorption of water vapor
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图 1 常温常压下空气中水汽的吸收谱线图
Figure 1. Absorption spectrum of water vapor in air at 296 K temperature and 0.1 MPa pressure
图 2 实验装置图。AOM：声光调制器
Figure 2. Schematic diagram of experimental setup for high reflectivity measurement at 2.9 μm. AOM: Acoustic-Optic Modulator
图 3 典型光腔衰荡信号及其单指数拟合曲线
Figure 3. A typical cavity ring-down signal and the corresponding single-exponential fit measured at 2.9 μm
图 5 腔镜表观反射率与初始腔腔长的关系
Figure 5. Average nominal reflectivity of the cavity mirrors measured versus the cavity length of the initial cavity
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