Study on Absolute Flatness Measurement of Silicon Single Crystalline Plane Mirrors Using Near-Infrared Interferometry

单晶硅平面镜是同步辐射光学元件中广泛使用的材料。本研究的目的是通过使用近红外激光器和探测器进行三平面法来测量这些反射镜的纳米精度的绝对平面度。硅对可见光是不透明的，这使我们无法使用可见光和硅作为透明参考进行干涉测量。我们研制了近红外移相斐索干涉仪，其中近红外（λ =1315 nm）半导体激光器和近红外CCD相机分别是光源和光探测器。近红外光对硅是透明的，这使得我们能够用硅平面镜作为透明参考来观察干涉条纹。然而，硅即使在近红外区域也具有高折射率。高反射率引起多光束干涉。这会使干涉条纹失真，从而导致 ...更多信息 o相移方法的测量误差。因此，作为抗反射膜的二氧化硅已经形成在反射镜上，以实现双光束干涉。其结果是，我们已经证实，干涉条纹的失真得到抑制。为了考察三平面法的测量精度，采用可见光三平面法测量了三个光学平面的中心线直线度。每个光学平面的绝对直线度精度为5 nm（峰谷高度）。作为下一步，平面度测量已被执行的近红外相移干涉法，其中干涉条纹的变化与扫描的参考沿着光轴被记录。为了抑制硅平面镜上的多光束干涉，我们注意到干涉条纹的对比度随腔长的增加而降低。通过对腔长的控制，实现了接近理想的双光束干涉。并且干涉测量的再现性为6.38nm（峰谷高度），低于λ/200。少

Single-crystalline silicon plane mirrors are widely used for the optical elements of synchrotron radiation. The purpose of this study is to measure the absolute flatness of those mirrors at nanometric accuracy by performing a three flat method with a near-infrared laser and a detector. Silicon is opaque to visible light, which prevents us from performing interference measurements with visible light and silicon as a transparent reference. We have developed the near-infrared phase-shifting Fizeau interferometer in which a near-infrared (λ =1315 nm) semiconductor laser and a near-infrared CCD camera is a source of light and a light detector, respectively. The near-infrared light is transparent to silicon, which enables us to observe interference fringes with a silicon plane mirror as a transparent reference. However, silicon has high refractive index even in the near-infrared region. A high reflectance induces a multiple-beam interference. This distorts interference fringes, which leads t … More o the measurement error of the phase-shifting method. Hence silicon dioxides acting as an anti-reflection film have been formed on the mirror in order to achieve a two-beam interference. As a result, we have confirmed that the distortion of interference fringes is suppressed. In order to investigate the measurement accuracy of a three flat method, the straightness of the center lines of three optical flats has been measured by the three flat method with visible light. And the accuracy of absolute straightness of each optical flat is 5 nm (peak-to-valley height). As a next step, flatness measurements have been performed by the near-infrared phase-shifting interferometry in which interference fringes changing with the scan of the reference along the light axis are recorded. In order to suppress the multiple-beam interference at silicon plane mirrors, we have paid attention to the fact that the contrast of fringes lowers as the cavity length becomes long. By controlling the cavity length, we have achieved a nearly ideal two-beam interference. And the reproducibility of the interferometry is 6.38 nm (peak-to-valley height) that is lower than λ/200. Less