Point diffraction interference system for the optical element evaluation of the extreme wavelength in the next generation

近年来，利用极紫外波段高亮度同步辐射的技术创新正在积极推进。为了有效地利用这种光，与传统可见光相比，开发生产和评估光学元件表面精度超过2位数的技术是很重要的。在传统干涉仪中，测量的是测试面与标准参比面之间的像差。因此，标准曲面的精度决定了测量精度。为了达到绝对的测量精度，有必要开发一种具有极高固有精度的新型干涉仪。点衍射干涉仪是基于衍射的，它允许使用半径与光波长相当的圆形孔径在特定的数值孔径上产生完美的球形波前。通过条纹扫描，可以显著提高干涉条纹的读取精度。本文介绍了由单模光纤和利用可见光激光源进行条纹扫描组成的点衍射干涉系统。可以得出以下结论。(1)得到了单模光纤的完美球面波前。(2)采用光纤，构建了多自由度、小检测元件的光学系统。(3)条纹扫描提高了干涉条纹的读取精度。(4)测量了商用球面反射镜与完美球面的像差，精度为nm。

Recently, the technological innovation using the extreme ultraviolet wavelength with high brightness of synchrotron radiation is actively advanced.In order to effectively utilize such light, it is important to develop the technology that produces and evaluates the optical element with surface accuracy over the 2 digits compared to conventional visible light. In the conventional interferometer, the aberration between the test surface and the standard reference surface is measured. Therefore the accuracy of standard surface determines the measurement accuracy. To achieve the absolute measurement accuracy, it is necessary to develop a new interferometer that has inherent very high accuracy The point diffraction interferometer is based on diffraction, which permits the generation of a perfect spherical wavefront over a specific numerical aperture by using a circular aperture with a radius comparable to the wavelength of light. Reading accuracy of the interference fringe is remarkably improved by the fringe scanning.This report describes the point diffraction interference system, which consisted of single-mode optical fiber and the fringe scanning using the laser source of visible light. The following conclusions can be drawn.(1) A perfect spherical wavefront from the single mode optical fiber was obtained.(2) By using optical fiber, the optical system with many degrees of freedom with small detecting element was constructed.(3) Reading accuracy of the interference fringe was improved by the fringe scanning.(4) The aberration from perfect spherical surface of a commercial spherical mirror was measured at the accuracy of the number nm.