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

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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.