CAREER: Breaking the Freeform Optics Metrology Barrier with Synthetic Wavelength Interferometry

自由曲面光学器件使用复杂的表面，这些表面不一定是轴对称的，可能非常复杂，并且具有许多应用。一个持续的问题是这种表面的测量（计量学）。现有的计量解决方案使用干涉测量法，但是遭受条纹可见度问题和其他误差，而诸如坐标测量机和轮廓测量法的触摸系统具有不足的准确度（即，与纳米尺度相对的微米尺度），并且可能损坏光学表面，尤其是对于较软的材料。PI将执行多方面的研究计划，旨在改善自由曲面光学的计量。他的方法的核心是两个光源的组合，每个光源具有不同的波长，以评估透镜的表面轮廓。这种方法将导致高纵横比表面的测量，其中干涉条纹通常间隔得太近而无法彼此区分。这通过考虑跟踪和回扫（在与光源相反的方向上经过相同的路径）以及两个测量之间的差异来增强。该方法使用将用于提取所需几何形状的数学模型。其结果是一种方法，可用于测量自由曲面光学，以达到比目前可能的更高的分辨率。该学院早期职业发展（CAREER）计划研究项目的一个广泛影响是为光学设计师和制造商提供可以测量先进自由曲面光学器件的计量方法。自由曲面光学器件是未来基于视觉的技术和轻便、便携、不显眼的移动的设备的基础。潜在的设备范围从用于监测环境变化的小型卫星到用于自动驾驶车辆的更好的光学传感器。这项研究将通过指导研究助理，将研究纳入本科生体验式学习，并与高中生进行外联，在大学一级产生教育影响。PI通过高中外展，通过研讨会，并通过广泛的播客和博客活动进行外展。这项研究将解决这个基本的研究问题：合成波长干涉测量（SWI）可以用来产生新的使能能力，在测量自由曲面光学？PI将检查（理论上和实验上）透射球f数和局部斜率偏离对渐晕的影响，以测量通常具有条纹模糊和回扫误差的光学器件。这也将通过应用合成波长干涉法（SWI），其中两个光波长相结合，使相位差可以检查和限制，传统的干涉法可以确定和改进。 该技术的关键方面是，可以生成合成波长，以解决长度可能远大于本构波长的条纹模糊性。这对于局部斜率偏离高的光学器件尤其重要。干涉仪中密集条纹图案的采样不足会导致条纹模糊，从而在测量中产生混叠。这些模糊性表现为来自高斜率偏离光学表面（如陡峭非球面和自由形式光学器件）的超过奈奎斯特采样限制。在这项研究中的调查将集中在两个主要方面：使用合成波长的色散效应和回扫误差从陡峭的斜坡导致返回的光通过光学系统中的不同的空间位置。Fizeau和Twyman-Green干涉仪配置都将被考虑，因为每个系统具有不同的参考表面位置，并且可能对色散和回扫误差产生不同的影响。合成波长的波长选择影响非模糊范围和扩展奈奎斯特采样限。这一点，以及与信号处理的耦合，为利用合成波长干涉测量法测量自由曲面光学器件提供了潜在的对比标准。

Freeform optics use complex surfaces that are not necessarily axisymmetric, potentially very complex, and have many applications. A continuing problem is the measurement (metrology) of such surfaces. Existing metrology solutions use interferometry, but suffer from fringe visibility issues and other errors, while touch systems such as coordinate measuring machines and profilometry have insufficient accuracy (i.e., micrometer scale as opposed to nanometer scale) and may damage the optical surface, especially for softer materials. The PI will execute a multi-faceted research program aimed at improving the metrology of freeform optics. Central to his approach is the combination of two light sources, each with a different wavelength, in order to evaluate the surface profile of the lens. This approach will lead to the measurement of high aspect ratio surfaces, where interference fringes would normally be spaced too close together to discern from each other. This is enhanced by the consideration of trace and retrace (going over the same path in opposite directions with the light source), and the differences between the two measurements. The approach uses mathematical models that will be used to extract the desired geometry. The result is a methodology that can be used in measuring free-form optics to a higher resolution than currently possible. One Broad Impact of this Faculty Early Career Development (CAREER) program research project is to empower optical designers and manufacturers with metrology methodologies that can measure advanced freeform optics. Freeform optics are the foundation for future vision-based technologies and lightweight, portable, unobtrusive mobile devices. Potential devices range from compact satellites for monitoring environmental changes to better optical sensors for autonomous vehicles. This research will have educational impact at the university level through mentoring research assistants, incorporating research in undergraduate student experiential learning, and outreach with high school students. The PI performs outreach via high school outreach, through workshops, and through extensive podcasting and blogging activities. This research will address this fundamental research question: Can synthetic wavelength interferometry (SWI) be used to produce new enabling capabilities in measuring freeform optics? The PI will examine (theoretically and experimentally) the impact of transmission sphere f-number and local slope departure on vignetting, in order to measure optics that normally have fringe ambiguity and retrace errors. This will be also investigated through the application of synthetic wavelength interferometry (SWI), where two light wavelengths are combined so that the phase difference can be examined and the limits to conventional interferometry can be identified and improved upon. The critical aspect of this technique is that the synthetic wavelength can be generated to resolve fringe ambiguities for lengths potentially much larger than the constitutive wavelengths. This is especially important for optics where the local slope departure is high. Fringe ambiguities arise from insufficient sampling of dense fringe patterns in interferometers, which create aliasing in the measurement. These ambiguities manifest from exceeding Nyquist sampling limits from high slope departure optical surfaces like steep aspheric and freeform optics. The investigations in this research will focus on two main aspects: dispersion effects from using synthetic wavelengths and retrace errors from steep slopes causing the returning light to pass through a different spatial location in the optical system. Both Fizeau and Twyman-Green interferometer configurations will be considered, as each system has a different placement of the reference surface and may cause different effects for both dispersion and retrace errors. The wavelength selection for the synthetic wavelength influences in both the non-ambiguity range and extending the Nyquist sampling limit. This, and the coupling with signal processing present potential contrasting criteria for utilizing synthetic wavelength interferometry for measuring freeform optics.