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

自由形状光学使用的复杂表面不一定是轴对称的，可能非常复杂，并有许多应用。一个持续存在的问题是此类表面的测量(计量学)。现有的测量解决方案使用干涉测量，但存在条纹可见性问题和其他误差，而坐标测量机和轮廓术等触摸系统精度不足(即微米级而不是纳米级)，可能会损坏光学表面，特别是对于较软的材料。PI将执行一个多方面的研究计划，旨在改善自由曲面光学的计量学。他的方法的核心是结合两个光源，每个光源都有不同的波长，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，在这种情况下，干涉条纹通常会间隔太近，彼此无法区分。这一点通过考虑跟踪和回溯(与光源在相反方向上沿同一路径)以及两种测量之间的差异而得到加强。该方法使用将用于提取所需几何图形的数学模型。其结果是一种方法，可以用来测量比目前可能的更高分辨率的自由形状光学元件。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能够测量高级自由形状光学元件的计量方法。自由形状光学技术是未来基于视觉的技术和轻便、便携、低调移动设备的基础。潜在的设备范围从用于监测环境变化的紧凑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，以及与高中生接触，在大学层面产生教育影响。PI通过高中外展、研讨会以及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)能否被用于在测量自由曲面光学中产生新的使能能力？PI将(从理论和实验上)检查透射球f数和局部斜率偏差对晕影的影响，以测量通常具有条纹模糊和回溯误差的光学元件。这也将通过应用合成波长干涉法(SWI)来研究，合成波长干涉法将两个光波长组合在一起，以便可以检查相位差，并可以识别和改进传统干涉测量的限制。这项技术的关键方面是，可以产生合成波长来解决可能比本征波长大得多的长度的条纹模糊。这对于局部斜率偏离很大的光学元件来说尤其重要。条纹模糊是由于干涉仪中密集的条纹图案采样不足而引起的，这会在测量中造成混叠。这些模糊性从高斜率偏离光学表面，如陡峭的非球面和自由形状的光学表面超过奈奎斯特采样限制来体现。这项研究将集中在两个主要方面：使用合成波长的色散效应和陡坡引起的回溯误差，使返回的光通过光学系统中的不同空间位置。Fizeau和Twyman-Green干涉仪配置都将被考虑，因为每个系统都有不同的参考面位置，并且可能会对色散和回扫误差造成不同的影响。合成波长的波长选择对非模糊范围和扩展奈奎斯特采样极限都有影响。这一点，以及与信号处理的耦合，为利用合成波长干涉法测量自由形状光学元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.